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In templates, where and why
do I have to put typename and template
on dependent names? What exactly are dependent names anyway? I have the following
code:
template typename Tail> // Tail will be a UnionNode too.
struct UnionNode : public
Tail {
// ...
template struct inUnion
{
// Q: where to add typename/template here?
typedef
Tail::inUnion dummy;
};
template< > struct
inUnion {
};
};
template T> // For the last node Tn.
struct UnionNode {
//
...
template struct inUnion {
char fail[ -2 +
(sizeof(U)%2) ]; // Cannot be instantiated for any U
};
template< > struct inUnion {
};
};
The
problem I have is in the typedef Tail::inUnion dummy
line. I'm fairly certain that inUnion is a dependent name, and
VC++ is quite right in choking on it. I also know that I should be able to add
template somewhere to tell the compiler that inUnion is a
template-id. But where exactly? And should it then assume that inUnion is a class
template, i.e. inUnion names a type and not a
function?
In order to parse a C++ program, the compiler
needs to know whether certain names are types or not. The following example demonstrates
that:
t *
f;
How
should this be parsed? For many languages a compiler doesn't need to know the meaning of
a name in order to parse and basically know what action a line of code does. In C++, the
above however can yield vastly different interpretations depending on what
t means. If it's a type, then it will be a declaration of a
pointer f. However if it's not a type, it will be a
multiplication. So the C++ Standard says at paragraph
(3/7):
Some names
denote types or templates. In general, whenever a name is encountered it is necessary to
determine whether that name denotes one of these entities before continuing to parse the
program that contains it. The process that determines this is called name
lookup.
How will
the compiler find out what a name t::x refers to, if
t refers to a template type parameter?
x could be a static int data member that could be multiplied or
could equally well be a nested class or typedef that could yield to a declaration. If a
name has this property - that it can't be looked up until the actual template arguments
are known - then it's called a dependent name (it "depends" on the
template parameters).
You might recommend to
just wait till the user instantiates the template:
Let's wait until the user instantiates the template, and then later
find out the real meaning of t::x * f;.
This will work
and actually is allowed by the Standard as a possible implementation approach. These
compilers basically copy the template's text into an internal buffer, and only when an
instantiation is needed, they parse the template and possibly detect errors in the
definition. But instead of bothering the template's users (poor colleagues!) with errors
made by a template's author, other implementations choose to check templates early on
and give errors in the definition as soon as possible, before an instantiation even
takes place.
So there has to be a way to tell
the compiler that certain names are types and that certain names aren't.
The "typename"
keyword
The answer is:
We decide how the compiler should parse this. If
t::x is a dependent name, then we need to prefix it by
typename to tell the compiler to parse it in a certain way. The
Standard says at (14.6/2):
A name used in a template declaration or definition and that is dependent on a
template-parameter is
assumed not to name a type unless the applicable name
lookup finds a type name or the name is qualified
by the keyword typename.
There are many
names for which typename is not necessary, because the compiler
can, with the applicable name lookup in the template definition, figure out how to parse
a construct itself - for example with T *f;, when
T is a type template parameter. But for t::x *
f; to be a declaration, it must be written as typename t::x
*f;. If you omit the keyword and the name is taken to be a non-type, but
when instantiation finds it denotes a type, the usual error messages are emitted by the
compiler. Sometimes, the error consequently is given at definition
time:
// t::x is taken
as non-type, but as an expression the following misses an
// operator between
the two names or a semicolon separating them.
t::x
f;
The
syntax allows typename only before qualified names -
it is therefor taken as granted that unqualified names are always known to refer to
types if they do so.
A similar gotcha exists for
names that denote templates, as hinted at by the introductory
text.
The "template"
keyword
Remember the initial quote
above and how the Standard requires special handling for templates as well? Let's take
the following innocent-looking example:
boost::function< int() >
f;
It might look
obvious to a human reader. Not so for the compiler. Imagine the following arbitrary
definition of boost::function and
f:
namespace
boost { int function = 0; }
int main() {
int f =
0;
boost::function< int() > f;
}
That's
actually a valid expression! It uses the less-than operator to
compare boost::function against zero
(int()), and then uses the greater-than operator to compare the
resulting bool against f. However as
you might well know, boost::function href="http://www.boost.org/doc/libs/1_54_0/doc/html/function.html"
rel="noreferrer">in real life is a template, so the compiler knows
(14.2/3):
After
name lookup (3.4) finds that a name is a template-name, if this name is followed by a
<, the < is
always taken as the beginning of a template-argument-list
and never as a name followed by the less-than
operator.
