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April 1, 2012 15:29
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C++11 Metaprogramming - Traits to check whether a function with a specific name can be called in the context defined by a signature
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// Here is a traits structure I wrote to be able to determine if a function with | |
// a given name can be called in the context defined by a signature. This was | |
// originally not possible in C++03 - or at least a one solution works everywhere | |
// did not exist AFAIK. | |
// | |
// Olivier Grant | |
// | |
#include <iostream> | |
#include <iomanip> | |
#include <type_traits> | |
#define DECLARE_HAS_CALLABLE_TRAIT( name_, func_ ) \ | |
\ | |
namespace detail { \ | |
\ | |
template< typename ClassT, typename SignatureT > \ | |
struct has_callable_helper_ ## name_; \ | |
\ | |
template< typename ClassT, typename RetT, typename ... ArgsVT > \ | |
struct has_callable_helper_ ## name_ <ClassT,RetT ( ArgsVT ... )> \ | |
{ \ | |
private: \ | |
template< typename T > \ | |
struct check_return \ | |
{ \ | |
typedef std::integral_constant< \ | |
bool, \ | |
std::is_same<RetT,void>::value || \ | |
std::is_convertible<T,RetT>::value \ | |
> result; \ | |
}; \ | |
\ | |
template< \ | |
typename T, \ | |
typename U = decltype(std::declval<T>().func_(std::declval<ArgsVT>()...)) \ | |
> \ | |
static \ | |
typename check_return<U>::result test( void * ); \ | |
\ | |
template< typename T > \ | |
static \ | |
std::false_type test( ... ); \ | |
\ | |
typedef decltype(test<ClassT>(0)) result; \ | |
\ | |
public: \ | |
static bool constexpr value = result::value; \ | |
}; \ | |
\ | |
} \ | |
\ | |
template< typename ClassT, typename SignatureT > \ | |
struct has_callable_ ## name_ \ | |
: public std::integral_constant< \ | |
bool, \ | |
detail::has_callable_helper_ ## name_< \ | |
ClassT, \ | |
SignatureT \ | |
>::value \ | |
> \ | |
{ }; | |
/////////////////////////////////////////////////////////////////////////////////////// | |
// Test related code. | |
/////////////////////////////////////////////////////////////////////////////////////// | |
DECLARE_HAS_CALLABLE_TRAIT(func, func) | |
#define CHECK_VALUE( call_, expected_ ) \ | |
{ \ | |
auto const res(call_); \ | |
\ | |
std::cout \ | |
<< (res == expected_ ? "Ok " : "Error") \ | |
<< " - " \ | |
<< #call_ \ | |
<< " = " \ | |
<< std::boolalpha << res \ | |
<< "\n" \ | |
; \ | |
} | |
struct Test0 | |
{ | |
}; | |
struct Test1 | |
{ | |
void func( ) | |
{ std::cerr << "Test1::func\n"; } | |
}; | |
struct Test2 | |
{ | |
int func( ) | |
{ std::cerr << "Test2::func\n"; return 0; } | |
}; | |
struct Test3 | |
{ | |
void func( double ) | |
{ std::cerr << "Test3::func\n"; } | |
}; | |
struct Test4 | |
{ | |
int func( double ) | |
{ std::cerr << "Test4::func\n"; return 0; } | |
}; | |
int main( ) | |
{ | |
CHECK_VALUE((has_callable_func<Test0,void ( )>::value), false) | |
CHECK_VALUE((has_callable_func<Test1,void ( )>::value), true) | |
CHECK_VALUE((has_callable_func<Test2,void ( )>::value), true) | |
CHECK_VALUE((has_callable_func<Test3,void ( )>::value), false) | |
CHECK_VALUE((has_callable_func<Test4,void ( )>::value), false) | |
CHECK_VALUE((has_callable_func<Test0,int ( )>::value), false) | |
CHECK_VALUE((has_callable_func<Test1,int ( )>::value), false) | |
CHECK_VALUE((has_callable_func<Test2,int ( )>::value), true) | |
CHECK_VALUE((has_callable_func<Test3,int ( )>::value), false) | |
CHECK_VALUE((has_callable_func<Test4,int ( )>::value), false) | |
CHECK_VALUE((has_callable_func<Test0,void ( double )>::value), false) | |
CHECK_VALUE((has_callable_func<Test1,void ( double )>::value), false) | |
CHECK_VALUE((has_callable_func<Test2,void ( double )>::value), false) | |
CHECK_VALUE((has_callable_func<Test3,void ( double )>::value), true) | |
CHECK_VALUE((has_callable_func<Test4,void ( double )>::value), true) | |
CHECK_VALUE((has_callable_func<Test0,int ( double )>::value), false) | |
CHECK_VALUE((has_callable_func<Test1,int ( double )>::value), false) | |
CHECK_VALUE((has_callable_func<Test2,int ( double )>::value), false) | |
CHECK_VALUE((has_callable_func<Test3,int ( double )>::value), false) | |
CHECK_VALUE((has_callable_func<Test4,int ( double )>::value), true) | |
return 0; | |
} |
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