Is it possible to write a template to check for a

Question

Is it possible to write a template that changes behavior depending on if a certain member function is defined on a class?

Here's a simple example of what I would want to write:

template<class T>
std::string optionalToString(T* obj)
{
    if (FUNCTION_EXISTS(T->toString))
        return obj->toString();
    else
        return "toString not defined";
}

So, if class T has toString() defined, then it uses it; otherwise, it doesn't. The magical part that I don't know how to do is the "FUNCTION_EXISTS" part.

Answer 1

The generic template that can be used for checking if some "feature" is supported by the type:

#include <type_traits>

template <template <typename> class TypeChecker, typename Type>
struct is_supported
{
    // these structs are used to recognize which version
    // of the two functions was chosen during overload resolution
    struct supported {};
    struct not_supported {};

    // this overload of chk will be ignored by SFINAE principle
    // if TypeChecker<Type_> is invalid type
    template <typename Type_>
    static supported chk(typename std::decay<TypeChecker<Type_>>::type *);

    // ellipsis has the lowest conversion rank, so this overload will be
    // chosen during overload resolution only if the template overload above is ignored
    template <typename Type_>
    static not_supported chk(...);

    // if the template overload of chk is chosen during
    // overload resolution then the feature is supported
    // if the ellipses overload is chosen the the feature is not supported
    static constexpr bool value = std::is_same<decltype(chk<Type>(nullptr)),supported>::value;
};

The template that checks whether there is a method foo that is compatible with signature double(const char*)

// if T doesn't have foo method with the signature that allows to compile the bellow
// expression then instantiating this template is Substitution Failure (SF)
// which Is Not An Error (INAE) if this happens during overload resolution
template <typename T>
using has_foo = decltype(double(std::declval<T>().foo(std::declval<const char*>())));

Examples

// types that support has_foo
struct struct1 { double foo(const char*); };            // exact signature match
struct struct2 { int    foo(const std::string &str); }; // compatible signature
struct struct3 { float  foo(...); };                    // compatible ellipsis signature
struct struct4 { template <typename T>
                 int    foo(T t); };                    // compatible template signature

// types that do not support has_foo
struct struct5 { void        foo(const char*); }; // returns void
struct struct6 { std::string foo(const char*); }; // std::string can't be converted to double
struct struct7 { double      foo(      int *); }; // const char* can't be converted to int*
struct struct8 { double      bar(const char*); }; // there is no foo method

int main()
{
    std::cout << std::boolalpha;

    std::cout << is_supported<has_foo, int    >::value << std::endl; // false
    std::cout << is_supported<has_foo, double >::value << std::endl; // false

    std::cout << is_supported<has_foo, struct1>::value << std::endl; // true
    std::cout << is_supported<has_foo, struct2>::value << std::endl; // true
    std::cout << is_supported<has_foo, struct3>::value << std::endl; // true
    std::cout << is_supported<has_foo, struct4>::value << std::endl; // true

    std::cout << is_supported<has_foo, struct5>::value << std::endl; // false
    std::cout << is_supported<has_foo, struct6>::value << std::endl; // false
    std::cout << is_supported<has_foo, struct7>::value << std::endl; // false
    std::cout << is_supported<has_foo, struct8>::value << std::endl; // false

    return 0;
}

http://coliru.stacked-crooked.com/a/83c6a631ed42cea4

Answer 2

MSVC has the __if_exists and __if_not_exists keywords (Doc). Together with the typeof-SFINAE approach of Nicola I could create a check for GCC and MSVC like the OP looked for.

Update: Source can be found Here

Answer 3

There are a lot of answers here, but I failed, to find a version, that performs real method resolution ordering, while not using any of the newer c++ features (only using c++98 features).
Note: This version is tested and working with vc++2013, g++ 5.2.0 and the onlline compiler.

So I came up with a version, that only uses sizeof():

template<typename T> T declval(void);

struct fake_void { };
template<typename T> T &operator,(T &,fake_void);
template<typename T> T const &operator,(T const &,fake_void);
template<typename T> T volatile &operator,(T volatile &,fake_void);
template<typename T> T const volatile &operator,(T const volatile &,fake_void);

struct yes { char v[1]; };
struct no  { char v[2]; };
template<bool> struct yes_no:yes{};
template<> struct yes_no<false>:no{};

template<typename T>
struct has_awesome_member {
 template<typename U> static yes_no<(sizeof((
   declval<U>().awesome_member(),fake_void()
  ))!=0)> check(int);
 template<typename> static no check(...);
 enum{value=sizeof(check<T>(0)) == sizeof(yes)};
};


struct foo { int awesome_member(void); };
struct bar { };
struct foo_void { void awesome_member(void); };
struct wrong_params { void awesome_member(int); };

static_assert(has_awesome_member<foo>::value,"");
static_assert(!has_awesome_member<bar>::value,"");
static_assert(has_awesome_member<foo_void>::value,"");
static_assert(!has_awesome_member<wrong_params>::value,"");

Live demo (with extended return type checking and vc++2010 workaround): http://cpp.sh/5b2vs

No source, as I came up with it myself.

