CODEWITHSUNDEEP
c++openglfunction-pointersc++14
Ethan Bierlein
Ethan Bierlein

Reputation: 3535

Why am I not able to use OpenGL functions when I include my loader header?

I'm currently trying to create a custom OpenGL function loader so I can start designing my own custom game engine. The header containing all of the loader code is formatted as follows:

#ifndef SWOGLL_H_
#define SWOGLL_H_
#pragma once

#include <Windows.h>
#include <gl/GL.h>
#include <gl/GLU.h>
#include <gl/glext.h>
#include <gl/wglext.h>

static PFNGLACTIVESHADERPROGRAMPROC glActiveShaderProgram;
static PFNGLACTIVETEXTUREPROC glActiveTexture;
// ...

static void* GetGLFunctionPointer(const char* functionName)
{
    void* functionPointer = (void*)wglGetProcAddress(functionName);
    if(
        functionPointer == (void*)0x0 ||
        functionPointer == (void*)0x1 ||
        functionPointer == (void*)0x2 ||
        functionPointer == (void*)0x3 ||
        functionPointer == (void*)-0x1
    )
    {
        HMODULE module = LoadLibraryA("opengl32.dll");
        functionPointer = (void*)GetProcAddress(module, functionName);
    }
    return functionPointer;
}


static void GenerateGLFunctionPointers()
{
    glActiveShaderProgram = (PFNGLACTIVESHADERPROGRAMPROC)GetGLFunctionPointer("glActiveShaderProgram");
    glActiveTexture = (PFNGLACTIVETEXTUREPROC)GetGLFunctionPointer("glActiveTexture");
    // ...
}

If I #include "SWOGLL.h" in my main.cpp file, the OpenGL functions work fine, assuming I've called GenerateGLFunctionPointers() after creating an OpenGL context.

However, if I create a separate file and #include "SWOGLL.h", the OpenGL functions don't work at all, even if the OpenGL context has already been created.

What's the problem here?

Upvotes: 1

Views: 988

Answers (1)

Dietrich Epp
Dietrich Epp

Reputation: 213318

You're declaring static functions and variables in a header file. Unless you have an exceptionally good reason to do something so bizarre, don't do that.

static PFNGLACTIVESHADERPROGRAMPROC glActiveShaderProgram;

static void* GetGLFunctionPointer(const char* functionName)

This will create a different copy of glActiveShaderProgram, GetGLFunctionPointer(), and everything else declared static in every C++ file that includes the header.

Remove static, and do one of the following things:

  1. Keep the declaration in the header, but move the definition to a C++ file.

  2. Keep the declaration and definition in the header, but mark it inline. This does not work with global variables.

The static keyword, in this case, makes it so the functions and variables declared static have "internal linkage", which means that you get a separate copy of them for every C++ file that defines them. This is different from declaring a member function static, which has a completely unrelated effect. It is somewhat unfortunate that the keyword static means different things in different places. C'est la vie.

Upvotes: 4

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