Now we
are back to the same problem as with typename. What if we can't
know yet whether the name is a template when parsing the code? We will need to insert
template immediately before the template name, as specified by
14.2/4. This looks
like:
t::template f();
// call a function
template
Template
names can not only occur after a :: but also after a
-> or . in a class member access.
You need to insert the keyword there
too:
this->template
f(); // call a function
template
/>
Dependencies
For
the people that have thick Standardese books on their shelf and that want to know what
exactly I was talking about, I'll talk a bit about how this is specified in the
Standard.
In template declarations some
constructs have different meanings depending on what template arguments you use to
instantiate the template: Expressions may have different types or values, variables may
have different types or function calls might end up calling different functions. Such
constructs are generally said to depend on template
parameters.
The Standard defines
precisely the rules by whether a construct is dependent or not. It separates them into
logically different groups: One catches types, another catches expressions. Expressions
may depend by their value and/or their type. So we have, with typical examples
appended:
- Dependent types
(e.g: a type template parameter
T)
- Value-dependent expressions
(e.g: a non-type template parameter
N)
- Type-dependent expressions
(e.g: a cast to a type template parameter
(T)0)
Most
of the rules are intuitive and are built up recursively: For example, a type constructed
as T[N] is a dependent type if N is a
value-dependent expression or T is a dependent type. The
details of this can be read in section (14.6.2/1) for dependent
types, (14.6.2.2) for type-dependent expressions and
(14.6.2.3) for value-dependent expressions.
Dependent
names
The Standard is a bit unclear about what
exactly is a dependent name. On a simple read
(you know, the principle of least surprise), all it defines as a dependent
name is the special case for function names below. But since clearly
T::x also needs to be looked up in the instantiation context,
it also needs to be a dependent name (fortunately, as of mid C++14 the committee has
started to look into how to fix this confusing definition).
To avoid this problem, I have resorted to a
simple interpretation of the Standard text. Of all the constructs that denote dependent
types or expressions, a subset of them represent names. Those names are therefore
"dependent names". A name can take different forms - the Standard
says:
A name is a
use of an identifier (2.11), operator-function-id (13.5), conversion-function-id
(12.3.2), or template-id (14.2) that denotes an entity or label (6.6.4,
6.1)
An
identifier is just a plain sequence of characters / digits, while the next two are the
operator + and operator type form. The
last form is template-name . All these are
names, and by conventional use in the Standard, a name can also include qualifiers that
say what namespace or class a name should be looked up
in.
A value dependent expression 1 +
N is not a name, but N is. The subset of all
dependent constructs that are names is called dependent name.
Function names, however, may have different meaning in different instantiations of a
template, but unfortunately are not caught by this general rule.
Dependent function
names
Not primarily a concern of this article,
but still worth mentioning: Function names are an exception that are handled separately.
An identifier function name is dependent not by itself, but by the type dependent
argument expressions used in a call. In the example f((T)0),
f is a dependent name. In the Standard, this is specified at
(14.6.2/1).
Additional
notes and examples
In enough cases
we need both of typename and template.
Your code should look like the
following
template typename Tail>
struct UnionNode : public Tail {
//
...
template struct inUnion {
typedef typename
Tail::template inUnion dummy;
};
//
...
};
The
keyword template doesn't always have to appear in the last part
of a name. It can appear in the middle before a class name that's used as a scope, like
in the following example
typename
t::template iterator::value_type
v;
In some cases, the
keywords are forbidden, as detailed
below
On
the name of a dependent base class you are not allowed to write
typename. It's assumed that the name given is a class type
name. This is true for both names in the base-class list and the constructor initializer
list:
template T>
struct derive_from_Has_type : /* typename */ SomeBase::type
{
};
In
using-declarations it's not possible to use template after the
last ::, and the C++ committee href="http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_closed.html#109"
rel="noreferrer">said not to work on a solution.
template T>
struct derive_from_Has_type : SomeBase {
using SomeBase::template type; // error
using typename
SomeBase::type; // typename *is* allowed
};
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