I keep reading it is poor practice to use the PHP close tag ?> at the end of the file. The header problem seems irrelevant in the following context (and this is the only good argument so far):
Modern versions of PHP set the output_buffering flag in php.ini If output buffering is enabled, you can set HTTP headers and cookies after outputting HTML because the returned code is not sent to the browser immediately.
Every good practice book and wiki starts with this 'rule' but nobody offers good reasons. Is there another good reason to skip the ending PHP tag?
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Sending headers earlier than the normal course may have far reaching consequences. Below are just a few of them that happened to come to my mind at the moment:
While current PHP releases may have output buffering on, the actual production servers you will be deploying your code on are far more important than any development or testing machines. And they do not always tend to follow latest PHP trends immediately.
You may have headaches over inexplicable functionality loss. Say, you are implementing some kind payment gateway, and redirect user to a specific URL after successful confirmation by the payment processor. If some kind of PHP error, even a warning, or an excess line ending happens, the payment may remain unprocessed and the user may still seem unbilled. This is also one of the reasons why needless redirection is evil and if redirection is to be used, it must be used with caution.
You may get "Page loading canceled" type of errors in Internet Explorer, even in the most recent versions. This is because an AJAX response/json include contains something that it shouldn't contain, because of the excess line endings in some PHP files, just as I've encountered a few days ago.
If you have some file downloads in your app, they can break too, because of this. And you may not notice it, even after years, since the specific breaking habit of a download depends on the server, the browser, the type and content of the file (and possibly some other factors I don't want to bore you with).
Finally, many PHP frameworks including href="http://symfony.com/doc/current/contributing/code/standards.html" rel="noreferrer">Symfony, href="http://framework.zend.com/manual/1.10/en/coding-standard.php-file-formatting.html" rel="noreferrer">Zend and Laravel (there is no mention of this in the href="https://github.com/laravel/framework/blob/master/CONTRIBUTING.md#coding-guidelines" rel="noreferrer">coding guidelines but it follows the suit) and the href="http://www.php-fig.org/psr/psr-2/" rel="noreferrer">PSR-2 standard (item 2.2) require omission of the closing tag. PHP manual itself ( href="http://php.net/basic-syntax.instruction-separation" rel="noreferrer">1, href="http://php.net/manual/en/language.basic-syntax.phptags.php" rel="noreferrer">2), href="http://make.wordpress.org/core/handbook/coding-standards/php/#remove-trailing-spaces" rel="noreferrer">Wordpress, href="https://www.drupal.org/coding-standards#phptags" rel="noreferrer">Drupal and many other PHP software I guess, advise to do so. If you simply make a habit of following the standard (and setup href="https://github.com/fabpot/PHP-CS-Fixer" rel="noreferrer">PHP-CS-Fixer for your code) you can forget the issue. Otherwise you will always need to keep the issue in your mind.
Bonus: a few gotchas (actually currently one) related to these 2 characters:
Even some well-known libraries may contain excess line endings after ?>. An example is Smarty, even the most recent versions of both 2.* and 3.* branch have this. So, as always, watch for third party code. Bonus in bonus: A regex for deleting needless PHP endings: replace (\s*\?>\s*)$ with empty text in all files that contain PHP code.
A Python range is an interesting object because it behaves like a generator when it comes to memory, but it otherwise behaves like a sequence and it additionally has O(1) time complexity for in operation, .index() and .count().
This is an example of a dynamic sequence, i.e. a href="https://docs.python.org/3/library/collections.abc.html#collections.abc.Sequence" rel="nofollow noreferrer">Sequence object that does not store its elements in memory.
How do I implement a dynamic sequence in Python?
The in operation, .index() and .count methods implemented in O(1) time, would, of course, be a nice addition.
This is not a duplicate of href="https://stackoverflow.com/questions/7875911/how-to-implement-a-minimal-class-that-behaves-like-a-sequence-in-python">How to implement a minimal class that behaves like a sequence in Python? as time / memory considerations are not discussed there.
How do I count the number of 1's a number will have in binary?
So let's say I have the number 45, which is equal to 101101 in binary and has 4 1's in it. What's the most efficient way to write an algorithm to do this?
