手动OpenGL扩展加载LNK2005和LNK1169

Question

我正在使用Visual C ++，并手动尝试加载OpenGL扩展。 但是，由于某些原因，从Khronos Groups提供的标头中定义指针会导致链接器错误，因此，即使在OpenGL上下文中定义这些函数，我也从未得到改变。 下面，我提供了代码的简化版本，其结构也会引起相同的问题。

//Test.cpp
#include "MyGL.h"
#include <iostream>

int main()
{
    std::cout << "Hello World!\n"; 
}

//MyGL.h
#pragma once
#include "MyGLOpenGL.h"

//MyGL.cpp
#include "MyGL.h"

//MyGLOpenGL.h
#pragma once

#include <windows.h> // Windows functions
#include <GL/gl.h> // Provided w/ Compiler
#include "GL/glext.h" // Put out by Khronos Group
#include "GL/wglext.h" // Put out by Khronos Group
#pragma comment(lib, "opengl32.lib") // Provided w/ Compiler



#ifndef GL_OPENGL
#define GL_OPENGL

void glInitPointers(); // Defines pointers to opengl functions
void* glGetAnyProcAddress(const char* name); // Gets a pointer to any OpenGL function

extern PFNWGLCHOOSEPIXELFORMATARBPROC wglChoosePixelFormatARB;
extern PFNWGLCREATECONTEXTATTRIBSARBPROC wglCreateContextAttribsARB;
#endif

//MyGLOpenGL.cpp
#include "MyGLOpenGL.h"

void glInitPointers() {
    wglChoosePixelFormatARB = (PFNWGLCHOOSEPIXELFORMATARBPROC)glGetAnyProcAddress("wglChoosePixelFormatARB"); //load function
    wglCreateContextAttribsARB = (PFNWGLCREATECONTEXTATTRIBSARBPROC)glGetAnyProcAddress("wglCreateContextAttribsARB"); //load function
    return;
}

void* glGetAnyProcAddress(const char* name) {
    void *pointer = (void *)wglGetProcAddress(name);
    if (pointer == 0 || (pointer == (void*)0x1) || (pointer == (void*)0x2) || (pointer == (void*)0x3) || (pointer == (void*)-1)) {
        HMODULE module = LoadLibraryW(L"opengl32.dll");
        pointer = (void *)GetProcAddress(module, name);
    }
    return pointer;
};

编译此代码会给我以下链接器错误：

1>MyGLOpenGL.obj : error LNK2001: unresolved external symbol "int (__stdcall* wglChoosePixelFormatARB)(struct HDC__ *,int const *,float const *,unsigned int,int *,unsigned int *)" (?wglChoosePixelFormatARB@@3P6GHPAUHDC__@@PBHPBMIPAHPAI@ZA)
1>MyGLOpenGL.obj : error LNK2001: unresolved external symbol "struct HGLRC__ * (__stdcall* wglCreateContextAttribsARB)(struct HDC__ *,struct HGLRC__ *,int const *)" (?wglCreateContextAttribsARB@@3P6GPAUHGLRC__@@PAUHDC__@@PAU1@PBH@ZA)
1>G:\Development\Test\Debug\Test.exe : fatal error LNK1120: 2 unresolved externals

由于此问题是无法解决的外部符号，因此我尝试删除extern关键字来获取这两个错误：

1>MyGLOpenGL.obj :error LNK2005: "int (__stdcall* wglChoosePixelFormatARB)(struct HDC__ *,int const *,float const *,unsigned int,int *,unsigned int *)" (?wglChoosePixelFormatARB@@3P6GHPAUHDC__@@PBHPBMIPAHPAI@ZA) already defined in MyGL.obj
1>MyGLOpenGL.obj : error LNK2005: "struct HGLRC__ * (__stdcall* wglCreateContextAttribsARB)(struct HDC__ *,struct HGLRC__ *,int const *)" (?wglCreateContextAttribsARB@@3P6GPAUHGLRC__@@PAUHDC__@@PAU1@PBH@ZA) already defined in MyGL.obj
1>Test.obj : error LNK2005: "int (__stdcall* wglChoosePixelFormatARB)(struct HDC__ *,int const *,float const *,unsigned int,int *,unsigned int *)" (?wglChoosePixelFormatARB@@3P6GHPAUHDC__@@PBHPBMIPAHPAI@ZA) already defined in MyGL.obj
1>Test.obj : error LNK2005: "struct HGLRC__ * (__stdcall* wglCreateContextAttribsARB)(struct HDC__ *,struct HGLRC__ *,int const *)" (?wglCreateContextAttribsARB@@3P6GPAUHGLRC__@@PAUHDC__@@PAU1@PBH@ZA) already defined in MyGL.obj
1>G:\Development\Test\Debug\Test.exe : fatal error LNK1169: one or more multiply defined symbols found

