Understanding how the default values of template parameters are declared

Question

In the std::basic_string documentation found at http://en.cppreference.com/w/cpp/string/basic_string, the basic_string class is declared as follows.

template<
    class CharT,
    class Traits = std::char_traits<CharT>,
    class Allocator = std::allocator<CharT>
> class basic_string;

However, in both GCC and Visual Studio the default values for the Traits and Allocator template parameters are not specified in the class declaration.

The following is from basic_string.h from GCC 4.9.2.

template<
    typename _CharT,
    typename _Traits,
    typename _Alloc
> class basic_string

Note the lack of default values for the _Traits and _Alloc template parameters.

What am I missing?

Answer 1

These classes are usually declared in a million places¹. Only one of those declarations will carry the default arguments, because it's an error otherwise.

For basic_string, in libstdc++, the default arguments are found in a forward declaration in bits/stringfwd.h (which is included by <string>, among other things):

  template<typename _CharT, typename _Traits = char_traits<_CharT>,
           typename _Alloc = allocator<_CharT> >
    class basic_string;

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_{¹ Not to be taken literally.}