Templated check for the existence of a class member function?

C++20 - requires expressions

With C++20 come concepts and assorted tools such as requires expressions which are a built-in way to check for a function existence. With them you could rewrite your optionalToString function as follows:

template<class T>
std::string optionalToString(T* obj)
{
    constexpr bool has_toString = requires(const T& t) {
        t.toString();
    };

    if constexpr (has_toString)
        return obj->toString();
    else
        return "toString not defined";
}

Pre-C++20 - Detection toolkit

N4502 proposes a detection toolkit for inclusion into the C++17 standard library that eventually made it into the library fundamentals TS v2. It most likely won't ever get into the standard because it has been subsumed by requires expressions since, but it still solves the problem in a somewhat elegant manner. The toolkit introduces some metafunctions, including std::is_detected which can be used to easily write type or function detection metafunctions on the top of it. Here is how you could use it:

template<typename T>
using toString_t = decltype( std::declval<T&>().toString() );

template<typename T>
constexpr bool has_toString = std::is_detected_v<toString_t, T>;

Note that the example above is untested. The detection toolkit is not available in standard libraries yet but the proposal contains a full implementation that you can easily copy if you really need it. It plays nice with the C++17 feature if constexpr:

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

C++14 - Boost.Hana

Boost.Hana apparently builds upon this specific example and provides a solution for C++14 in its documentation, so I'm going to quote it directly:

[...] Hana provides a is_valid function that can be combined with C++14 generic lambdas to obtain a much cleaner implementation of the same thing:

auto has_toString = hana::is_valid([](auto&& obj) -> decltype(obj.toString()) { });

This leaves us with a function object has_toString which returns whether the given expression is valid on the argument we pass to it. The result is returned as an IntegralConstant, so constexpr-ness is not an issue here because the result of the function is represented as a type anyway. Now, in addition to being less verbose (that's a one liner!), the intent is much clearer. Other benefits are the fact that has_toString can be passed to higher order algorithms and it can also be defined at function scope, so there is no need to pollute the namespace scope with implementation details.

Boost.TTI

Another somewhat idiomatic toolkit to perform such a check - even though less elegant - is Boost.TTI, introduced in Boost 1.54.0. For your example, you would have to use the macro BOOST_TTI_HAS_MEMBER_FUNCTION. Here is how you could use it:

#include <boost/tti/has_member_function.hpp>

// Generate the metafunction
BOOST_TTI_HAS_MEMBER_FUNCTION(toString)

// Check whether T has a member function toString
// which takes no parameter and returns a std::string
constexpr bool foo = has_member_function_toString<T, std::string>::value;

Then, you could use the bool to create a SFINAE check.

Explanation

The macro BOOST_TTI_HAS_MEMBER_FUNCTION generates the metafunction has_member_function_toString which takes the checked type as its first template parameter. The second template parameter corresponds to the return type of the member function, and the following parameters correspond to the types of the function's parameters. The member value contains true if the class T has a member function std::string toString().

Alternatively, has_member_function_toString can take a member function pointer as a template parameter. Therefore, it is possible to replace has_member_function_toString<T, std::string>::value by has_member_function_toString<std::string T::* ()>::value.

Yes, with SFINAE you can check if a given class does provide a certain method. Here's the working code:

#include <iostream>

struct Hello
{
    int helloworld() { return 0; }
};

struct Generic {};    

// SFINAE test
template <typename T>
class has_helloworld
{
    typedef char one;
    struct two { char x[2]; };

    template <typename C> static one test( decltype(&C::helloworld) ) ;
    template <typename C> static two test(...);    

public:
    enum { value = sizeof(test<T>(0)) == sizeof(char) };
};
    
int main(int argc, char *argv[])
{
    std::cout << has_helloworld<Hello>::value << std::endl;
    std::cout << has_helloworld<Generic>::value << std::endl;
    return 0;
}

I've just tested it with Linux and gcc 4.1/4.3. I don't know if it's portable to other platforms running different compilers.

