Have an instance of CustomDerivedClass in your testApp class, have a draw method in CustomDerivedClass and call it in your testApp::draw method. Something like this :
class CustomDerivedClass : public CustomClass
{
public:
void draw();
};
class testApp : public ofBaseApp
{
public:
void setup();
void draw();
CustomDerivedClass myCustomDerivedClassInstance;
};
void testApp::setup()
{
}
void testApp::draw()
{
myCustomDerivedClassInstance.draw();
}