function_view cannot bind to const lambda

[nitronoid]

@Naios

---

Expected Behavior

The const-ness of the callable should be propagated to the erased function pointer

Actual Behavior

In file included from main.cpp:1:0:
function2.hpp: In instantiation of ‘static void* fu2::abi_400::detail::type_erasure::address_taker<T, <template-parameter-1-2> >::take(O&&) [with O = const main()::<lambda()>&; T = main()::<lambda()>; <template-parameter-1-2> = void]’:
function2.hpp:1184:51:   required from ‘constexpr fu2::abi_400::detail::type_erasure::erasure<false, Config, fu2::abi_400::detail::property<IsThrowing, HasStrongExceptGuarantee, Args ...> >::erasure(T&&) [with T = const main()::<lambda()>&; Config = fu2::abi_400::detail::config<false, true, fu2::capacity_default>; bool IsThrowing = true; bool HasStrongExceptGuarantee = false; Args = {int()}]’
function2.hpp:1376:72:   required from ‘constexpr fu2::abi_400::detail::function<Config, fu2::abi_400::detail::property<IsThrowing, HasStrongExceptGuarantee, Args ...> >::function(T&&) [with T = const main()::<lambda()>&; fu2::abi_400::detail::function<Config, fu2::abi_400::detail::property<IsThrowing, HasStrongExceptGuarantee, Args ...> >::enable_if_not_convertible_to_this<T>* <anonymous> = 0; fu2::abi_400::detail::function<Config, fu2::abi_400::detail::property<IsThrowing, HasStrongExceptGuarantee, Args ...> >::enable_if_can_accept_all_t<T>* <anonymous> = 0; fu2::abi_400::detail::function<Config, fu2::abi_400::detail::property<IsThrowing, HasStrongExceptGuarantee, Args ...> >::assert_wrong_copy_assign_t<T>* <anonymous> = 0; fu2::abi_400::detail::function<Config, fu2::abi_400::detail::property<IsThrowing, HasStrongExceptGuarantee, Args ...> >::assert_no_strong_except_guarantee_t<T>* <anonymous> = 0; Config = fu2::abi_400::detail::config<false, true, fu2::capacity_default>; bool IsThrowing = true; bool HasStrongExceptGuarantee = false; Args = {int()}]’
main.cpp:9:42:   required from here
function2.hpp:249:26: error: invalid conversion from ‘const void*’ to ‘void*’ [-fpermissive]
     return std::addressof(obj);

Steps to Reproduce

#include "function2.hpp"
int main()
{
  const auto callable = []{ 
      return 5;
  };  

  fu2::function_view<int()> view(callable);
}

g++ -std=c++14 main.cpp

Your Environment

>$ g++ --version
g++ (GCC) 7.3.1 20180303 (Red Hat 7.3.1-5)
Copyright (C) 2017 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

[Naios]

From my point of view this is the intended behaviour here because there is no way to access
a method qualified as const from a non const context without ignoring the qualifier.

If you want to create a function_view to a const callable object use fu2::function_view<int() const> view(callable);

Implementing it in a way you are proposing would heavily violate the const guarantees provided by the library, because then the constness of an erased object is not propageted to the callable erasure anymore.

[nitronoid]

Hi, thanks for the fast reply. Firstly, the method you suggest fails with the same error:

#include "function2.hpp"

int main()
{
  const auto callable = []{ 
      return 5;
  };  

  fu2::function_view<int() const> view(callable);
}

Secondly I disagree with your final remark. If it is guaranteed that casting any pointer to void* and back will yield the same pointer, and you are not modifying the stored pointer, then I don't see where the violation occurs? This code appears valid to me:

#include "function2.hpp"
#include <iostream>

using func_t = int (*const)();

int main()
{
  const auto callable = [](){
      return 5;
  };

  func_t fptr = callable;
  void* ptr = (void*)(fptr);
  auto cptr = (func_t)(ptr);
  std::cout<<cptr()<<'\n';
}

I'm am however happy to be told otherwise, thanks

[ladnir]

See folly/function for a basic explanation. It's about halfway down and I quote the beginning:

Other than copyability, there is one more significant difference between std::function and folly::Function, and it concerns const-correctness. std::function does not enforce const-correctness: it allows you to store mutable callables (i.e. callables that may change their inner state when executed, such as a mutable lambda) and call them in a const context (i.e. when you only have access to a const reference to the std::function object). For example ...

[nitronoid]

That does make sense, so could you explain why my const example doesn't compile?

[Naios]

Seems like there is a bug in the implementation, I'll provide a fix for that soon.

[wak-google]

Any update on this? I was testing and noticed

  const auto callable = [] { 
      return 5;
  };  
  fu2::function_view<int() const> view(callable);

still doesn't work

[Naios]

@wak-google Sadly I did not look into this yet.
Mainly I thought this issue has not a super high priority.

[wak-google]

Any update? 😃

[Naios]

Fixed in fa0b708