Multiple definitions in C++ [duplicate] - c++

Question

This question already has answers here:
How do I create a header-only library?
(4 answers)
Closed 9 years ago.
In a file called gl_ext.h I have the following:
#ifndef GLEXT_H_INCLUDED
#define GLEXT_H_INCLUDED
#include <stdexcept>
#ifdef WIN32
#include <Windows.h>
#include <GL/GL.h>
#include <GL/glext.h>
# define glGetProcAddress(arg) wglGetProcAddress(arg)
#elif __linux__
#include <GL/gl.h>
#include <GL/glext.h>
# include <GL/glx.h>
# define glGetProcAddress(arg) glXGetProcAddress((const GLubyte*)arg)
#endif
PFNGLCREATESHADERPROC glCreateShader = 0;
namespace glext
{
bool load_gl_extensions()
{
static bool loaded = false;
if (loaded) {
return true;
}
if (!glCreateShader) {
glCreateShader =
(PFNGLCREATESHADERPROC)(glGetProcAddress("glCreateShader"));
if (!glCreateShader) {
throw "Failed to load glCreateShader";
}
}
}
}
#endif
When building from within qt creator using the following .pro file
QT += core gui opengl
TEMPLATE = app
TARGET = GLExtensions
INCLUDEPATH += .
LIBS += -lGL
HEADERS += gl_ext.h \
qtrenderer.h
SOURCES += main.cpp \
qtrenderer.cpp
The usage of this "header library" is as follows:
main.cpp
#include <QtGui/QApplication>
#include "qtrenderer.h"
int main(int argc, char * argv[]) {
QApplication app(argc, argv);
QtRenderer *renderer = new QtRenderer();
renderer->show();
app.exec();
}
qtrenderer.h
#ifndef QTRENDERER_H_INCLUDED
#define QTRENDERER_H_INCLUDED
#include <QtCore/QObject>
#include <QtOpenGL/QGLWidget>
#include <gl_ext.h>
class QtRenderer : public QGLWidget
{
Q_OBJECT
private:
QtRenderer(const QtRenderer &other);
QtRenderer &operator = (const QtRenderer &other);
protected:
virtual void paintGL();
virtual void initializeGL();
public:
QtRenderer();
~QtRenderer();
public slots:
virtual void updateGL();
};
#endif
qtrenderer.cpp
#include "qtrenderer.h"
QtRenderer::QtRenderer() :
QGLWidget() {
}
QtRenderer::~QtRenderer() {
}
void QtRenderer::initializeGL() {
try {
glext::load_gl_extensions();
} catch (...) {
throw std::runtime_error("Failed to load needed extensions.");
}
}
void QtRenderer::paintGL() {
swapBuffers();
}
void QtRenderer::updateGL() {
paintGL();
}
When building this source code using
gcc (GCC) 4.7.2 20120921 (Red Hat 4.7.2-2)
I get the following build errors:
qtrenderer.o: In function `glext::load_gl_extensions()':
/home/mehoggan/Devel/test_gl/./gl_ext.h:28: multiple definition of `glCreateShader'
main.o:/home/mehoggan/Devel/test_gl/./gl_ext.h:28: first defined here
Why is this so?

Answer 1

Well, the header gl_ext.h is included multiple times. Remember that #include is like replacing the #include statement with the content of the file in a copy&paste manner.
You should put the implementation of load_gl_extensions() into a .cpp file, and put only the declaration into the header file.
gl_ext.h:
//...
extern PFNGLCREATESHADERPROC glCreateShader;
namespace glext
{
bool load_gl_extensions();
}
gl_ext.cpp:
#include "gl_ext.h"
PFNGLCREATESHADERPROC glCreateShader = 0;
namespace glext
{
bool load_gl_extensions()
{
static bool loaded = false;
if (loaded) {
return true;
}
if (!glCreateShader) {
glCreateShader =
(PFNGLCREATESHADERPROC)(glGetProcAddress("glCreateShader"));
if (!glCreateShader) {
throw "Failed to load glCreateShader";
}
}
}
}
extern tells the compiler that the variable/function pointer (glCreateShader) is placed in a different compilation unit (every .cpp file is compiled as a different unit). The linker then inserts the correct memory-adress of your variable. Maybe you should do some research on how C++ compilation and linkage works.

Answer 2

The solution to the problem consisted of making the method inlined and declaring the function pointer as static.