boost.png (6897 bytes)Header boost/utility.hpp

The entire contents of the header <boost/utility.hpp> are in namespace boost.

Contents

Class template result_of

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 new language feature recently 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: You can use boost::tr1_result_of, which is also defined in <boost/utility/result_of.hpp>. Alternatively, you can define the macro BOOST_RESULT_OF_USE_TR1, which causes result_of to use the protocol described above instead of decltype. If you choose to follow the protocol, take care to ensure that the result_type and result<> members accurately represent the return type of operator() given a call expression.

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 declytpe fallback may workaround certain problems at the cost of portability. For example:

This implementation of result_of requires class template partial specialization, the ability to parse function types properly, and support for SFINAE. If result_of is not supported by your compiler, including the header boost/utility/result_of.hpp will define the macro BOOST_NO_RESULT_OF.

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

Usage guidelines for boost::result_of

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

  1. If you are targeting C++11 and are not concerned about portability to non-compliant compilers or previous versions of the standard, then use std::result_of. If std::result_of meets your needs, then there's no reason to stop using it.
  2. If you are targeting C++11 but may port your code to legacy compilers at some time in the future, then use boost::result_of with decltype. When decltype is used boost::result_of and std::result_of are usually interchangeable. See the documentation on known differences between boost::result_of and C++11 result_of.
  3. If compiler portability is required, use boost::result_of with the TR1 protocol.

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 &

Usage guidelines for the TR1 result_of protocol

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.

Known differences between boost::result_of and TR1 result_of

 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.

Known differences between boost::result_of and C++11 result_of

When using decltype, boost::result_of implements most of the C++11 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

Macro BOOST_BINARY

The macro BOOST_BINARY is used for the representation of binary literals. It takes as an argument a binary number arranged as an arbitrary amount of 1s and 0s in groupings of length 1 to 8, with groups separated by spaces. The type of the literal yielded is determined by the same rules as those of hex and octal literals (2.13.1p1). By implementation, this macro expands directly to an octal literal during preprocessing, so there is no overhead at runtime and the result is useable in any place that an octal literal would be.

In order to directly support binary literals with suffixes, additional macros of the form BOOST_BINARY_XXX are also provided, where XXX is a standard integer suffix in all capital letters. In addition, LL and ULL suffixes may be used for representing long long and unsigned long long types in compilers which provide them as an extension.

The BOOST_BINARY family of macros resides in the header <boost/utility/binary.hpp> which is automatically included by <boost/utility.hpp>.

Contributed by Matt Calabrese.

Example

void foo( int );

void foo( unsigned long );

void bar()
{
  int value1 = BOOST_BINARY( 100 111000 01 1 110 );

  unsigned long value2 = BOOST_BINARY_UL( 100 001 ); // unsigned long

  long long value3 = BOOST_BINARY_LL( 11 000 ); // long long if supported

  assert(    BOOST_BINARY( 10010 )
          &  BOOST_BINARY( 11000 )
          == BOOST_BINARY( 10000 )
        );

  foo( BOOST_BINARY( 1010 ) ); // calls the first foo

  foo( BOOST_BINARY_LU( 1010 ) ); // calls the second foo
}

Revised  04 September, 2008

© Copyright Beman Dawes 1999-2003.

Distributed under the Boost Software License, Version 1.0. See www.boost.org/LICENSE_1_0.txt