dictionary - Why does the C++ map type argument require an empty constructor when using []?

itemprop="text">

See
also
href="https://stackoverflow.com/questions/695372/c-standard-list-and-default-constructible-types">C++
standard list and default-constructible
types

Not
a major issue, just annoying as I don't want my class to ever be instantiated without
the particular arguments.

#include
            

This
gives me the following g++
error:

/usr/include/c++/4.3/bits/stl_map.h:419: error: no matching function for call
to
‘MyClass()’

This
compiles fine if I add a default constructor; I am certain it's not caused by incorrect
syntax.

Answer

This issue comes with operator[]. Quote from
SGI documentation:

data_type& operator[](const key_type& k) -
Returns a reference to the object
that is associated with a
particular

key. If the map does not already
contain
such an object, operator[]
inserts the default
object

data_type().

If
you don't have default constructor you can use insert/find
functions.
Following example works
fine:

myMap.insert( std::map<
            int, MyClass >::value_type ( 1, MyClass(1) ) );

myMap.find( 1
            )->second;