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Instead of writing an algorithm to do this its best to use the built in function. href="http://docs.oracle.com/javase/7/docs/api/java/lang/Integer.html#bitCount%28int%29" rel="noreferrer">Integer.bitCount()
What makes this especially efficient is that the JVM can treat this as an intrinsic. i.e. recognise and replace the whole thing with a single machine code instruction on a platform which supports it e.g. Intel/AMD
To demonstrate how effective this optimisation is
public static void main(String... args) { perfTestIntrinsic();
perfTestACopy(); }
private static void perfTestIntrinsic() { long start = System.nanoTime(); long countBits = 0; for (int i = 0; i < Integer.MAX_VALUE; i++)
countBits += Integer.bitCount(i); long time = System.nanoTime() - start; System.out.printf("Intrinsic: Each bit count took %.1f ns, countBits=%d%n", (double) time / Integer.MAX_VALUE, countBits); }
private static void perfTestACopy() { long start2 = System.nanoTime(); long countBits2 = 0;
for (int i = 0; i < Integer.MAX_VALUE; i++) countBits2 += myBitCount(i);
long time2 = System.nanoTime() - start2;
System.out.printf("Copy of same code: Each bit count took %.1f ns, countBits=%d%n", (double) time2 / Integer.MAX_VALUE, countBits2); }
// Copied from Integer.bitCount() public static int myBitCount(int i) { // HD, Figure 5-2 i = i - ((i >>> 1) & 0x55555555); i = (i & 0x33333333) + ((i >>> 2) & 0x33333333); i = (i + (i >>> 4)) & 0x0f0f0f0f;
i = i + (i >>> 8); i = i + (i >>> 16); return i & 0x3f; }
prints
Intrinsic: Each bit count took 0.4 ns, countBits=33285996513 Copy of same code: Each bit count took 2.4 ns, countBits=33285996513
Each bit count using the intrinsic version and loop takes just 0.4 nano-second on average. Using a copy of the same code takes 6x longer (gets the same result)
The problem is with subfolders. If I manually go to: rel="nofollow noreferrer">http://www.website.com/myfolder it pushes me to: https://website.com/myfolder/.php (404 error)
My goal is to always remove the "www" and redirect to SSL for the root and all subfolders. Obviously without the above error where it somehow leaves the ".php"
main.o(.text+0x1ed): In function `main': : undefined reference to `avergecolumns' collect2: ld returned 1 exit status
when I gcc *.o. I'm not quite sure what causes this error. Other posters have explained it as the function is not found or the function is empty. If someone could clarify or refine, it would be greaty appreciated!
Here is my function's code(I'm trying to calculate the average of the column in 2D arrays):
#include "my.h"
void averagecolumns (int x, int y, int** a) { int i; int j; float sum;
float colAvg;
sum = 0; colAvg = 0;
printf("i. The column averages are: \n");
for(i = 0; i < x; i++) { for(j = 0; j < y; j++)
{ sum += a[i][j]; colAvg = sum / (float)x; }
printf("Column: %3d, Average: %6.2f", j, colAvg);
sum = 0;
colAvg = 0;
}
The relavent parts of main are:
#include "my.h"
int main (int argc, char* argv[])
{ int** a; float** colAvg;
int ROWS; int COLS; int i; int j; int** table; FILE* fpmyfile; int closeResult;
Also, is this a linker or a compile error (I wasn't sure which tag to add).
Answer
It's a linker error. ld is the linker, so if you get an error message ending with "ld returned 1 exit status", that tells you that it's a linker error.
The error message tells you that none of the object files you're linking against contains a definition for avergecolumns. The reason for that is that the function you've defined is called averagecolumns (in other words: you misspelled the function name when calling the function (and presumably in the header file as well - otherwise you'd have gotten a different error at compile time)).
When I press the Android HOME key, I can see the "Desktop" and my app icon. Then I press my app icon and my application launches twice. I don't want open my app twice.
How my program works:
I have 4 Activities (A, B, C, D).
A - The Main Activity: It is the first to open. It opens the other activity that has a lot of buttons. It's like a Java's main() method. I show a SplashScreen and I call another Activity. Then I finish my activity "A".
B - The Menu Screen: In this activity, I have some buttons, like a menu. I have a configuration button, update button, and Login Button. When I click the login button, I finish this activity and open the Login Screen (Activity "C").
C - The Login Screen: The user writes the Login and Password. If the login is successful, I finish this activity and open the Activity "D".
D - The application main screen: It stays opened all the time and launches another Activities. I finish this when I want close my application.
P.S.: I tried change the launchMode flag (androidManifest.xml), but didn't work.
I have some code and when it executes, it throws a NullReferenceException, saying:
Object reference not set to an instance of an object.
What does this mean, and what can I do to fix this error?
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Bottom Line
You are trying to use something that is null (or Nothing in VB.NET). This means you either set it to null, or you never set it to anything at all.
Like anything else, null gets passed around. If it is null in method "A", it could be that method "B" passed a nullto method "A".
null can have different meanings:
Object variables which are uninitialized and hence point to nothing. In this case, if you access properties or methods of such objects, it causes a NullReferenceException.
The developer is using null intentionally to indicate there is no meaningful value available. Note that C# has the concept of nullable datatypes for variables (like database tables can have nullable fields) - you can assign null to them to indicate there is no value stored in it, for example int? a = null; where the question mark indicates it is allowed to store null in variable a. You can check that either with if (a.HasValue) {...} or with if (a==null) {...}. Nullable variables, like a this example, allow to access the value via a.Value explicitly, or just as normal via a. Note that accessing it via a.Value throws an InvalidOperationException instead of a NullReferenceException if a is null - you should do the check beforehand, i.e. if you have another on-nullable variable int b; then you should do assignments like if (a.HasValue) { b = a.Value; } or shorter if (a != null) { b = a; }.