When running the Live demo on the g++ compiler, please note that array sizes of 0 are allowed, meaning that the static_assert used will not trigger a compiler error, even when it fails.
A commonly used work-around is to replace the 'typedef' in the macro with 'extern'.

Answer 4

Strange nobody suggested the following nice trick I saw once on this very site :

template <class T>
struct has_foo
{
    struct S { void foo(...); };
    struct derived : S, T {};

    template <typename V, V> struct W {};

    template <typename X>
    char (&test(W<void (X::*)(), &X::foo> *))[1];

    template <typename>
    char (&test(...))[2];

    static const bool value = sizeof(test<derived>(0)) == 1;
};

You have to make sure T is a class. It seems that ambiguity in the lookup of foo is a substitution failure. I made it work on gcc, not sure if it is standard though.

Answer 5

Here is my version that handles all possible member function overloads with arbitrary arity, including template member functions, possibly with default arguments. It distinguishes 3 mutually exclusive scenarios when making a member function call to some class type, with given arg types: (1) valid, or (2) ambiguous, or (3) non-viable. Example usage:

#include <string>
#include <vector>

HAS_MEM(bar)
HAS_MEM_FUN_CALL(bar)

struct test
{
   void bar(int);
   void bar(double);
   void bar(int,double);

   template < typename T >
   typename std::enable_if< not std::is_integral<T>::value >::type
   bar(const T&, int=0){}

   template < typename T >
   typename std::enable_if< std::is_integral<T>::value >::type
   bar(const std::vector<T>&, T*){}

   template < typename T >
   int bar(const std::string&, int){}
};

Now you can use it like this:

int main(int argc, const char * argv[])
{
   static_assert( has_mem_bar<test>::value , "");

   static_assert( has_valid_mem_fun_call_bar<test(char const*,long)>::value , "");
   static_assert( has_valid_mem_fun_call_bar<test(std::string&,long)>::value , "");

   static_assert( has_valid_mem_fun_call_bar<test(std::vector<int>, int*)>::value , "");
   static_assert( has_no_viable_mem_fun_call_bar<test(std::vector<double>, double*)>::value , "");

   static_assert( has_valid_mem_fun_call_bar<test(int)>::value , "");
   static_assert( std::is_same<void,result_of_mem_fun_call_bar<test(int)>::type>::value , "");

   static_assert( has_valid_mem_fun_call_bar<test(int,double)>::value , "");
   static_assert( not has_valid_mem_fun_call_bar<test(int,double,int)>::value , "");

   static_assert( not has_ambiguous_mem_fun_call_bar<test(double)>::value , "");
   static_assert( has_ambiguous_mem_fun_call_bar<test(unsigned)>::value , "");

   static_assert( has_viable_mem_fun_call_bar<test(unsigned)>::value , "");
   static_assert( has_viable_mem_fun_call_bar<test(int)>::value , "");

   static_assert( has_no_viable_mem_fun_call_bar<test(void)>::value , "");

   return 0;
}

Here is the code, written in c++11, however, you can easily port it (with minor tweaks) to non-c++11 that has typeof extensions (e.g. gcc). You can replace the HAS_MEM macro with your own.

#pragma once

#if __cplusplus >= 201103

#include <utility>
#include <type_traits>

#define HAS_MEM(mem)                                                                                     \
                                                                                                     \
template < typename T >                                                                               \
struct has_mem_##mem                                                                                  \
{                                                                                                     \
  struct yes {};                                                                                     \
  struct no  {};                                                                                     \
                                                                                                     \
  struct ambiguate_seed { char mem; };                                                               \
  template < typename U > struct ambiguate : U, ambiguate_seed {};                                   \
                                                                                                     \
  template < typename U, typename = decltype(&U::mem) > static constexpr no  test(int);              \
  template < typename                                 > static constexpr yes test(...);              \
                                                                                                     \
  static bool constexpr value = std::is_same<decltype(test< ambiguate<T> >(0)),yes>::value ;         \
  typedef std::integral_constant<bool,value>    type;                                                \
};


