Unable to call a template member function of a template class from another template class

Question

I'm getting compiler errors when attempting to access a template member function of a template class from within another template class. A function call to getSubmatrix results in the compiler warnings, "expression result unused", about the parameters (0, 0), finally failing with the compiler error "reference to non-static member function must be called".

The matrix class:

template <std::size_t m, std::size_t n, typename T, std::size_t rowPadding = 0>
class Matrix
{
public:
    ...

    template <std::size_t p, std::size_t q>
    Matrix<p, q, T, n - q>& getSubmatrix(std::size_t column, std::size_t row)
    {
        ...
    }
};

The transform class:

template <typename T>
class Transform
{
public:
    ...

    Matrix<4, 4, T> matrix() const
    {
        ...

        Matrix<4, 4, T> result;
        result.getSubmatrix<3, 3>(0, 0) = Matrix<3, 3, T>();

        ...
    }
};

Note that changing the matrix type to Matrix<4, 4, float> rather than Matrix<4, 4, T> will result in valid compilation. The compiler is clang 4.0 and the language version is C++11.

Answer 1

You should add keyword template:

result.template getSubmatrix<3, 3>(0, 0) = Matrix<3, 3, T>();
//     ^^^^^^^^

Without it, compiler will think that < is a comparison operator.

PS. In that case, g++ produces a little more understandable error:

error: invalid operands of types <unresolved overloaded function type> and int to binary operator<