c++ - Correct way to declare/define custom cout-like object -

i created own std::cout-like object writes both std::cout , log file.

i'm defining in header file, i'm getting unused variable warnings.

header file <mylib/log.h>

static lout { }; static lout lo;  template<typename t> inline lout& operator<<(lout& mlout, const t& mvalue) {     std::string str{tostr(mvalue)};     std::cout << str;     getlogstream() << str;     return mlout; } 

usage:

#include <mylib/log.h> ... lo << "hello!" << std::endl; 

should lo static? should lo extern?

kudos explaining correct way of declaring cout-like object , showing how main standard library implementations it.

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edit: cout-like object, mean global variable available after including corresponding header.

std::cout declared follows:

namespace std {     extern ostream cout; } 

it regular global variable; can same thing yourself. put extern declaration of variable in header; define same variable in source file , link application:

// mylog.h extern mylog mylog;  // mylog.cpp mylog mylog(someparams);