Question

I have a class imgmanager that allows me to load all my images exactly once, it’s quite nice, and while prototyping I had all of my files in one place, so I didn’t have to worry about cyclical definitions. However after separating all of my classes I have a problem.

My Header File

#ifndef IMAGEMANAGER_H   
#define IMAGEMANAGER_H
#include "Img.h"
#include <vector>
#include <map>
#include <string>

class imgmanager{
 protected:
 std::vector<sf::Image*> images;
 std::map<std::string,int> positions;
 public:
 sf::Image* addimg(std::string path); //relative to resources
 sf::Image* getimg(std::string path);
 int size();
 virtual ~imgmanager();
 sf::Image* operator[](int);
}imagemgr;

#endif

With the instance created after the } and before the ; my compiler complains at me:

So I ask: What should I do to have a global instance of my imagemgr class? Should I just make a global header file and create an instance? (in this particular case I can just make a global variable in my main.cpp, none of the headers require the instance)

Answer 1

Don’t create object instances in headers.

Create your object instance in one source file.

If you need to access it across multiple Translation Units, put this in your header:

extern imgmanager imagemgr; // declaration

This will inform all code that can “see” the header that there exists a so-named object; but it will still only actually be defined in the one source file where you wrote:

imgmanager imagemgr; // definition

(This is analogous to the way in which you declare functions in a header, but define them in precisely one source file:

void f(); // declaration
void f() { ... } // definition

)

---

The above general advice dutifully imparted, I would now question the rationale of having a class at all if you’re only going to use one, single, global instance of it. Either make it a “singleton” class, or use free functions in a namespace instead.