This question is old, but with C++11 we got a new way to check for a functions existence (or existence of any non-type member, really), relying on SFINAE again:

template<class T>
auto serialize_imp(std::ostream& os, T const& obj, int)
    -> decltype(os << obj, void())
{
  os << obj;
}

template<class T>
auto serialize_imp(std::ostream& os, T const& obj, long)
    -> decltype(obj.stream(os), void())
{
  obj.stream(os);
}

template<class T>
auto serialize(std::ostream& os, T const& obj)
    -> decltype(serialize_imp(os, obj, 0), void())
{
  serialize_imp(os, obj, 0);
}

Now onto some explanations. First thing, I use expression SFINAE to exclude the serialize(_imp) functions from overload resolution, if the first expression inside decltype isn't valid (aka, the function doesn't exist).

The void() is used to make the return type of all those functions void.

The 0 argument is used to prefer the os << obj overload if both are available (literal 0 is of type int and as such the first overload is a better match).

---

Now, you probably want a trait to check if a function exists. Luckily, it's easy to write that. Note, though, that you need to write a trait yourself for every different function name you might want.

#include <type_traits>

template<class>
struct sfinae_true : std::true_type{};

namespace detail{
  template<class T, class A0>
  static auto test_stream(int)
      -> sfinae_true<decltype(std::declval<T>().stream(std::declval<A0>()))>;
  template<class, class A0>
  static auto test_stream(long) -> std::false_type;
} // detail::

template<class T, class Arg>
struct has_stream : decltype(detail::test_stream<T, Arg>(0)){};

Live example.

And on to explanations. First, sfinae_true is a helper type, and it basically amounts to the same as writing decltype(void(std::declval<T>().stream(a0)), std::true_type{}). The advantage is simply that it's shorter.
Next, the struct has_stream : decltype(...) inherits from either std::true_type or std::false_type in the end, depending on whether the decltype check in test_stream fails or not.
Last, std::declval gives you a "value" of whatever type you pass, without you needing to know how you can construct it. Note that this is only possible inside an unevaluated context, such as decltype, sizeof and others.

---

Note that decltype is not necessarily needed, as sizeof (and all unevaluated contexts) got that enhancement. It's just that decltype already delivers a type and as such is just cleaner. Here's a sizeof version of one of the overloads:

template<class T>
void serialize_imp(std::ostream& os, T const& obj, int,
    int(*)[sizeof((os << obj),0)] = 0)
{
  os << obj;
}

The int and long parameters are still there for the same reason. The array pointer is used to provide a context where sizeof can be used.

C++ allows SFINAE to be used for this (notice that with C++11 features this is simplier because it supports extended SFINAE on nearly arbitrary expressions - the below was crafted to work with common C++03 compilers):

#define HAS_MEM_FUNC(func, name)                                        \
    template<typename T, typename Sign>                                 \
    struct name {                                                       \
        typedef char yes[1];                                            \
        typedef char no [2];                                            \
        template <typename U, U> struct type_check;                     \
        template <typename _1> static yes &chk(type_check<Sign, &_1::func > *); \
        template <typename   > static no  &chk(...);                    \
        static bool const value = sizeof(chk<T>(0)) == sizeof(yes);     \
    }

The above template and macro tries to instantiate a template, giving it a member function pointer type, and the actual member function pointer. If the types do not fit, SFINAE causes the template to be ignored. Usage like this:

HAS_MEM_FUNC(toString, has_to_string);

template<typename T> void
doSomething() {
   if(has_to_string<T, std::string(T::*)()>::value) {
      ...
   } else {
      ...
   }
}

But note that you cannot just call that toString function in that if branch. Since the compiler will check for validity in both branches, that would fail for cases the function doesn't exist. One way is to use SFINAE once again (enable_if can be obtained from boost, too):

template<bool C, typename T = void>
struct enable_if {
  typedef T type;
};

template<typename T>
struct enable_if<false, T> { };

HAS_MEM_FUNC(toString, has_to_string);

template<typename T> 
typename enable_if<has_to_string<T, 
                   std::string(T::*)()>::value, std::string>::type
doSomething(T * t) {
   /* something when T has toString ... */
   return t->toString();
}

template<typename T> 
typename enable_if<!has_to_string<T, 
                   std::string(T::*)()>::value, std::string>::type
doSomething(T * t) {
   /* something when T doesnt have toString ... */
   return "T::toString() does not exist.";
}

Have fun using it. The advantage of it is that it also works for overloaded member functions, and also for const member functions (remember using std::string(T::*)() const as the member function pointer type then!).