OpenGL glutInit 2

The error is caused by defining display inside main. In standard C++, a function definition cannot be nested inside another function, so the compiler reports a syntax error at the { after void display(void).

Move display out of main, then register it with glutDisplayFunc and glutIdleFunc.

#include <iostream>
#include <GL/gl.h>
#include <GL/glut.h>
#include <GL/freeglut.h>
#include <cmath>

using namespace std;

void display()
{
    glClearColor(0.f, 0.f, 0.f, 1.f);
    glClear(GL_COLOR_BUFFER_BIT);
    glLoadIdentity();
    glFlush();
}

int main(int argc, char** argv)
{
    glutInit(&argc, argv);
    glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB);
    glutInitWindowSize(500, 500);
    glutCreateWindow("Hello World");
    glutDisplayFunc(display);
    glutIdleFunc(display);
    glutMainLoop();
    return 0;
}

Also correct these points:

1. glutInit is normally called with arguments: glutInit(&argc, argv);
2. Header casing and paths should be consistent, for example #include <GL/gl.h> rather than mixing Gl and GL.
3. glLoadIdentity and glFlush are valid OpenGL calls in the display function.

References:

• OpenGL
• OpenGL Correctness Tips
• OpenGL Reference