Forward declaration of function messes up, saying args are undeclared identifiers

Question

This is my code:

• mainheader.h

  void displaygrid(int (* _grid)[10][10] , string msg );
  

• something.cpp

  #include <string>
  #include "mainheader.h"
  
  void displaygrid(int (* _grid)[10][10], string msg = "" )
  {
     //body goes here
  }
  

Compiler says at line of forward declaration in header:

_grid and string are undeclared identifiers

however since its just prototype, it shouldnt be worried about existence of _grid, right ?

Also, header is included after the string , but it doesnt know about string as well. It looks like its trying to call function instead of forward declaring. What's wrong?

Answer 1

For string you have to use std::string.

Answer 2

It needs to know about __grid in the header as well as string - forward declare or not. You need to at least place a struct __grid; somewhere above it or the likes so it has some sort of clue of what it is. Same for string...

Answer 3

You need to #include <string> in your mainheader.h and refer to string as std::string. You also need to include whatever is necessary to get _grid:

//mainheader.h
#include <string>
void displaygrid(int (* _grid)[10][10] , std::string msg );

Related post here.

Answer 4

You also want the default argument in the header file, not in the implementation, because you want code that calls it to know that msg can be defaulted.