c++ compile-time check function arguments

Question

I\'m searching a way to check function arguments in compile-time if it\'s possible to do for compiler.

To be more specific: assume that we have some class Matrix.

Answer 1

The way that linear algebra packages tend to do this is to use templates for fixed size matrices, as in:

template<int x, int y> class Matrix { ... }

and an extra class for matrices that can change size at runtime

class DynamicMatrix {...}

You still have to rely on the programmer actually using the first option when they want fixed size matrices, but the template version makes it easy to generate a compiler error when x or y are zero.

Answer 2

Run-time:

Matrix(int width, int height):
    x_size{width},
    y_size{height}
{
        assert(x_size>0);
        assert(y_size>0);
}

Compile-time (Actually you couldn't do it with function arguments. You can use template ways):

template <size_t WIDTH, size_t HEIGHT>
class Matrix
{
    const size_t x_size = WIDTH;
    const size_t y_size = HEIGHT;

    static_assert(WIDTH > 0, "Width must > 0");
    static_assert(HEIGHT > 0, "Height must > 0");
};

Answer 3

To get a compile time error you would need a template:

template <int width, int height>
class MatrixTemplate : public Matrix
{
    static_assert(0 < width, "Invalid Width");
    static_assert(0 < height, "Invalid Height");
    public:
    MatrixTemplate()
    : Matrix(width, height)
    {}
};

(Btw.: I suggest unsigned types for indices)

If you do not have static_assert (here I switch to unsigned):

template <unsigned width, unsigned height>
class MatrixTemplate : public Matrix
{
    public:
    MatrixTemplate()
    : Matrix(width, height)
    {}
};

template <> class MatrixTemplate<0, 0> {};
template <unsigned height> class MatrixTemplate<0, height> {};   
template <unsigned width> class MatrixTemplate<width, 0> {};

There is no support for empty matrices (MatrixTemplate<0, 0>), here. But it should be an easy task to adjust the static_asserts or class MatrixTemplate<0. 0>.

Answer 4

You may add a method like this:

template <int WIDTH, int HEIGHT>
Matrix CreateMatrix()
{
    static_assert(WIDTH > 0, "WIDTH > 0 failed");
    static_assert(HEIGHT > 0, "HEIGHT > 0 failed");

    return Matrix(WIDTH, HEIGHT);
}

int main() {
    Matrix m(0, 2);     // no static check
    Matrix m2 = CreateMatrix<0,2>(); // static check

    return 0;
}