我还已经确保“ #pragma一次”在everfile文件中，头文件保护，在Visual Studio的其他依赖项中添加“ OpenGL32.lib”，为lib添加一个pragma注释，在其中将指针设置为null声明，即使在搜索问题后，我也无能为力。 就我而言，请使用GLEW或任何其他扩展加载库，因为这正是我要创建的内容。

Answer 1

在C和C ++中， 声明和定义之间存在重要区别。 形式的声明

extern <type> <symbol>;

声明 某处有某个符号，但实际上并没有使它存在。 就像对编译器的承诺一样，该符号将在其他地方定义。 当你写

extern PFNWGLCHOOSEPIXELFORMATARBPROC wglChoosePixelFormatARB;
extern PFNWGLCREATECONTEXTATTRIBSARBPROC wglCreateContextAttribsARB;

您实际上并没有为这些函数指针创建变量，而只是为程序“在目录中列出了它们”。 您实际上必须在某处定义它们。 即在某些编译单元中，您必须编写（没有外部代码）。

PFNWGLCHOOSEPIXELFORMATARBPROC wglChoosePixelFormatARB;
PFNWGLCREATECONTEXTATTRIBSARBPROC wglCreateContextAttribsARB;

正如@EricLopushansky所暗示的，这与初始化完全无关 。 这是关于它们正在被实际定义的一切。 由于这些是全局作用域，因此即使您没有明确地将其写为= 0 ，它们也将始终初始化为= 0 。

Answer 2

OpenGL Wiki在文章“ 加载OpenGL函数”中对此解释得不是很好，但只需执行以下操作即可。 在cpp文件中将指针设置为null将解决此问题。 例如：

PFNWGLCHOOSEPIXELFORMATARBPROC wglChoosePixelFormatARB = NULL;

对于任何有兴趣的人，以下是GLEW的相关代码，其用途相同：

//glew.h
#ifdef GLEW_STATIC
#  define GLEWAPI extern
#else
#  ifdef GLEW_BUILD
#    define GLEWAPI extern __declspec(dllexport)
#  else
#    define GLEWAPI extern __declspec(dllimport)
#  endif
#endif
#define GLEW_FUN_EXPORT GLEWAPI

//wglew.h
#define WGLEW_GET_FUN(x) x
#define wglChoosePixelFormatARB WGLEW_GET_FUN(__wglewChoosePixelFormatARB)
typedef BOOL (WINAPI * PFNWGLCHOOSEPIXELFORMATARBPROC) (HDC hdc, const int* piAttribIList, const FLOAT *pfAttribFList, UINT nMaxFormats, int *piFormats, UINT *nNumFormats);
#define WGLEW_FUN_EXPORT GLEW_FUN_EXPORT
WGLEW_FUN_EXPORT PFNWGLCHOOSEPIXELFORMATARBPROC __wglewChoosePixelFormatARB;

//glew.c
#  define glewGetProcAddress(name) wglGetProcAddress((LPCSTR)name)
PFNWGLCHOOSEPIXELFORMATARBPROC __wglewChoosePixelFormatARB = NULL;
r = ((wglChoosePixelFormatARB = (PFNWGLCHOOSEPIXELFORMATARBPROC)glewGetProcAddress((const GLubyte*)"wglChoosePixelFormatARB")) == NULL) || r;


//I would be using this static so it would evaluate in my code to
//MyGLOpenGL.h
#define wglChoosePixelFormatARB __wglewChoosePixelFormatARB
typedef BOOL (WINAPI * PFNWGLCHOOSEPIXELFORMATARBPROC) (HDC hdc, const int* piAttribIList, const FLOAT *pfAttribFList, UINT nMaxFormats, int *piFormats, UINT *nNumFormats);
extern PFNWGLCHOOSEPIXELFORMATARBPROC __wglewChoosePixelFormatARB;

//MyGLOpenGL.cpp
PFNWGLCHOOSEPIXELFORMATARBPROC __wglewChoosePixelFormatARB = NULL;
wglChoosePixelFormatARB = (PFNWGLCHOOSEPIXELFORMATARBPROC)wglGetProcAddress((const GLubyte*)"wglChoosePixelFormatARB")