The rest of this article goes into more detail and shows mistakes that many programmers often make which can lead to a NullReferenceException.
More Specifically
The runtime throwing a NullReferenceExceptionalways means the same thing: you are trying to use a reference, and the reference is not initialized (or it was once initialized, but is no longer initialized).
This means the reference is null, and you cannot access members (such as methods) through a null reference. The simplest case:
string foo = null; foo.ToUpper();
This will throw a NullReferenceException at the second line because you can't call the instance method ToUpper() on a string reference pointing to null.
How do you find the source of a NullReferenceException? Apart from looking at the exception itself, which will be thrown exactly at the location where it occurs, the general rules of debugging in Visual Studio apply: place strategic breakpoints and rel="noreferrer">inspect your variables, either by hovering the mouse over their names, opening a (Quick)Watch window or using the various debugging panels like Locals and Autos.
If you want to find out where the reference is or isn't set, right-click its name and select "Find All References". You can then place a breakpoint at every found location and run your program with the debugger attached. Every time the debugger breaks on such a breakpoint, you need to determine whether you expect the reference to be non-null, inspect the variable and and verify that it points to an instance when you expect it to.
By following the program flow this way, you can find the location where the instance should not be null, and why it isn't properly set.
Some common scenarios where the exception can be thrown:
Generic
ref1.ref2.ref3.member
If ref1 or ref2 or ref3 is null, then you'll get a NullReferenceException. If you want to solve the problem, then find out which one is null by rewriting the expression to its simpler equivalent:
var r1 = ref1; var r2 = r1.ref2; var r3 = r2.ref3; r3.member
Specifically, in HttpContext.Current.User.Identity.Name, the HttpContext.Current could be null, or the User property could be null, or the Identity property could be null.
Indirect
public class Person { public int Age { get; set; } } public class Book {
public Person Author { get; set; } } public class Example { public void Foo() {
Book b1 = new Book(); int authorAge = b1.Author.Age; // You never initialized the Author property. // there is no Person to get an Age from.
} }
If you want to avoid the child (Person) null reference, you could initialize it in the parent (Book) object's constructor.
Nested Object Initializers
The same applies to nested object initializers:
Book b1 = new Book { Author = { Age = 45 } };
This translates to
Book b1 = new Book(); b1.Author.Age = 45;
While the new keyword is used, it only creates a new instance of Book, but not a new instance of Person, so the Author the property is still null.
Nested Collection Initializers
public class Person { public ICollection Books { get; set; } } public class Book { public string Title { get; set; } }
The nested collection initializers behave the same:
Person p1 = new Person { Books = { new Book { Title = "Title1" }, new Book { Title = "Title2" },
} };
This translates to
Person p1 = new Person(); p1.Books.Add(new Book { Title = "Title1" }); p1.Books.Add(new Book { Title = "Title2" });
The new Person only creates an instance of Person, but the Books collection is still null. The collection initializer syntax does not create a collection for p1.Books, it only translates to the p1.Books.Add(...) statements.
Array
int[] numbers = null; int n = numbers[0]; // numbers is null. There is no array to index.
Array Elements
Person[] people = new Person[5]; people[0].Age = 20 // people[0] is null. The array was allocated but not // initialized. There is no Person to set the Age for.
Jagged Arrays
long[][] array = new long[1][]; array[0][0] = 3; // is null because only the first dimension is yet initialized. // Use array[0] = new long[2]; first.
Collection/List/Dictionary
Dictionary int> agesForNames = null; int age = agesForNames["Bob"]; // agesForNames is null. // There is no Dictionary to perform the lookup.
Range Variable (Indirect/Deferred)
public class Person { public string Name { get; set; } }
var people = new List(); people.Add(null); var names = from p in people select p.Name; string firstName = names.First(); // Exception is thrown here, but actually occurs // on the line above. "p" is null because the
// first element we added to the list is null.
Events
public class Demo { public event EventHandler StateChanged;
protected virtual void OnStateChanged(EventArgs e) { StateChanged(this, e); // Exception is thrown here
// if no event handlers have been attached // to StateChanged event
}
}
Bad Naming Conventions:
If you named fields differently from locals, you might have realized that you never initialized the field.
This can be solved by following the convention to prefix fields with an underscore:
private Customer _customer;
ASP.NET Page Life cycle:
public partial class Issues_Edit : System.Web.UI.Page {
protected TestIssue myIssue;
protected void Page_Load(object sender, EventArgs e) { if (!IsPostBack) { // Only called on first load, not when button clicked myIssue = new TestIssue();
// if the "FirstName" session value has not yet been set, // then this line will throw a NullReferenceException string firstName = Session["FirstName"].ToString();
ASP.NET MVC empty view models
If the exception occurs when referencing a property of @Model in an ASP.NET MVC view, you need to understand that the Model gets set in your action method, when you return a view. When you return an empty model (or model property) from your controller, the exception occurs when the views access it:
// Controller
public class Restaurant:Controller { public ActionResult Search()
{ return View(); // Forgot the provide a Model here.
