Undefined reference to [closed]

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I'm doing an opengl/qt3 assignment, but I'm running into an undefined reference error:

Renderer.h:
...
#include "Mesh.h"
... Mesh mesh;

Renderer.cpp: 
...
mesh.load("ball.obj"); 
...

Mesh.h:
...
bool load(string filename);
...

Mesh.cpp:
#include "Mesh.h"
...
bool Mesh::load(string filename) { ... }
...

but the compiler complains about the line mesh.load("ball.obj") as an undefined reference... What is going on? Thank you in advance!

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Forgot to add the files to QT Designer. Solved!