Question

I’m wondering if it’s safe to cast a double (*)(double) to double(*)(...), this is going to be used to generalize a code which may have pointers to multiple functions.

so far I’ve stored every thing that is going to be passed to the function in a vector, and I’m wondering if there is a way to call a function (while passing the correct number of arguments) in a generalized code? I mean something this:

//while initializing
mFunction = sin;
//later in code
double (*generalized)(...) = mFunction;
for(i=0;i<args.size();i++)
    pusharg(args[i]);
call(generalized);

–edit–

if there is not a valid way to do it using c++ is it possible to saftly call the function using assembly?

Answer 1

You cannot directly assign a double(*)(double) to double(*)(...), but you could reinterpret_cast it. The cast is explicitly allowed by §5.2.10[expr.reinterpret.cast]/6, but calling the casted function pointer will cause undefined behavior:

A function pointer can be explicitly converted to a function pointer of a different type.

The effect of calling a function through a pointer to a function type that is not the same as the type used in the definition of the function is undefined.

Except that converting a prvalue of type “pointer to T1” to the type “pointer to T2” (where T1 and T2 are function types) and back to its original type yields the original pointer value, the result of such a pointer conversion is unspecified.

It is easy to see why it leads to UB — what if we call generalized(1.0, 2.0, 3.0)? This will likely corrupt the call stack. But it is fine if you cast the generalized back to a double(*)(double) before you call it.