} }
// Razor view @foreach (var restaurantSearch in Model.RestaurantSearch) // Throws.
{ }
@Model.somePropertyName
WPF Control Creation Order and Events
WPF controls are created during the call to InitializeComponent in the order they appear in the visual tree. A NullReferenceException will be raised in the case of early-created controls with event handlers, etc. , that fire during InitializeComponent which reference late-created controls.
Changing the order of the declarations in the XAML (i.e., listing label1 before comboBox1, ignoring issues of design philosophy, would at least resolve the NullReferenceException here.
Cast with as
var myThing = someObject as Thing;
This doesn't throw an InvalidCastException but returns a null when the cast fails (and when someObject is itself null). So be aware of that.
LINQ FirstOrDefault() and SingleOrDefault()
The plain versions First() and Single() throw exceptions when there is nothing. The "OrDefault" versions return null in that case. So be aware of that.
foreach
foreach throws when you try to iterate null collection. Usually caused by unexpected null result from methods that return collections.
List list = null; foreach(var v in list) { } // exception
More realistic example - select nodes from XML document. Will throw if nodes are not found but initial debugging shows that all properties valid:
foreach (var node in myData.MyXml.DocumentNode.SelectNodes("//Data"))
/>
Explicitly check for null and ignore null values.
If you expect the reference sometimes to be null, you can check for it being null before accessing instance members:
void PrintName(Person p) { if (p != null) {
Console.WriteLine(p.Name);
} }
Explicitly check for null and provide a default value.
Methods call you expect to return an instance can return null, for example when the object being sought cannot be found. You can choose to return a default value when this is the case:
string GetCategory(Book b) { if (b == null) return "Unknown"; return b.Category; }
Explicitly check for null from method calls and throw a custom exception.
You can also throw a custom exception, only to catch it in the calling code:
string GetCategory(string bookTitle) { var book = library.FindBook(bookTitle); // This may return null if (book == null) throw new BookNotFoundException(bookTitle); // Your custom exception return book.Category; }
Use Debug.Assert if a value should never be null, to catch the problem earlier than the exception occurs.
When you know during development that a method maybe can, but never should return null, you can use Debug.Assert() to break as soon as possible when it does occur:
string GetTitle(int knownBookID) { // You know this should never return null. var book = library.GetBook(knownBookID);
// Exception will occur on the next line instead of at the end of this method.
Debug.Assert(book != null, "Library didn't return a book for known book ID.");
// Some other code
return book.Title; // Will never throw NullReferenceException in Debug mode. }
Though this check href="https://stackoverflow.com/questions/3021538/debug-assert-appears-in-release-mode">will not end up in your release build, causing it to throw the NullReferenceException again when book == null at runtime in release mode.
Use GetValueOrDefault() for nullable value types to provide a default value when they are null.
DateTime? appointment = null; Console.WriteLine(appointment.GetValueOrDefault(DateTime.Now)); // Will display the default value provided (DateTime.Now), because appointment is null.
appointment = new DateTime(2022, 10, 20); Console.WriteLine(appointment.GetValueOrDefault(DateTime.Now)); // Will display the appointment date, not the default
Use the null coalescing operator: ?? [C#] or If() [VB].
The shorthand to providing a default value when a null is encountered:
IService CreateService(ILogger log, Int32? frobPowerLevel) { var serviceImpl = new MyService(log ?? NullLog.Instance);
// Note that the above "GetValueOrDefault()" can also be rewritten to use // the coalesce operator: serviceImpl.FrobPowerLevel = frobPowerLevel ?? 5; }
Use the null condition operator: ?. or ?[x] for arrays (available in C# 6 and VB.NET 14):
This is also sometimes called the safe navigation or Elvis (after its shape) operator. If the expression on the left side of the operator is null, then the right side will not be evaluated, and null is returned instead. That means cases like this:
var title = person.Title.ToUpper();
If the person does not have a title, this will throw an exception because it is trying to call ToUpper on a property with a null value.
In C# 5 and below, this can be guarded with:
var title = person.Title == null ? null : person.Title.ToUpper();
Now the title variable will be null instead of throwing an exception. C# 6 introduces a shorter syntax for this:
var title = person.Title?.ToUpper();
This will result in the title variable being null, and the call to ToUpper is not made if person.Title is null.
Of course, you still have to check title for null or use the null condition operator together with the null coalescing operator (??) to supply a default value:
// regular null check
int titleLength = 0; if (title != null)
titleLength = title.Length; // If title is null, this would throw NullReferenceException
// combining the `?` and the `??` operator int titleLength = title?.Length ?? 0;
Likewise, for arrays you can use ?[i] as follows:
int[] myIntArray=null; var i=5; int? elem = myIntArray?[i]; if (!elem.HasValue) Console.WriteLine("No value");
This will do the following: If myIntArray is null, the expression returns null and you can safely check it. If it contains an array, it will do the same as: elem = myIntArray[i]; and returns the ith element.
