How to make a variadic macro for std::cout?

Question

How would I make a macro that took a variable amount of arguments, and prints its out using std::cout? Sorry if this is a noob question, couldn\'t find anything that clarifi

Answer 1

When using variadic macros you need __VA_ARGS__ to expand all the arguments. The problem however is that those arguments are comma-separated. Presumabely it just separates arguments to a function call, but since macros works with just text you can actually use __VA_ARGS__ in other contexts as well, where a comma-separated list makes sense.

The trick you can employ is to define your own comma operator for std::ostream (the type of std::cout). For example:

#include
#define LOG(...) std::cout , __VA_ARGS__ , std::endl

template 
std::ostream& operator,(std::ostream& out, const T& t) {
  out << t;
  return out;
}

//overloaded version to handle all those special std::endl and others...
std::ostream& operator,(std::ostream& out, std::ostream&(*f)(std::ostream&)) {
  out << f;
  return out;
}

int main() {
  LOG("example","output","filler","text");
  return 0;
}

Now, the LOG invocation will expand to:

std::cout , "example" , "output" , "filler" , "text" , std::endl;

and the commas will invoke the overloaded comma operators.

If you don't like overloaded operator, polluting all std::ostream-s, you can encapsulate std::cout with your own special logger class.