Why does predeclaring std::basic_string<T> break boost::regex?

Question

Using the Microsoft Visual Studio 2012 compiler, with boost 1.53, I had some working code that used:

#include <boost/regex.hpp>

Later I added the following two lines to a header file that was included before regex.hpp:

namespace std {template<class T> class basic_string<T>;}
typedef std::basic_string<TCHAR> tstring;

Now I get a pile of compile errors, but if regex.hpp is included before these other lines, there are no errors.

The first few of many errors are:

c:\program files (x86)\boost\boost_1_53\boost\regex\v4\instances.hpp(106): error C2027: use of undefined type 'std::basic_string<_Elem,_Traits,_Alloc>'
19>          with
19>          [
19>              _Elem=char16_t,
19>              _Traits=std::char_traits<unsigned short>,
19>              _Alloc=std::allocator<char16_t>
19>          ]
19>c:\program files (x86)\boost\boost_1_53\boost\regex\v4\instances.hpp(106): error C2065: 'const_iterator' : undeclared identifier
19>c:\program files (x86)\boost\boost_1_53\boost\regex\v4\instances.hpp(106): error C2923: 'boost::match_results' : 'const_iterator' is not a valid template type argument for parameter 'BidiIterator'
19>c:\program files (x86)\microsoft visual studio 11.0\vc\include\xutility(364): error C2825: '_Iter': must be a class or namespace when followed by '::'
19>          c:\program files (x86)\boost\boost_1_53\boost\regex\v4\iterator_traits.hpp(116) : see reference to class template instantiation 'std::iterator_traits<_Iter>' being compiled
19>          with
19>          [
19>              _Iter=int
19>          ]
19>          c:\program files (x86)\boost\boost_1_53\boost\regex\v4\match_results.hpp(68) : see reference to class template instantiation 'boost::re_detail::regex_iterator_traits<T>' being compiled
19>          with
19>          [
19>              T=int
19>          ]
19>          c:\program files (x86)\boost\boost_1_53\boost\regex\v4\instances.hpp(106) : see reference to class template instantiation 'boost::match_results<BidiIterator>' being compiled
19>          with
19>          [
19>              BidiIterator=int
19>          ]
19>c:\program files (x86)\microsoft visual studio 11.0\vc\include\xutility(364): error C2039: 'iterator_category' : is not a member of '`global namespace''
19>c:\program files (x86)\microsoft visual studio 11.0\vc\include\xutility(364): error C2146: syntax error : missing ';' before identifier 'iterator_category'
19>c:\program files (x86)\microsoft visual studio 11.0\vc\include\xutility(364): error C2602: 'std::iterator_traits<_Iter>::iterator_category' is not a member of a base class of 'std::iterator_traits<_Iter>'
19>          with
19>          [
19>              _Iter=int
19>          ]
19>          c:\program files (x86)\microsoft visual studio 11.0\vc\include\xutility(364) : see declaration of 'std::iterator_traits<_Iter>::iterator_category'
19>          with
19>          [
19>              _Iter=int
19>          ]
19>c:\program files (x86)\microsoft visual studio 11.0\vc\include\xutility(364): error C2868: 'std::iterator_traits<_Iter>::iterator_category' : illegal syntax for using-declaration; expected qualified-name
19>          with
19>          [
19>              _Iter=int
19>          ]
19>c:\program files (x86)\microsoft visual studio 11.0\vc\include\xutility(365): error C2825: '_Iter': must be a class or namespace when followed by '::'
19>c:\program files (x86)\microsoft visual studio 11.0\vc\include\xutility(365): error C2039: 'value_type' : is not a member of '`global namespace''
19>c:\program files (x86)\microsoft visual studio 11.0\vc\include\xutility(365): error C2146: syntax error : missing ';' before identifier 'value_type'
19>c:\program files (x86)\microsoft visual studio 11.0\vc\include\xutility(365): error C2602: 'std::iterator_traits<_Iter>::value_type' is not a member of a base class of 'std::iterator_traits<_Iter>'

Please can anyone tell me what is wrong with my predeclaration of std::basic_string?

Answer 1

Please can anyone tell me what is wrong with my predeclaration of std::basic_string?

It has three template parameters:

template<class charT, class traits = char_traits<charT>,
    class Allocator = allocator<charT> >
class basic_string;

But there's no reason to declare standard types yourself and, as you've demonstrated, a good reason not to. So don't do that; include the header if you need the declaration.

Answer 2

You've omitted a couple of template parameters from your declaration. The correct one looks like:

template<class charT, class traits = char_traits<charT>,
         class Allocator = allocator<charT> >
class basic_string;

Don't try to do this on your own though. Use <string>.