Use null context (available in C# 8):
Introduced in C# 8 there null context's and nullable reference types perform static analysis on variables and provides a compiler warning if a value can be potentially null or have been set to null. The nullable reference types allows types to be explicitly allowed to be null.
The nullable annotation context and nullable warning context can be set for a project using the Nullable element in your csproj file. This element configures how the compiler interprets the nullability of types and what warnings are generated. Valid settings are:
enable: The nullable annotation context is enabled. The nullable warning context is enabled. Variables of a reference type, string for example, are non-nullable. All nullability warnings are enabled.
disable: The nullable annotation context is disabled. The nullable warning context is disabled. Variables of a reference type are oblivious, just like earlier versions of C#. All nullability warnings are disabled.
safeonly: The nullable annotation context is enabled. The nullable warning context is safeonly. Variables of a reference type are nonnullable. All safety nullability warnings are enabled.
warnings: The nullable annotation context is disabled. The nullable warning context is enabled. Variables of a reference type are oblivious. All nullability warnings are enabled.
safeonlywarnings: The nullable annotation context is disabled. The nullable warning context is safeonly.
Variables of a reference type are oblivious. All safety nullability warnings are enabled.
A nullable reference type is noted using the same syntax as nullable value types: a ? is appended to the type of the variable.
Special techniques for debugging and fixing null derefs in iterators
C# supports "iterator blocks" (called "generators" in some other popular languages). Null dereference exceptions can be particularly tricky to debug in iterator blocks because of deferred execution:
public IEnumerable GetFrobs(FrobFactory f, int count) { for (int i = 0; i < count; ++i) yield return f.MakeFrob(); } ... FrobFactory factory = whatever; IEnumerable frobs = GetFrobs(); ... foreach(Frob frob in frobs) { ... }
If whatever results in null then MakeFrob will throw. Now, you might think that the right thing to do is this:
// DON'T DO THIS public IEnumerable GetFrobs(FrobFactory f, int count) { if (f == null) throw new ArgumentNullException("f", "factory must not be null"); for (int i = 0; i < count; ++i)
yield return f.MakeFrob(); }
Why is this wrong? Because the iterator block does not actually run until the foreach! The call to GetFrobs simply returns an object which when iterated will run the iterator block.
By writing a null check like this you prevent the null dereference, but you move the null argument exception to the point of the iteration, not to the point of the call, and that is very confusing to debug.
The correct fix is:
// DO THIS public IEnumerable GetFrobs(FrobFactory f, int count) { // No yields in a public method that throws! if (f == null) throw new ArgumentNullException("f", "factory must not be null"); return GetFrobsForReal(f, count); } private IEnumerable GetFrobsForReal(FrobFactory f, int count) {
// Yields in a private method
Debug.Assert(f != null); for (int i = 0; i < count; ++i) yield return f.MakeFrob(); }
That is, make a private helper method that has the iterator block logic, and a public surface method that does the null check and returns the iterator. Now when GetFrobs is called, the null check happens immediately, and then GetFrobsForReal executes when the sequence is iterated.
If you examine the reference source for LINQ to Objects you will see that this technique is used throughout. It is slightly more clunky to write, but it makes debugging nullity errors much easier. Optimize your code for the convenience of the caller, not the convenience of the author.
A note on null dereferences in unsafe code
C# has an "unsafe" mode which is, as the name implies, extremely dangerous because the normal safety mechanisms which provide memory safety and type safety are not enforced. You should not be writing unsafe code unless you have a thorough and deep understanding of how memory works.
In unsafe mode, you should be aware of two important facts:
dereferencing a null pointer produces the same exception as dereferencing a null reference
dereferencing an invalid non-null pointer can produce that exception
in some circumstances
To understand why that is, it helps to understand how .NET produces null dereference exceptions in the first place. (These details apply to .NET running on Windows; other operating systems use similar mechanisms.)
Memory is virtualized in Windows; each process gets a virtual memory space of many "pages" of memory that are tracked by the operating system. Each page of memory has flags set on it which determine how it may be used: read from, written to, executed, and so on. The lowest page is marked as "produce an error if ever used in any way".
Both a null pointer and a null reference in C# are internally represented as the number zero, and so any attempt to dereference it into its corresponding memory storage causes the operating system to produce an error. The .NET runtime then detects this error and turns it into the null dereference exception.
That's why dereferencing both a null pointer and a null reference produces the same exception.
What about the second point? Dereferencing any invalid pointer that falls in the lowest page of virtual memory causes the same operating system error, and thereby the same exception.
Why does this make sense? Well, suppose we have a struct containing two ints, and an unmanaged pointer equal to null. If we attempt to dereference the second int in the struct, the CLR will not attempt to access the storage at location zero; it will access the storage at location four. But logically this is a null dereference because we are getting to that address via the null.
If you are working with unsafe code and you get a null dereference exception, just be aware that the offending pointer need not be null. It can be any location in the lowest page, and this exception will be produced.
