Boost C++ Libraries

The class template result_of helps determine the type of a call expression. For example, given an lvalue f of type F and lvalues t1,t2, ..., tN of types T1, T2, ..., TN, respectively, the type result_of<F(T1, T2, ..., TN)>::type defines the result type of the expression f(t1, t2, ...,tN).

This implementation permits the type F to be a function pointer, function reference, member function pointer, or class type. By default, N may be any value between 0 and 16. To change the upper limit, define the macro BOOST_RESULT_OF_NUM_ARGS to the maximum value for N. Class template result_of resides in the header <boost/utility/result_of.hpp>.

If your compiler's support for decltype is adequate, result_of automatically uses it to deduce the type of the call expression, in which case result_of<F(T1, T2, ..., TN)>::type names the type decltype(boost::declval<F>()(boost::declval<T1>(), boost::declval<T2>(), ..., boost::declval<TN>())), as in the following example.

struct functor {
    template<class T>
    T operator()(T x)
    {
        return x;
    }
};

typedef boost::result_of<functor(int)>::type type; // type is int

You can test whether result_of is using decltype by checking if the macro BOOST_RESULT_OF_USE_DECLTYPE is defined after including result_of.hpp. You can also force result_of to use decltype by defining BOOST_RESULT_OF_USE_DECLTYPE prior to including result_of.hpp.

If decltype is not used, then automatic result type deduction of function objects is not possible. Instead, result_of uses the following protocol to allow the programmer to specify a type. When F is a class type with a member type result_type, result_of<F(T1, T2, ..., TN)>::type is F::result_type. When F does not contain result_type, result_of<F(T1, T2, ..., TN)>::type is F::result<F(T1, T2, ..., TN)>::type when N > 0 or void when N = 0.

Note that it is the responsibility of the programmer to ensure that function objects accurately advertise their result type via this protocol, as in the following example.

struct functor {
    template <class> struct result;

    template<class F, class T>
    struct result<F(T)> {
        typedef T type;
    };

    template<class T>
    T operator()(T x)
    {
        return x;
    }
};

typedef boost::result_of<functor(int)>::type type; // type is int

Since decltype is a language feature standardized in C++11, if you are writing a function object to be used with result_of, for maximum portability, you might consider following the above protocol even if your compiler has proper decltype support.

If you wish to continue to use the protocol on compilers that support decltype, there are two options:

Additionally, boost::result_of provides a third mode of operation, which some users may find convenient. When BOOST_RESULT_OF_USE_TR1_WITH_DECLTYPE_FALLBACK is defined, boost::result_of behaves as follows. If the function object has a member type result_type or member template result<>, then boost::result_of will use the TR1 protocol.

Otherwise, boost::result_of will use decltype. Using TR1 with a decltype fallback may workaround certain problems at the cost of portability. For example:

For additional information about result_of, see the C++ Library Technical Report, N1836, or, for motivation and design rationale, the result_of proposal.

The following are general suggestions about when and how to use boost::result_of.

Regardless of how you configure boost::result_of, it is important to bear in mind that the return type of a function may change depending on its arguments, and additionally, the return type of a member function may change depending on the cv-qualification of the object. boost::result_of must be passed the appropriately cv-qualified types in order to deduce the corresponding return type.

For example:

struct functor {
    int& operator()(int);
    int const& operator()(int) const;

    float& operator()(float&);
    float const& operator()(float const&);
};

typedef boost::result_of<
    functor(int)
>::type type1; // type1 is int &

typedef boost::result_of<
    const functor(int)
>::type type2; // type2 is int const &

typedef boost::result_of<
    functor(float&)
>::type type3; // type3 is float &

typedef boost::result_of<
    functor(float const&)
>::type type4; // type4 is float const &

On compliant C++11 compilers, boost::result_of can use decltype to deduce the type of any call expression, including calls to function objects. However, on pre-C++11 compilers or on compilers without adequate decltype support, additional scaffolding is needed from function objects as described above. The following are suggestions about how to use the TR1 protocol.

