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How do I get the return type of a member function without an object? (4 answers)

Deduce return type of member function (1 answer)

Closed 18 mins ago.

Is there a way to determine a return type of a member function pointer?

法典样本:

///// my library
void my_func(auto mptr) { // have to use `auto`
  // some logic based on a return type of mptr: int, string, A, etc.
}

///// client code
struct A {
  int foo();
  std::string bar(int);
};

class B{
public:
  A func(int, double);
};
// ... and many other classes

my_func(&A::foo);
my_func(&A::bar);
my_func(&B::func);
// ... many other calls of my_func()

我需要“填入”<代码>my_func()。

Edit: I can t use std::result_of/std::invoke_result as I don t know the full list of parameters of mptr. It s not important with which params a method is supposed to be called as I m not calling it. I would like to avoid creating an object of base class of mptr even if I m able to determine it (using declval is ok).

Answer 1

您可使用部分模板专业确定<代码>mptr的返回类型:

template <typename T>
struct ReturnType;

template <typename Object, typename Return, typename... Args>
struct ReturnType<Return (Object::*)(Args...)>
{
    using Type = Return;
};

void my_func(auto mptr) {
  typename ReturnType<decltype(mptr)>::Type obj;
}

http://coliru.stacked-crooked.com/a/e7761cff81061d3d”rel=“noretinger”>Live Demo

Answer 2

You can write a function that deduces the type of a member function pointer, and returns the deduced return type. Note that only a declaration, and no definition is needed

template <typename C, typename Ret, typename... Args>
auto ret_type(Ret (C::*)(Args...)) -> Ret;

void my_func(auto mptr) 
{ 
  using type = decltype(ret_type(mptr));
}

我认为,这比专业化解决办法更容易阅读。

参看。

也可以通过增加超载荷来核算cv-qualifiers。 e.g.

template <typename C, typename Ret, typename... Args>
auto ret_type(Ret (C::*)(Args...) const) -> Ret;

参看 demo。

Answer 3

这里,我所谈到的情况似乎大多在工作。

template <typename TYPE, typename FUNCPTR, typename...ARGS>
using mfunc_return_t = decltype((std::declval<TYPE>().*std::declval<FUNCPTR>())(std::declval<ARGS&>()...));

Then I use it in a function which calls a provided member function.

template <typename FUNCPTR, typename TYPE, typename...ARGS>
mfunc_return_t<TYPE, FUNCPTR, ARGS...> SafeCall(TYPE* ptr, FUNCPTR func, ARGS&...args)
{
    if (ptr != nullptr)
        return (ptr->*func)(args...);
    using ReturnType = mfunc_return_t<TYPE, FUNCPTR, ARGS...>;
    return DefaultValue<ReturnType>();
}

我对处理类型,包括算术类型、点工、真空和其他可缺省建筑类型,有几处缺陷。

Then I can do things like

return SafeCall(pointer, &MyClass::FunctionName, arg1, arg2);

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