I was looking at some of the changes made to the Java SE API with 1.8, and I when looking at the new method href="http://docs.oracle.com/javase/8/docs/api/java/util/Map.html#merge-K-V-java.util.function.BiFunction-" rel="noreferrer">Map.merge it shows an example of how to use it with the line
map.merge(key, msg, String::concat)
I understand how to use a lambda expressions to create anonymous functional interfaces, but this seems to use a method as a BiFunction. I like to understand and use obscure java syntaxes, but I can't find any mention of this one anywhere.
When I try to run the rel="noreferrer">splice method on the array I get a TypeError: array.splice is not a function. My intent is to remove the an_object "key" and all its content from the array variable.
How can I make that?
/>
Note: When I run the console.log(array[an_object]) (the same is valid for another_object and all other objects) I get:
Now, you can delete the entry in the object with the delete statement:
delete myObject[an_object]; // Returns true / false if the deletion is a success / failure console.log(myObject[an_object]) // Returns undefined
/>
Now, that said, this will not work like you'd expect. myObject[an_object] will contain "abc" Don't use objects as keys. Use strings, instead. This is because of the fact that any parameter entered in the [] will be converted to string. So actually, you're entering myObject["[object Object]"]
I need to turn a linked list class for ints that I wrote into a class template. I'm having issues with the constructor and destructor for a struct nested in the List Class, called node.
Layout:
template class List {
public: //Stuff that's not important to this question
private: struct Node { Node(T value); // constructor ~Node(); // destructor Node *next; // pointer to the next Node T data; // the actual data in the node static int nodes_alive; // count of nodes still allocated
};
};
Implementation:
template typename List::Node::Node(T value) { data = value;
next = 0; }
template typename List::Node::~Node() {
--nodes_alive; }
Errors:
Expected ';' at end of declaration
typename List::Node::Node(T value)
Expected an identifier or template ID after '::'
typename List::Node::~Node()
Expected the class name after '~' to name a destructor
typename List::Node::~Node()
Not really sure what's going on here. My implementation is in a separate file included at the bottom of the header file. Any help would be greatly appreciated.
style="font-weight: bold;">
Answer
It's simple: Get rid of the typename keyword. Since you're writing a constructor/destructor and there is no return type, it is not needed.
rel="nofollow noreferrer">The ideal format of the email I wish to automate. Certain parts blacked out for privacy.
I am trying to create an email template for business use that can be sent using VBA because the ultimate goal is that the user can only fill in the blanks via userform therefore the text remains unchanged. I already have the userform, and have coded in such a way that the proper blanks are filled in in the text portion and are included in the email body, however I have not figured out how to include the table as well.
Here is what I have so far as my attempt to add the table:
Sub SendEmail(what_address As String, subject_line As String, mail_body As String, claim_info As Range)
Dim olApp As Outlook.Application Set olApp = CreateObject("Outlook.Application")
Dim olMail As Outlook.MailItem Set olMail = olApp.CreateItem(olMailItem)
Dim mail_body_message As String Dim tracking_number As String Dim amount_paid As String Dim date_paid As String Dim payment_due As String Dim claim As Range
Set claim = Nothing On Error Resume Next 'Only send the visible cells in the selection. Set claim = Selection.SpecialCells(xlCellTypeVisible) Set claim = Sheets("Sheet1").RangeToHtml("B2:C9").SpecialCells(xlCellTypeVisible, xlTextValues) On Error GoTo 0
When building my C++ program, I'm getting the error message
undefined reference to 'vtable...
What is the cause of this problem? How do I fix it?
/>
It so happens that I'm getting the error for the following code (The class in question is CGameModule.) and I cannot for the life of me understand what the problem is. At first, I thought it was related to forgetting to give a virtual function a body, but as far as I understand, everything is all here. The inheritance chain is a little long, but here is the related source code. I'm not sure what other information I should provide.
Note: The constructor is where this error is happening, it'd seem.
So, I've figured out the issue and it was a combination of bad logic and not being totally familiar with the automake/autotools world. I was adding the correct files to my Makefile.am template, but I wasn't sure which step in our build process actually created the makefile itself. So, I was compiling with an old makefile that had no idea about my new files whatsoever.
Thanks for the responses and the link to the GCC FAQ. I will be sure to read that to avoid this problem occurring for a real reason.
I need a good explanation (references are a plus) on Python's slice notation.
To me, this notation needs a bit of picking up.
It looks extremely powerful, but I haven't quite got my head around it.
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It's pretty simple really:
a[start:stop] # items start through stop-1 a[start:] # items start through the rest of the array a[:stop] # items from the beginning through stop-1 a[:] # a copy of the whole array
There is also the step value, which can be used with any of the above:
a[start:stop:step] # start through not past stop, by step
The key point to remember is that the :stop value represents the first value that is not in the selected slice. So, the difference between stop and start is the number of elements selected (if step is 1, the default).
