store a matrix created by a function opengl - opengl

Question

I was wondering if there is an easy way to store a created matrix into a variable to be later called by glLoadMatrix(), the point being to reset the matrix to an earlier state.
For example:
void reshape(int w, int h){
glViewport(0, 0, (GLsizei)w, (GLsizei)h);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
gluPerspective(dist, (GLfloat)w / (GLfloat)h, 1.0, 20.0);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
glTranslatef(x_shift, y_shift, z_shift);
}
The goal being to store the matrix produced into a separate variable to effectively save the current matrix state before multiplying on top of it.

Answer 1

I already recommendet to stop using the GL matrix stack at all. It is depracted and has been removed from the core profile of modern OpenGL. Libraries like glm could be used instead.
However, what you ask for can be done with the matrix stack. Just use glGetFloatv() with GL_MODELVIEW matrix, GL_PROJECTION_MATRIX or GL_TEXTURE_MATRIX to query the current the respective matrix:
GLfloat m[16];
glGetFloatv(GL_MODELVIEW_MATRIX, m);
// other GL code modifying the MODELVIEW matrix
// ...
glMatrixMode(GL_MODELVIEW); // make sure we restore to MODELVIEW
glLoadMatrixf(m); // restore it

Answer 2

With the legacy functionality you are using, matrices are managed as a stack. Or to be precise, as a separate stack for each of the 4 matrix modes, where the most commonly used ones are GL_MODELVIEW and GL_PROJECTION.
The most direct way to save/restore matrices is by using the actual stack operations on this matrix stack, which is done with glPushMatrix() and glPopMatrix(), where Push and Pop follow the usual stack semantics:
Push: Adds a stack entry by copying the current top entry.
Pop: Removes the current top entry.
Let's say you first set up a matrix 1, then matrix 2, and then want to restore matrix 1. For illustration, I'll set up matrix 1 as a rotation matrix, and matrix 2 as a translation matrix, but they can of course be anything.
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
glRotatef(...);
// This is our matrix 1, now on top of the stack.
glPushMatrix();
// Push a new stack entry, to save matrix 1.
glLoadIdentity();
glTranslatef(...);
// Now we have matrix 2 on top of the stack.
glPopMatrix();
// Since we popped the top entry (matrix 2) off the stack, matrix 1 is
// now the active matrix at the top of the stack again.
You can also use glGetFloatf(GL_MODELVIEW_MATRIX, ...) and glGetFloatf(GL_PROJECTION_MATRIX, ...) to get the current matrix, and later set them again with glLoadMatrix(). But if you care about performance at all, you should really avoid glGet*() calls.