c++ - cout is declared in iostream, but where it is defined?

Question

When I try to see definition of cout, I land to iostream file where it is declared as,

extern _CRTDATA2 ostream cout;

So where it is defined? Because extern is just declaration and not definition.

Answer 1

Global symbols are defined in a run-time library that you link with your applications. For example, in gcc you pass the compiler option -lstdc++ that will link your application with the libstdc++.a library. That is where all such symbols reside.

Though, this is specific to your compiler/run-time library version and will vary. Microsoft Visual C++ may behave differently but the idea is the same: the symbols are inside the precompiled libraries that are delivered with your C++ compiler.

With GNU you can type:

nm -g libstdc++.a

to see the symbols inside the library. The output may look like this (among lots of other lines):

ios_init.o:
                 U _ZSt3cin
globals_io.o:
0000000000000000 D _ZSt3cin
0000000000000000 D _ZSt4cerr
0000000000000000 D _ZSt4clog
0000000000000000 D _ZSt4cout
0000000000000000 D _ZSt4wcin
0000000000000000 D _ZSt5wcerr
0000000000000000 D _ZSt5wclog
0000000000000000 D _ZSt5wcout