harpocrates | Runtime Evironment library

^{From the error message I suppose that sha1_hash is overloaded.}

void (*sha1_hash)(const std::vector&, std::vector&) = harpocrates::hashing::sha1_hash; works because when taking the address of an overloaded function overload resolution is performed, the overload whose signature matches the type of sha1_hash is selected, i.e. void (*)(const std::vector&, std::vector&).

In all these contexts, the function selected from the overload set is the function whose type matches the pointer to function, reference to function, or pointer to member function type that is expected by target: the object or reference being initialized, the left-hand side of the assignment, function or operator parameter, the return type of a function, the target type of a cast, or the type of the template parameter, respectively.

You can also use static_cast to specify explicitly.