c++ - Understanding how the default values of template parameters are declared -

in std::basic_string documentation found @ http://en.cppreference.com/w/cpp/string/basic_string, basic_string class declared follows.

template<     class chart,     class traits = std::char_traits<chart>,     class allocator = std::allocator<chart> > class basic_string; 

however, in both gcc , visual studio default values traits , allocator template parameters not specified in class declaration.

the following basic_string.h gcc 4.9.2.

template<     typename _chart,     typename _traits,     typename _alloc > class basic_string 

note lack of default values _traits , _alloc template parameters.

what missing?

these classes declared in million places¹. 1 of declarations carry default arguments, because it's error otherwise.

for basic_string, in libstdc++, default arguments found in forward declaration in bits/stringfwd.h (which included <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 taken literally.}