Correct way to declare/define custom cout-like object

c++, c++11, extern, global-variables, static

Solution

`std::cout` is simply declared as follows:

namespace std {
    extern ostream cout;
}

It is a regular global variable; you can do the same thing yourself. Put an `extern` declaration of your variable in a header; then define the same variable in a source file and link it to your application:

// mylog.h
extern MyLog mylog;

// mylog.cpp
MyLog mylog(someparams);

Problem

I created my own `std::cout`-like object that writes both to `std::cout` and to a log file. I'm currently defining it like this in a header file, but 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` be `static`? Should `lo` be `extern`? Kudos for explaining the correct way of declaring a `cout`-like object and showing how the main standard library implementations do it. Edit: by `cout`-like object, I mean a global variable that is always available after including the corresponding header.

Original source