struct functor {
    typedef int result_type;
    result_type operator()(int);
};

struct functor {
    template<class> struct result;

    template<class F>
    struct result<F(int)> {
        typedef int& type;
    };
    result<functor(int)>::type operator()(int);

    template<class F>
    struct result<const F(int)> {
        typedef int const& type;
    };
    result<const functor(int)>::type operator()(int) const;
};

struct functor {
    template<class> struct result;

    template<class F, class T>
    struct result<F(T)>
        : boost::remove_cv<
              typename boost::remove_reference<T>::type
          >
    {};

    template<class T>
    T operator()(T const&amp; x) const;
};

When using decltype, boost::result_of ignores the TR1 protocol and instead deduces the return type of function objects directly via decltype. In most situations, users will not notice a difference, so long as they use the protocol correctly. The following are situations in which the type deduced by boost::result_of is known to differ depending on whether decltype or the TR1 protocol is used.

TR1 protocol misusage: When using the TR1 protocol, boost::result_of cannot detect whether the actual type of a call to a function object is the same as the type specified by the protocol, which allows for the possibility of inadvertent mismatches between the specified type and the actual type. When using decltype, these subtle bugs may result in compilation errors. For example:

struct functor {
   typedef short result_type;
   int operator()(short);
};

#ifdef BOOST_RESULT_OF_USE_DECLTYPE

BOOST_STATIC_ASSERT((
   boost::is_same<boost::result_of<functor(short)>::type, int>::value
));

#else

BOOST_STATIC_ASSERT((
   boost::is_same<boost::result_of<functor(short)>::type, short>::value
));

#endif

Note that the user can force boost::result_of to use the TR1 protocol even on platforms that support decltype by defining BOOST_RESULT_OF_USE_TR1.

Nullary function objects: When using the TR1 protocol, boost::result_of cannot always deduce the type of calls to nullary function objects, in which case the type defaults to void. When using decltype, boost::result_of always gives the actual type of the call expression. For example:

struct functor {
   template<class> struct result {
       typedef int type;
   };
   int operator()();
};

#ifdef BOOST_RESULT_OF_USE_DECLTYPE

BOOST_STATIC_ASSERT((
   boost::is_same<boost::result_of<functor()>::type, int>::value
));

#else

BOOST_STATIC_ASSERT((
   boost::is_same<boost::result_of<functor()>::type, void>::value
));

#endif

Note that there are some workarounds for the nullary function problem. So long as the return type does not vary, result_type can always be used to specify the return type regardless of arity. If the return type does vary, then the user can specialize boost::result_of itself for nullary calls.

Non-class prvalues and cv-qualification: When using the TR1 protocol, boost::result_of will report the cv-qualified type specified by result_type or the result template regardless of the actual cv-qualification of the call expression. When using decltype, boost::result_of will report the actual type of the call expression, which is not cv-qualified when the expression is a non-class prvalue. For example:

struct functor {
   template<class> struct result;
   template<class F, class T> struct result<F(const T)> {
       typedef const T type;
   };

   const short operator()(const short);
   int const & operator()(int const &);
};

// Non-prvalue call expressions work the same with or without decltype.

BOOST_STATIC_ASSERT((
   boost::is_same<
       boost::result_of<functor(int const &)>::type,
       int const &
::value
));

// Non-class prvalue call expressions are not actually cv-qualified,
// but only the decltype-based result_of reports this accurately.

#ifdef BOOST_RESULT_OF_USE_DECLTYPE

BOOST_STATIC_ASSERT((
   boost::is_same<
       boost::result_of<functor(const short)>::type,
       short
::value
));

#else

BOOST_STATIC_ASSERT((
   boost::is_same<
       boost::result_of<functor(const short)>::type,
       const short
::value
));

#endif

When using decltype, boost::result_of implements most of the C++11 std::result_of specification. One known exception is that boost::result_of does not implement the requirements regarding pointers to member data.

Created by Doug Gregor. Contributions from Daniel Walker, Eric Niebler, Michel Morin and others.