Question

Basically, I have a matrix class like this (with a lot of operator overloads and other functions removed):

template
<
    uint32 TRows,
    uint32 TCols
>
struct Matrix
{
    float values[TRows][TCols];

    inline explicit Matrix()
    {
    }

    inline Matrix<TRows - 1, TCols - 1> minor(const uint32 col, const uint32 row)
    {
        Matrix<TRows - 1, TCols - 1> matrix;
        for(int i = 0; i < TRows; ++i)
            for(int j = 0; j < TCols; ++j)
            {
                if(i == col || j == row) continue;
                matrix.values[i - (i > col)][j - (j > row)] = this->values[i][j];
            }
        return matrix;
    }

    inline float determinant()
    {
        if(TRows != TCols) throw DimensionError("Matrix is not square");

        float det = 0;

        if(TRows <= 0)
            det = 0;
        else if(TRows == 1)
            det = this->values[0][0];
        else if(TRows == 2)
            det = this->values[0][0] * this->values[1][1] - this->values[1][0] * this->values[0][1];
        else
            for(int j = 0; j < TCols; ++j)
                det += (j % 2 ? -1 : 1) * this->values[0][j] * this->minor(0, j).determinant();

        return det;
    }
}

I don’t understand why, for the line det += (j % 2 ? -1 : 1) * this->values[0][j] * this->minor(0, j).determinant();, GCC attempts to generate an immense amount of functions:

matrix.cpp:95:   instantiated from `float Matrix<TRows, TCols>::determinant() [with unsigned int TRows = -49u, unsigned int TCols = -49u]'
matrix.cpp:95:   instantiated from `float Matrix<TRows, TCols>::determinant() [with unsigned int TRows = -48u, unsigned int TCols = -48u]'
matrix.cpp:95:   instantiated from `float Matrix<TRows, TCols>::determinant() [with unsigned int TRows = -47u, unsigned int TCols = -47u]'
matrix.cpp:95:   instantiated from `float Matrix<TRows, TCols>::determinant() [with unsigned int TRows = -46u, unsigned int TCols = -46u]'
matrix.cpp:95:   instantiated from `float Matrix<TRows, TCols>::determinant() [with unsigned int TRows = -45u, unsigned int TCols = -45u]'
matrix.cpp:95:   instantiated from `float Matrix<TRows, TCols>::determinant() [with unsigned int TRows = -44u, unsigned int TCols = -44u]'
matrix.cpp:95:   instantiated from `float Matrix<TRows, TCols>::determinant() [with unsigned int TRows = -43u, unsigned int TCols = -43u]'
matrix.cpp:95:   instantiated from `float Matrix<TRows, TCols>::determinant() [with unsigned int TRows = -42u, unsigned int TCols = -42u]'
matrix.cpp:95:   instantiated from `float Matrix<TRows, TCols>::determinant() [with unsigned int TRows = -41u, unsigned int TCols = -41u]'
matrix.cpp:95:   instantiated from `float Matrix<TRows, TCols>::determinant() [with unsigned int TRows = -40u, unsigned int TCols = -40u]'
matrix.cpp:95:   instantiated from `float Matrix<TRows, TCols>::determinant() [with unsigned int TRows = -39u, unsigned int TCols = -39u]'
matrix.cpp:95:   instantiated from `float Matrix<TRows, TCols>::determinant() [with unsigned int TRows = -38u, unsigned int TCols = -38u]'
matrix.cpp:95:   instantiated from `float Matrix<TRows, TCols>::determinant() [with unsigned int TRows = -37u, unsigned int TCols = -37u]'
matrix.cpp:95:   instantiated from `float Matrix<TRows, TCols>::determinant() [with unsigned int TRows = -36u, unsigned int TCols = -36u]'

This is probably some error in my code, but I can’t for the life of me see where I’m going wrong. Help would be much appreciated!

Answer 1

Matrix::minor(0, j) returns a matrix of size (N-1, N-1).
Calling for its determinant makes the process recursive, which means you are generating a minor (and determinant method) for all p integers, starting from N down to … -infinity ?

Have you added a specialization for the case where N==1 or N==0 ?
You have to make sure recursion stops !

Try adding something like :

template<>
struct Matrix<1,1>
{
  float values[1][1];

  float determinant(){ return values[0][0]; }
};

EDIT :
Recursion has to be stopped at compile time. Adding an if statement inside the method does not prevent the compiler from compiling all possible paths.