Error in OpenGL code

[Robert_d1968]

In this section of code there is an Error...

 

while (!window->Closed())
    {
        iVec2 sz = subwindow->ClientSize();                        "It states that this is not declared"  What should it be declared as?
        glViewport(0, 0, sz.x, sz.y);
        glClearColor(0.15f, 0.15f, 0.15f, 1.0f);
        glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

        glBegin(GL_TRIANGLES);
        glColor3f(1, 0, 0);
        glVertex3f(0, 0.5, 0);
        glColor3f(0, 1, 0);
        glVertex3f(0.5, -0.5, 0);
        glColor3f(0, 0, 1);
        glVertex3f(-0.5, -0.5, 0);
        glEnd();

        SwapBuffers(hdc);
    }

Thanks for any help, Robert