The other feature is that start or stop may be a negative number, which means it counts from the end of the array instead of the beginning. So:
a[-1] # last item in the array a[-2:] # last two items in the array
a[:-2] # everything except the last two items
Similarly, step may be a negative number:
a[::-1] # all items in the array, reversed a[1::-1] # the first two items, reversed a[:-3:-1] # the last two items, reversed a[-3::-1] # everything except the last two items, reversed
Python is kind to the programmer if there are fewer items than you ask for. For example, if you ask for a[:-2] and a only contains one element, you get an empty list instead of an error. Sometimes you would prefer the error, so you have to be aware that this may happen.
Relation to slice() object
The slicing operator [] is actually being used in the above code with a slice() object using the : notation (which is only valid within []), i.e.:
a[start:stop:step]
is equivalent to:
a[slice(start, stop, step)]
Slice objects also behave slightly differently depending on the number of arguments, similarly to range(), i.e. both slice(stop) and slice(start, stop[, step]) are supported. To skip specifying a given argument, one might use None, so that e.g. a[start:] is equivalent to a[slice(start, None)] or a[::-1] is equivalent to a[slice(None, None, -1)].
While the :-based notation is very helpful for simple slicing, the explicit use of slice() objects simplifies the programmatic generation of slicing.
In most of the movies and TV-series I have watched, whenever there is some kind of "technology" involved (say, like a tracking device or hacking something important), there is a lot of unnecessary, impractical key-pressing and fancy colors and sounds (a lot of 1's and 0's going on the screen and a lot of "beep"s).
For example, take the TV-series href="http://www.imdb.com/title/tt0808096/">Primeval. Their device for tracking anomalies have a lot of the things said above.
Why do movies and TV-series employ this kind of false-looking tech? It would have been much easier to use a real OS (like MacOS for normal things and Linux for hacking-kind-of things), maybe with a custom-made software suited to the task. It would have been more realistic.
So is there any specific reason for this?
Answer
Well, I think there is perfect reason for it: audience appeal. Of course those things are totally unrealistic and over the top, but show a simple black-and-white console to the audience or a basic database application and they will just find it boring to look at or think there is not much to it.
Of course it bothers the hell out of those who know better, but the average guy that uses his computer for YouTube, Facebook and maybe Office is just more pleased with colorful displays and stunning graphics in a simple database query and is more likely to accept that there is something interesting going on. It's about conveying the information inherent in those abstract and hard to grasp processes in an interesting and entertaining way, thus sacrificing some realism for the sake of story-telling.
(And of course all hackers fluently communicate in 0s and 1s, which makes them so awesome. ;-))
I'm new to android but not to sockets. I have a GUI with a button that runs the UAVServer thread. When I click it, the server should listen for the client. The line of code...
Socket client = serverSocket.accept();
...Should block until a client connects. But it just crashes saying "Unfortunately, DroidUAV has stopped."
public class UAVServer extends Thread {
private String TAG = UAVServer.class.getSimpleName();
@Override public void run() { ServerSocket serverSocket; try { serverSocket = new ServerSocket(12345); Log.d(TAG, "Fails at the next line of code");
This seems pretty straight forward, why would it be crashing? Also how does android handle blocking ports, this might have something to do with it. If it was on the wifi, I figure it would be up to the router to unblock ports. But what if I was just on 4G?
NOTE: uses-permission android:name="android.permission.INTERNET" is in my manifest file.
Edit: I must be doing something wrong, because I just tried to make it run the client instead and it crashes on this line...
Socket s = new Socket("192.168.1.102",4444);
Edit: Added errors...
FATAL EXCEPTION: main java.lang.IllegalStateException: Could not execute method of the activity at android.view.View$1.onClick(View.java:3071) at android.view.View.performClick(View.java:3538) at android.widget.CompoundButton.performClick(CompoundButton.java:103) at android.view.View$PerformClick.run(View.java:14319) at android.os.Handler.handleCallback(Handler.java:608)
at android.os.Handler.dispatchMessage(Handler.java:92) at android.os.Looper.loop(Looper.java:156) at android.app.ActivityThread.main(ActivityThread.java:5099) at java.lang.reflect.Method.invokeNative(Native Method) at java.lang.reflect.Method.invoke(Method.java:511) at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:991)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:758) at dalvik.system.NativeStart.main(Native Method) Caused by: java.lang.reflect.InvocationTargetException at java.lang.reflect.Method.invokeNative(Native Method)
at java.lang.reflect.Method.invoke(Method.java:511) at android.view.View$1.onClick(View.java:3066) ... 12 more Caused by: android.os.NetworkOnMainThreadException at android.os.StrictMode$AndroidBlockGuardPolicy.onNetwork(StrictMode.java:1190)
at libcore.io.BlockGuardOs.accept(BlockGuardOs.java:54) at java.net.PlainSocketImpl.accept(PlainSocketImpl.java:98) at java.net.ServerSocket.implAccept(ServerSocket.java:202) at java.net.ServerSocket.accept(ServerSocket.java:127) at com.example.droiduav.UAVServer.run(UAVServer.java:19)
at com.example.droiduav.MainActivity.onToggleServer(MainActivity.java:31) ... 15 more
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Well, the problem you are facing has to do with the following exception that can be seen in your LogCat trace:
What it means is pretty straightforward guessable from the name. Don't perform any Network code on the Main UI Thread. Solution: href="http://developer.android.com/training/basics/network-ops/connecting.html#AsyncTask" rel="nofollow noreferrer">Start your own thread for that (see href="http://developer.android.com/training/basics/network-ops/connecting.html#AsyncTask" rel="nofollow noreferrer">this Android Developer article).
