This error is caused because the linker in gcc is unable to find the functions you have defined in your headers in your actual code. So if you have a header which looks like this:

#ifndef MESH_H
#define MESH_H
 
class Mesh
{
public:
    Mesh();
    ~Mesh();
    virtual void draw();
};
 
#endif // MESH_H

You must at least have these functions defined in your .cpp file:

#include "mesh.h"
 
Mesh::Mesh() {
}
 
Mesh::~Mesh() {
}
 
void Mesh::draw() {
}

After this, make sure you clean your compile environment to make sure no object files are being misinterpreted by the compiler. If you are using Qt or a project with a Makefile, you could just run these three commands (the first only applies to Qt projects).

qmake
make clean
make