Use OpenGL context in an external class in Qt - c++

Question

I have a window in Qt that inherits from QGLViewer. If I create any shader program in that class, QGLShaderProgram myShader everything runs fine.
However I start moving some rendering calls to classes outside the class that has the draw()call and things are broken.
The application compiles fine without errors, but when executing it I received an error The program has unexpectedly finished.
I found around that from Qt4 to Qt5 the shader class changed, being QOpenGLShaderProgram the one used in Qt5. I gave it a try and the same problem occur, nevertheless I got a different error message QOpenGLFunctions created with a non-current context.
Which makes me think that when calling OpenGL functions from a class that has no direct relation to the class that actually does the drawing the OpenGL context is "lost".
How can I make the context visible across all the classes? In general my code looks like
MyViewer.hpp
class MyViewer : public QGLViewer
{
MyViewer(const QGLFormat format);
~MyViewer();
protected:
init();
draw()
{
// Clear color buffer and depth buffer
// Do stuff
m_cube.render();
}
private:
...
...
Cube m_cube;
};
Cube.cpp
class Cube
{
public:
Cube()
{
m_shaderProgram.addShaderFromSourceFile(QGLShader::Vertex, ":/vertex.glsl");
m_shaderProgram.addShaderFromSourceFile(QGLShader::Fragment, ":/fragment.glsl");
m_shaderProgram.link();
//Initialize VAO and VBOs
}
void render(){ // render OpenGL calls }
private:
QGLShaderProgram m_shaderProgram;
};

Answer 1

Open gl contexts are global but you can explicit share a context between 2 Viewers like so
QGLViewer ( QGLContext * context,
QWidget * parent = 0,
const QGLWidget * shareWidget = 0,
Qt::WindowFlags flags = 0
)
Acoording to ducumentation
Same as QGLViewer(), but a QGLContext can be provided so that viewers
share GL contexts, even with QGLContext sub-classes (use shareWidget
otherwise).
So first check the order in witch you classes are created. Because the Cube could be calling opengl functions whilest your vieuwer is still incomplete
If you call opengl functions before the QGLviewer has created a context you get an error.
if so a quick work around would be to create a new Qglcontext in your construtor of cube and pass it back to they viewer.
Else do this
Cube() {}; // empty cube constructor
void InitShaders()
{
m_shaderProgram.addShaderFromSourceFile(QGLShader::Vertex, ":/vertex.glsl");
m_shaderProgram.addShaderFromSourceFile(QGLShader::Fragment, ":/fragment.glsl");
m_shaderProgram.link();
//Initialize VAO and VBOs
}
and in the constructor of my viewer do
MyViewer(const QGLFormat format){
cube.initShaders();
}
I have not tested this code but it should alter the order of initialization.