C++11 Lambda functions implicit conversion to bool vs. std::function

Question

Consider this simple example code:

#include 
#include 

void f(bool _switch) {
    std::cout << \"Nothing really\" &l         


        

Answer 1

namespace details{
  template
  struct invoke {};
  template
  struct invoke()(std::declval()...)
  ))>{
    using type=decltype(std::declval()(std::declval()...));
  };
}
templatestruct invoke:details::invoke{};

template
struct invoke_test:std::false_type {};
template
struct invoke_test::type,
      R
    >::value
  >::type
>:std::true_type {};
template
struct invoke_test::type ) )
>:std::true_type{};
template
constexpr bool invokable() {
  return invoke_test::value;
}

this gives us a pseudo-concept invokable.

We can then use it like:

template
typename std::enable_if()>::type
f(F&&){
  std::cout << "can be invoked\n";
}
void f(bool) {
  std::cout << "is bool\n";
}

and bob is your uncle.

The real problem is that std::function 's constructor does not do a similar test, and instead claims (falsely) that it can be constructed from anything at all. This is a flaw in the standard, one I suspect will be fixed once concepts are standardized.