C++ – Check if a class has a member function of a given signature

cc++11sfinaetemplates

I'm asking for a template trick to detect if a class has a specific member function of a given signature.

The problem is similar to the one cited here
http://www.gotw.ca/gotw/071.htm
but not the same: in the item of Sutter's book he answered to the question that a class C MUST PROVIDE a member function with a particular signature, else the program won't compile. In my problem I need to do something if a class has that function, else do "something else".

A similar problem was faced by boost::serialization but I don't like the solution they adopted: a template function that invokes by default a free function (that you have to define) with a particular signature unless you define a particular member function (in their case "serialize" that takes 2 parameters of a given type) with a particular signature, else a compile error will happens. That is to implement both intrusive and non-intrusive serialization.

I don't like that solution for two reasons:

  1. To be non intrusive you must override the global "serialize" function that is in boost::serialization namespace, so you have IN YOUR CLIENT CODE to open namespace boost and namespace serialization!
  2. The stack to resolve that
    mess was 10 to 12 function invocations.

I need to define a custom behavior for classes that has not that member function, and my entities are inside different namespaces (and I don't want to override a global function defined in one namespace while I'm in another one)

Can you give me a hint to solve this puzzle?

Best Answer

Here's a possible implementation relying on C++11 features. It correctly detects the function even if it's inherited (unlike the solution in the accepted answer, as Mike Kinghan observes in his answer).

The function this snippet tests for is called serialize:

#include <type_traits>

// Primary template with a static assertion
// for a meaningful error message
// if it ever gets instantiated.
// We could leave it undefined if we didn't care.

template<typename, typename T>
struct has_serialize {
    static_assert(
        std::integral_constant<T, false>::value,
        "Second template parameter needs to be of function type.");
};

// specialization that does the checking

template<typename C, typename Ret, typename... Args>
struct has_serialize<C, Ret(Args...)> {
private:
    template<typename T>
    static constexpr auto check(T*)
    -> typename
        std::is_same<
            decltype( std::declval<T>().serialize( std::declval<Args>()... ) ),
            Ret    // ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
        >::type;  // attempt to call it and see if the return type is correct

    template<typename>
    static constexpr std::false_type check(...);

    typedef decltype(check<C>(0)) type;

public:
    static constexpr bool value = type::value;
};

Usage:

struct X {
     int serialize(const std::string&) { return 42; } 
};

struct Y : X {};

std::cout << has_serialize<Y, int(const std::string&)>::value; // will print 1
Related Topic