Why?
Network code can take a "long time" (assume several seconds) to execute. If you would call all your network code on the main thread, it will freeze the UI in the mean time (this is - as one could guess - not really preferable and could and will result in href="http://developer.android.com/training/articles/perf-anr.html" rel="nofollow noreferrer">ANR exceptions).
see also these questions as they are related to your problem:
In href="http://www.imdb.com/title/tt2170049/" rel="nofollow">True Blood: Authority Always Wins, Alexander Drew's character was great and fantastic, he was aged 9 years before being turned (according to href="http://trueblood.wikia.com/wiki/Alexander_Drew" rel="nofollow">trueblood.wikia). But it also says that he serves the Vampire Authority as a Chancellor.
According to href="http://trueblood.wikia.com/wiki/Alexander_Drew" rel="nofollow">trueblood.wikia:
Alexander Drew is an angelic looking, yet impetuous vampire. He was
turned at the tender age of 9, and serves the Vampire Authority as a
Chancellor. He enjoys smoking cigarettes and taunting everyone around him, including his fellow Chancellors.
Other vampires such as rel="nofollow">Salome and href="http://trueblood.wikia.com/wiki/Godric" rel="nofollow">Godric are aged 2000+ years. So, how does it possible for Alexander to become a chancellor of the Vampire authority?
Answer
href="http://trueblood.wikia.com/wiki/Alexander_Drew" rel="nofollow">As the wiki states, Alexander was aged 9 years old when he was turned.
Just like all vampires, they do not age beyond what age they were when turned. For all we know he could be beyond age of all vampires including Russell (2800 years old), Eric (1100+), and especially Bill (176).
This detail has not been made known yet, however but we can assume he is a very old vampire, if not, very experienced to be on the council.
If you come from a class-based, statically typed object-oriented language (like Java, C++ or C#) I assume that you are trying to create a variable or method associated to a "type" but not to an instance.
An example using a "classical" approach, with constructor functions maybe could help you to catch the concepts of basic OO JavaScript:
function MyClass () { // constructor function var privateVariable = "foo"; // Private variable
this.publicVariable = "bar"; // Public variable
this.privilegedMethod = function () { // Public Method
alert(privateVariable); }; }
// Instance method will be available to all instances but only load once in memory
MyClass.prototype.publicMethod = function () {
alert(this.publicVariable); };
// Static variable shared by all instances MyClass.staticProperty = "baz";
var myInstance = new MyClass();
staticProperty is defined in the MyClass object (which is a function) and has nothing to do with its created instances, JavaScript treats functions as href="http://en.wikipedia.org/wiki/First-class_function" rel="nofollow noreferrer">first-class objects, so being an object, you can assign properties to a function.
UPDATE: ES6 introduced the ability to href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Classes" rel="nofollow noreferrer">declare classes through the class keyword. It is syntax sugar over the existing prototype-based inheritance.
The href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Classes/static" rel="nofollow noreferrer">static keyword allows you to easily define static properties or methods in a class.
Let's see the above example implemented with ES6 classes:
class MyClass { // class constructor, equivalent to // the function body of a constructor constructor() { const privateVariable = 'private value'; // Private variable at the constructor scope this.publicVariable = 'public value'; // Public property
this.privilegedMethod = function() { // Public Method with access to the constructor scope variables
console.log(privateVariable); }; }
// Prototype methods: publicMethod() {
console.log(this.publicVariable); }
// Static properties shared by all instances
static staticProperty = 'static value';
static staticMethod() {
console.log(this.staticProperty); } }
// We can add properties to the class prototype MyClass.prototype.additionalMethod = function() {
First, it's not a good practice to name classes with a lowercase, so try renaming to something like MakeGUI instead of makeGUI.
The problem with your commented code is that it creates a new instance of makeGUI every time the button is clicked and the action listener is invoked. The result is that when you click on the close button, a new frame is created, then an inner one and this inner one gets immediately closed. The only thing you'd be doing is creating more and more frames. You should keep the instance as a state, for instance as a class member:
class MakeGUI { JFrame smallframe; JButton close = new JButton("CLOSE ME"); MakeGUI() { frame f1 = new frame(); smallframe = new JFrame(); //want to close this one JPanel jp = new JPanel(new FlowLayout()); smallframe.setSize(300, 300);
class action implements ActionListener { public void actionPerformed(ActionEvent e) { if (e.getSource() == close) { // use this instead of dispose smallframe.dispatchEvent(new WindowEvent(smallframe, WindowEvent.WINDOW_CLOSING));
