c++ linux compile error: undefined reference to `main'

Question

I have looked for this error and tried a few of the solutions, but have not found anything, at this point I would just like to compile it.

The error I am getting is:

/usr/lib/gcc/i486-linux-gnu/4.4.1/../../../../lib/crt1.o: In function `_start':
/build/buildd/eglibc-2.10.1/csu/../sysdeps/i386/elf/start.S:115: undefined reference to `main'

using:

g++ -lglut Solar.cpp

the code is here:

using namespace std;
#include <stdio.h>
#include <GL/glut.h>
#include "math.h"

class Solar {

  int main(){
    initializeGL();
    //Stars Alpha = new Stars(5.0);
    //Stars *Alpha = new Stars(5.0);
    //Planets *Awe = new Planets(.6,2,30,"Awe",0.0,0.0,0.0);
    paintGL();
    return 0;
  }



  vid initializeGL(){
      glShadeModel(GL_SMOOTH);

      glClearColor(0.0f, 0.0f, 0.0f, 0.0f);
      glClearDepth(1.0f);

      glEnable(GL_DEPTH_TEST);
      glDepthFunc(GL_LEQUAL);

      glHint(GL_PERSPECTIVE_CORRECTION_HINT, GL_NICEST);

      // lighting stuff
      GLfloat ambient[] = {0.0, 0.0, 0.0, 1.0};
      GLfloat diffuse[] = {0.9, 0.9, 0.9, 1.0};
      GLfloat specular[] = {0.4, 0.4, 0.4, 1.0};
      GLfloat position0[] = {1.0, 1.0, 1.0, 0.0};
      glLightfv( GL_LIGHT0, GL_POSITION, position0 );
      glLightfv( GL_LIGHT0, GL_AMBIENT, ambient );
      glLightfv( GL_LIGHT0, GL_DIFFUSE, diffuse );
      glLightfv( GL_LIGHT0, GL_SPECULAR, specular );
      GLfloat position1[] = {-1.0, -1.0, -1.0, 0.0};
      glLightfv( GL_LIGHT1, GL_POSITION, position1 );
      glLightfv( GL_LIGHT1, GL_AMBIENT, ambient );
      glLightfv( GL_LIGHT1, GL_DIFFUSE, diffuse );
      glLightfv( GL_LIGHT1, GL_SPECULAR, specular );

      glEnable( GL_LIGHTING );
      glEnable( GL_LIGHT0 );
      glEnable( GL_LIGHT1 );
      glEnable( GL_COLOR_MATERIAL );

      /* Draws the Grid*/
      drawRGrid();
  }

  void resizeGL( int width, int height ){
    height = height?height:1;

    glViewport( 0, 0, (GLint)width, (GLint)height );

    // update projection matrix
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    gluPerspective(45.0f,(GLfloat)width/(GLfloat)height,.10f,200.0f);

    // modeview matrix is simply identity
    glMatrixMode(GL_MODELVIEW);
    glLoadIdentity();
  }

  void paintGL(){

    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

    //set camera position using gluLookAt
    glLoadIdentity();
    gluLookAt(0.0f,0.0f,0.0f,0.0f,0.0f,-200.0f,0.0f,1.0f,0.0f);

  }


  void doCircle(double x, double y, double radius){
    glEnable(GL_BLEND);
    double y1=y+radius;
    double x1=x;
    glBegin(GL_LINE_STRIP);
    for(double angle=0.0f;angle<=(2.0f*3.14159);angle+=0.01f){
      double x2=x+(radius*(float)sin((double)angle));
      double y2=y+(radius*(float)cos((double)angle));
      glColor3f(1.0,1.0,1.0); //White
      glVertex2d(x1,y1);
      y1=y2;
      x1=x2;
    }
    glEnd();
    glDisable(GL_BLEND);
  }

  void drawRGrid(){
    float xCirc = 0;
    float yCirc = 0;
    int numCircles = 5;
    int threesixty = 360;
    int numLines = 20;

    //draws my circles
    for (int i=0; i < numCircles;i++ ){
      doCircle(1.0,1.0,i);
    }

    //draws my lines
    for (int j=0; j < threesixty / numLines;j+= numLines){
      // multiply by numCircles to get to extend to 
      // that length
      drawLines(sin(j)*numCircles,sin(j)*numCircles);
    }

  }

  void drawLines(float xCirc, float yCirc){
    glBegin(GL_LINES);
    glVertex2f(0,0);
    glVertex2f(xCirc,yCirc);
    glEnd();
  }



};

Any help will be great appreciated!

Answer 1

You have declared main() as a member function.

The main() function called when the application starts up needs to be a non-member function in the global namespace.

A good introductory C++ book would explain this.

Answer 2

You declared main in class Solar. main is supposed to be a free function, i.e. not contained within a class.

Answer 3

main cannot be found because the function you wrote is called Solar::main (in truth, it has a much funnier name). You need to move it below class Solar. Then you probably want to change class Solar into struct Solar until you introduce member variables. At last, you may want to rewrite main to be

extern "C" int main (int /* argc */, char *const * /* argv */) {
    Solar solar;
    solar.initializeGL();
    //Stars Alpha = new Stars(5.0);
    //Stars *Alpha = new Stars(5.0);
    //Planets *Awe = new Planets(.6,2,30,"Awe",0.0,0.0,0.0);
    solar.paintGL();
    return 0;
}