CODEWITHSUNDEEP
c++g++c-preprocessorstdnm
eXXXXXXXXXXX2
eXXXXXXXXXXX2

Reputation: 1580

Undefined symbols after header inclusion of std c++ library

#include <iostream>

int main()
{
    std::cout << 1.0;
    return 0;
}

I use command g++ -E main.cpp for getting this compile unit after preprocessor run. After preprocessing it contains about 20k lines. And it contains this part of code with definition of operator<<(double __f).

namespace std __attribute__ ((__visibility__ ("default")))
{

# 55 "/usr/include/c++/4.7/ostream" 3
  template<typename _CharT, typename _Traits>
    class basic_ostream : virtual public basic_ios<_CharT, _Traits>
    {
    public:

      typedef _CharT char_type;
      typedef typename _Traits::int_type int_type;
      typedef typename _Traits::pos_type pos_type;
      typedef typename _Traits::off_type off_type;
      typedef _Traits traits_type;


      typedef basic_streambuf<_CharT, _Traits> __streambuf_type;
      typedef basic_ios<_CharT, _Traits> __ios_type;
      typedef basic_ostream<_CharT, _Traits> __ostream_type;
      typedef num_put<_CharT, ostreambuf_iterator<_CharT, _Traits> >
             __num_put_type;
      typedef ctype<_CharT> __ctype_type;
# 81 "/usr/include/c++/4.7/ostream" 3
      explicit
      basic_ostream(__streambuf_type* __sb)
      { this->init(__sb); }

      //.........................................................

      __ostream_type&
      operator<<(double __f)
      { return _M_insert(__f); }

      //.........................................................
    };

}

Next, run g++ -c main.cpp and nm main.o

000000000000005a t _GLOBAL__sub_I_main
000000000000001d t _Z41__static_initialization_and_destruction_0ii
                 U _ZNSolsEd
                 U _ZNSt8ios_base4InitC1Ev
                 U _ZNSt8ios_base4InitD1Ev
                 U _ZSt4cout
0000000000000000 r _ZStL19piecewise_construct
0000000000000000 b _ZStL8__ioinit
                 U __cxa_atexit
                 U __dso_handle
0000000000000000 T main

_ZNSolsEd is mangled name for operator<<(double) and that line has "U" The symbol is undefined. But definition for this operator is in compiled source. What is reason for that?

Upvotes: 2

Views: 304

Answers (1)

Marc Glisse
Marc Glisse

Reputation: 7925

This line:

extern template class basic_ostream<char>;

tells the compiler that the function is already available elsewhere and it should not emit any code for it (except possibly for inlining), like inline does in C99.

Upvotes: 1

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