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  1. #ifndef MPL_IS_CALLABLE_HPP
  2. #define MPL_IS_CALLABLE_HPP
  3.  
  4. #include <type_traits>
  5.  
  6. namespace mpl
  7. {
  8.     namespace // implementation detail
  9.     {
  10.         // build R (*)(Args...) from R (Args...)
  11.         // compile error if signature is not a valid function signature
  12.         template <typename, typename>
  13.         struct build_free_function;
  14.  
  15.         template <typename F, typename R, typename ... Args>
  16.         struct build_free_function<F, R (Args...)>
  17.         { using type = R (*)(Args...); };
  18.  
  19.         // build R (C::*)(Args...) from R (Args...)
  20.         //       R (C::*)(Args...) const from R (Args...) const
  21.         //       R (C::*)(Args...) volatile from R (Args...) volatile
  22.         // compile error if signature is not a valid member function signature
  23.         template <typename, typename>
  24.         struct build_class_function;
  25.  
  26.         template <typename C, typename R, typename ... Args>
  27.         struct build_class_function<C, R (Args...)>
  28.         { using type = R (C::*)(Args...); };
  29.  
  30.         template <typename C, typename R, typename ... Args>
  31.         struct build_class_function<C, R (Args...) const>
  32.         { using type = R (C::*)(Args...) const; };
  33.  
  34.         template <typename C, typename R, typename ... Args>
  35.         struct build_class_function<C, R (Args...) volatile>
  36.         { using type = R (C::*)(Args...) volatile; };
  37.  
  38.         // determine whether a class C has an operator() with signature S
  39.         template <typename C, typename S>
  40.         struct is_functor_with_signature
  41.         {
  42.             typedef char (& yes)[1];
  43.             typedef char (& no)[2];
  44.  
  45.             // helper struct to determine that C::operator() does indeed have
  46.             // the desired signature; &C::operator() is only of type 
  47.             // R (C::*)(Args...) if this is true
  48.             template <typename T, T> struct check;
  49.  
  50.             // T is needed to enable SFINAE
  51.             template <typename T> static yes deduce(check<
  52.                 typename build_class_function<C, S>::type, &T::operator()> *);
  53.             // fallback if check helper could not be built
  54.             template <typename> static no deduce(...);
  55.  
  56.             static bool constexpr value = sizeof(deduce<C>(0)) == sizeof(yes);
  57.         };
  58.  
  59.         // determine whether a free function pointer F has signature S
  60.         template <typename F, typename S>
  61.         struct is_function_with_signature
  62.         {
  63.             // check whether F and the function pointer of S are of the same
  64.             // type
  65.             static bool constexpr value = std::is_same<
  66.                 F, typename build_free_function<F, S>::type
  67.             >::value;
  68.         };
  69.  
  70.         // C is a class, delegate to is_functor_with_signature
  71.         template <typename C, typename S, bool>
  72.         struct is_callable_impl
  73.             : std::integral_constant<
  74.                 bool, is_functor_with_signature<C, S>::value
  75.               >
  76.         {};
  77.  
  78.         // F is not a class, delegate to is_function_with_signature
  79.         template <typename F, typename S>
  80.         struct is_callable_impl<F, S, false>
  81.             : std::integral_constant<
  82.                 bool, is_function_with_signature<F, S>::value
  83.               >
  84.         {};
  85.     }
  86.  
  87.     // Determine whether type Callable is callable with signature Signature.
  88.     // Compliant with functors, i.e. classes that declare operator(); and free
  89.     // function pointers: R (*)(Args...), but not R (Args...)!
  90.     template <typename Callable, typename Signature>
  91.     struct is_callable
  92.         : is_callable_impl<
  93.             Callable, Signature,
  94.             std::is_class<Callable>::value
  95.           >
  96.     {};
  97.  
  98.     namespace // tests
  99.     {
  100.         struct A { void operator()(); };
  101.         struct B {};
  102.         struct C { 
  103.             int operator()(int &, void **) const; 
  104.             int operator()(double);
  105.         };
  106.         void a();
  107.         int b;
  108.         int c(int &, void **);
  109.         int c(double);
  110.  
  111. #define MPL_IS_CALLABLE_POSITIVE "should be recognized as callable"
  112. #define MPL_IS_CALLABLE_NEGATIVE "should not be recognized as callable"
  113.         static_assert(is_callable<A, void ()>::value, MPL_IS_CALLABLE_POSITIVE);
  114.         static_assert(!is_callable<B, void ()>::value, MPL_IS_CALLABLE_NEGATIVE);
  115.         static_assert(is_callable<C, int (int &, void **) const>::value, MPL_IS_CALLABLE_POSITIVE);
  116.         static_assert(is_callable<C, int (double)>::value, MPL_IS_CALLABLE_POSITIVE);
  117.         static_assert(is_callable<decltype(&a), void ()>::value, MPL_IS_CALLABLE_POSITIVE);
  118.         static_assert(!is_callable<decltype(&b), void ()>::value, MPL_IS_CALLABLE_NEGATIVE);
  119.         static_assert(is_callable<decltype(static_cast<int (*)(int &, void **)>(&c)), int (int &, void **)>::value, MPL_IS_CALLABLE_POSITIVE);
  120.         static_assert(is_callable<decltype(static_cast<int (*)(double)>(&c)), int (double)>::value, MPL_IS_CALLABLE_POSITIVE);
  121. #undef MPL_IS_CALLABLE_POSITIVE
  122. #undef MPL_IS_CALLABLE_NEGATIVE
  123.     }
  124. }
  125.  
  126. #endif // MPL_IS_CALLABLE_HPP
Compilation error #stdin compilation error #stdout 0s 0KB
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stdin
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Standard input is empty
compilation info 
/usr/lib/gcc/i486-linux-gnu/4.8/../../../i386-linux-gnu/crt1.o: In function `_start':
(.text+0x18): undefined reference to `main'
collect2: error: ld returned 1 exit status
stdout
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Standard output is empty
https://ideone.com/7PWdiv
language:
C++14 (gcc 8.3)
created:
11 years ago
visibility:
public

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