Correct way to declare/define custom cout-like object

Question

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.

Answer 1

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);