Pretend you have an awesome templated square matrix class, parameterized on the value type and its size. It would probably look something like this:

// the template class

Once you have the matrix, you’d want to specialize it for specific sizes (say N=4). Its also a good idea to do this so you can add more constructors (doubt you want to keep creating arrays to pass in!). Anyhow, your implementation might end up looking something like

// derieved TMatrix4<T>

Guess what? It won’t work! But before we can get to that, the compiler is going to try to confuse us into thinking the problem is somewhere else! Take the following code. It should work, right?

TMatrix4<float> a = b + c;

Ha! The compiler throws a really nasty error, which I’ll paraphrase here:

“No match for operator= in ‘n=TMatrix<T,N>::operator + ( const TMatrixT,N>& ) const [with T= float, int N=4]“

“What black magic is this?”, I hear you say. Without going into details, the compiler is lying to you – while the compiler is erroring out on the assignment operator it is actually trying to find the constructor. (Return Value Optimization). So, implementing the copy constructor in the derieved TMatrix4 class should solve the problem, right?

Wrong!

// i need to recreate the error for you guys

When you created TMatrix4, you created a templated class that depended TMatrix<T,N>. When you call operator +, it is using TMatrix<T,N>::+ which has a return type of TMatrix<T,N>. Open returning from operator +, it needs to construct a new object – but the compiler cannot tell that the returned object is TMatrix4<T>. So the solution:

TMatrix4( const TMatrix<T,4>& mat )