#How do I check for constness when using member function pointers?

44 messages · Page 1 of 1 (latest)

astral thorn
thin veldtBOT
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astral thorn
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C++14 only pls

raven roost
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i don't quite understand what the purpose of this contraption would be

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but apart from that

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why would it not be valid to call a non-const member function of a Demo on a non-const Demo?

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and ofc you can't call a non-const member function of a Demo on a const Demo

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so if you ask me, looks like you just screwed up the asserts there?

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note: build yourself something like std::void_t and you can avoid those helpers

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also, be advised that just because calling a member function pointer on a given object is valid doesn't mean the class actually has a member function of a matching signature

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so this thing really doesn't check whatever the name would suggest it's supposed to be checking

astral thorn
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!f

thin veldtBOT
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#include <iostream>
#include <type_traits>

template<typename ThisType, typename MemberFuncType, typename... Args>
std::true_type has_member_function_helper(
    decltype((std::declval<ThisType>().*std::declval<MemberFuncType>())(
        std::declval<Args>()...))* returnParam = nullptr);

template<typename ThisType, typename MemberFuncType, typename... Args>
std::false_type has_member_function_helper(...);

template<typename ThisType, typename MemberFuncType, typename... Args>
struct has_member_function
    : decltype(has_member_function_helper<ThisType, MemberFuncType, Args...>(
          nullptr)) {};

class Demo {
 public:
  int stuff(int a, int b) const { return a + b; }

  int stuff2(int a, int b) { return a - b; }
};

int main() {
  using Func1Type = int (Demo::*)(int, int) const;
  using Func2Type = int (Demo::*)(int, int);

  static_assert(has_member_function<Demo, Func2Type, int, int>::value,
                "Error 1");
  static_assert(
      has_member_function<const Demo, Func2Type, int, int>::value == false,
      "Error 2");

  static_assert(has_member_function<Demo, Func1Type, int, int>::value,
                "Error 3");
  static_assert(has_member_function<const Demo, Func1Type, int, int>::value,
                "Error 4");

  return 0;
}
Kolio98
raven roost
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yeah

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now change Demo to

class Demo {};
```and watch it compile just fine.
astral thorn
raven roost
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just to make sure that's what you intended

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because the name would suggest that's not what you intended

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your has_member_function doesn't check whether the thing has a member function of that signature

astral thorn
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wtf

raven roost
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all it checks is whether it's possible to invoke a member function through a member function pointer

raven roost
astral thorn
raven roost
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you don't

astral thorn
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wut

raven roost
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there's no way to check whether a given class has any member function of a given signature

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at least not as far as i'd be aware

astral thorn
raven roost
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you would need to know the name of the member function

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at which point, might as well just make it an operator () and use one of the invocable traits or smth…

astral thorn
raven roost
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ah those were added in 17

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well anyways, just make it an operator () and check whether you can (args...) an object of the given type

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ofc that also doesn't check for an exact signature

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if you wanted to check that, i guess you could try to see if you can convert &class::fun to a member function pointer of the given signature

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not 100% sure if that will work in the face of overloading

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but there's an exception there to make it work for function pointers, i'd assume that there's a smilar rule for member function pointers

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apart from all that: if you ask me, just don't.

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nothing good has ever come off doing this sort of stuff

astral thorn
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I wanted to learn about the limitations here

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not use this in real life

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!solved