#define HAS_MEM_FUN_CALL(memfun)                                                                         \
                                                                                                     \
template < typename Signature >                                                                       \
struct has_valid_mem_fun_call_##memfun;                                                               \
                                                                                                     \
template < typename T, typename... Args >                                                             \
struct has_valid_mem_fun_call_##memfun< T(Args...) >                                                  \
{                                                                                                     \
  struct yes {};                                                                                     \
  struct no  {};                                                                                     \
                                                                                                     \
  template < typename U, bool = has_mem_##memfun<U>::value >                                         \
  struct impl                                                                                        \
  {                                                                                                  \
     template < typename V, typename = decltype(std::declval<V>().memfun(std::declval<Args>()...)) > \
     struct test_result { using type = yes; };                                                       \
                                                                                                     \
     template < typename V > static constexpr typename test_result<V>::type test(int);               \
     template < typename   > static constexpr                            no test(...);               \
                                                                                                     \
     static constexpr bool value = std::is_same<decltype(test<U>(0)),yes>::value;                    \
     using type = std::integral_constant<bool, value>;                                               \
  };                                                                                                 \
                                                                                                     \
  template < typename U >                                                                            \
  struct impl<U,false> : std::false_type {};                                                         \
                                                                                                     \
  static constexpr bool value = impl<T>::value;                                                      \
  using type = std::integral_constant<bool, value>;                                                  \
};                                                                                                    \
                                                                                                     \
template < typename Signature >                                                                       \
struct has_ambiguous_mem_fun_call_##memfun;                                                           \
                                                                                                     \
template < typename T, typename... Args >                                                             \
struct has_ambiguous_mem_fun_call_##memfun< T(Args...) >                                              \
{                                                                                                     \
  struct ambiguate_seed { void memfun(...); };                                                       \
                                                                                                     \
  template < class U, bool = has_mem_##memfun<U>::value >                                            \
  struct ambiguate : U, ambiguate_seed                                                               \
  {                                                                                                  \
    using ambiguate_seed::memfun;                                                                    \
    using U::memfun;                                                                                 \
  };                                                                                                 \
                                                                                                     \
  template < class U >                                                                               \
  struct ambiguate<U,false> : ambiguate_seed {};                                                     \
                                                                                                     \
  static constexpr bool value = not has_valid_mem_fun_call_##memfun< ambiguate<T>(Args...) >::value; \
  using type = std::integral_constant<bool, value>;                                                  \
};                                                                                                    \
                                                                                                     \
template < typename Signature >                                                                       \
struct has_viable_mem_fun_call_##memfun;                                                              \
                                                                                                     \
template < typename T, typename... Args >                                                             \
struct has_viable_mem_fun_call_##memfun< T(Args...) >                                                 \
{                                                                                                     \
  static constexpr bool value = has_valid_mem_fun_call_##memfun<T(Args...)>::value                   \
                             or has_ambiguous_mem_fun_call_##memfun<T(Args...)>::value;              \
  using type = std::integral_constant<bool, value>;                                                  \
};                                                                                                    \
                                                                                                     \
template < typename Signature >                                                                       \
struct has_no_viable_mem_fun_call_##memfun;                                                           \
                                                                                                     \
template < typename T, typename... Args >                                                             \
struct has_no_viable_mem_fun_call_##memfun < T(Args...) >                                             \
{                                                                                                     \
  static constexpr bool value = not has_viable_mem_fun_call_##memfun<T(Args...)>::value;             \
  using type = std::integral_constant<bool, value>;                                                  \
};                                                                                                    \
                                                                                                     \
template < typename Signature >                                                                       \
struct result_of_mem_fun_call_##memfun;                                                               \
                                                                                                     \
template < typename T, typename... Args >                                                             \
struct result_of_mem_fun_call_##memfun< T(Args...) >                                                  \
{                                                                                                     \
  using type = decltype(std::declval<T>().memfun(std::declval<Args>()...));                          \
};

#endif

---

Answer 6

I wrote an answer to this in another thread that (unlike the solutions above) also checks inherited member functions:

SFINAE to check for inherited member functions

Here are some example from that solution:

Example1:

We are checking for a member with the following signature: T::const_iterator begin() const

template<class T> struct has_const_begin
{
    typedef char (&Yes)[1];
    typedef char (&No)[2];

    template<class U> 
    static Yes test(U const * data, 
                    typename std::enable_if<std::is_same<
                             typename U::const_iterator, 
                             decltype(data->begin())
                    >::value>::type * = 0);
    static No test(...);
    static const bool value = sizeof(Yes) == sizeof(has_const_begin::test((typename std::remove_reference<T>::type*)0));
};

Please notice that it even checks the constness of the method, and works with primitive types, as well. (I mean has_const_begin<int>::value is false and doesn't cause a compile-time error.)

Example 2

Now we are looking for the signature: void foo(MyClass&, unsigned)

template<class T> struct has_foo
{
    typedef char (&Yes)[1];
    typedef char (&No)[2];

    template<class U>
    static Yes test(U * data, MyClass* arg1 = 0,
                    typename std::enable_if<std::is_void<
                             decltype(data->foo(*arg1, 1u))
                    >::value>::type * = 0);
    static No test(...);
    static const bool value = sizeof(Yes) == sizeof(has_foo::test((typename std::remove_reference<T>::type*)0));
};

Please notice that MyClass doesn't has to be default constructible or to satisfy any special concept. The technique works with template members, as well.

I am eagerly waiting opinions regarding this.