Using C++11 I am trying to get the type of function arguments from a function name, granted the function signature is unambiguous. Ultimately, I am trying to check that an argument is exactly the type I want before I call a function.
So far I can do it with non-member functions thanks to this solution: Unpacking arguments of a functional parameter to a C++ template class
I have slightly edited it to get the following class:
template<typename T>
struct FunctionSignatureParser; // unimplemented primary template
template<typename Result, typename...Args>
struct FunctionSignatureParser<Result(Args...)>
{
using return_type = Result;
using args_tuple = std::tuple<Args...>;
template <size_t i> struct arg
{
typedef typename std::tuple_element<i, args_tuple>::type type;
};
};
For instance I can check the type of a function at compile-time:
short square(char x) { // 8-bit integer as input, 16-bit integer as output
return short(x)*short(x);
}
int main() {
char answer = 42;
static_assert(std::is_same<char, FunctionSignatureParser<decltype(square)>::arg<0>::type>::value, "Function 'square' does not use an argument of type 'char'");
static_assert(std::is_same<short, FunctionSignatureParser<decltype(square)>::return_type>::value, "Function 'square' does not return a value of type 'short'");
short sqrAnswer = square(answer);
std::cout << "The square of " << +answer << " is " << +sqrAnswer << std::endl;
return 0;
}
But when I want to check the type of member functions, some compilers are not happy with it:
struct Temperature
{
double degrees;
Temperature add(double value);
};
int main() {
Temperature t{16};
double increment{8};
static_assert(std::is_same<double, FunctionSignatureParser<decltype(t.add)>::arg<0>::type>::value, "Method 'Temperature::add' does not use an argument of type 'double'");
std::cout << t.degrees << u8" \u00b0C + " << increment << " == " << t.add(increment).degrees << u8" \u00b0C" << std::endl;
return 0;
}
Here is what gcc 6.3 has to say:
error: invalid use of non-static member function ‘Temperature Temperature::add(double)’
Here is what clang 4.0 has to say:
error: reference to non-static member function must be called
I have tried those options, to no avail:
decltype(Temperature::add)
decltype(std::declval<Temperature>().add)
What’s the problem with non-static member functions inside a decltype
? Since expressions inside a decltype
are not evaluated, the static
qualifier should not matter, right?
For the record, MSVC12 succeeds in this situation. I cannot tell if the Microsoft compiler is right or wrong, though. (please let’s not make this thread a compiler war)
Also, if you have a solution to argument checking that does not involve the initial approach, I am open for it as well.
You need another template specialization for member functions, something like this:
template<typename ClassType, typename Result, typename...Args>
struct FunctionSignatureParser<Result(ClassType::*)(Args...)>
{
using return_type = Result;
using args_tuple = std::tuple<Args...>;
template <size_t i> struct arg
{
typedef typename std::tuple_element<i, args_tuple>::type type;
};
};
And it can be used like this:
static_assert(std::is_same<double, FunctionSignatureParser<decltype(&Temperature::add)>::arg<0>::type>::value, "Method 'Temperature::add' does not use an argument of type 'double'");
This works with gcc 7.2, haven't tested other compilers