More C++ Idioms/Boost mutant

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Boost mutant
[edit | edit source]

Intent[edit | edit source]

Reorder the data members of a plain old data (POD) type when all members are of the same type, without physically reorganizing or copying the data items.

Also Known As[edit | edit source]

Motivation[edit | edit source]

The need of this idiom is best motivated using Boost's Bimap.[1] Boost.Bimap is a bidirectional maps library for C++. In bimap<X,Y>, values of types X and Y can both serve as keys. The implementation of such a data structure can be optimized using the boost mutant idiom.

Solution and Sample Code[edit | edit source]

Boost mutant idiom makes use of reinterpret_cast and depends heavily on the assumption that the memory layouts of two different structures with identical data members (types and order) are interchangeable. Although the C++ standard does not guarantee this property, virtually all the compilers satisfy it. Moreover, the mutant idiom is standard if only POD types are used.[2] The following example shows how the boost mutant idiom works.

template <class Pair>
struct Reverse
{
    typedef typename Pair::first_type  second_type;
    typedef typename Pair::second_type first_type;
    second_type second;
    first_type first;
};

template <class Pair>
Reverse<Pair> & mutate(Pair & p)
{
  return reinterpret_cast<Reverse<Pair> &>(p);
}

int main(void)
{
  std::pair<double, int> p(1.34, 5);
  std::cout << "p.first = " << p.first 
            << ", p.second = "  << p.second << std::endl
            << "mutate(p).first = " << mutate(p).first 
            << ", mutate(p).second = "  << mutate(p).second << std::endl;
}

Given a std::pair<X,Y> object of POD data members only, the layout of the Reverse<std::pair<X,Y>> is identical to that of pair's on most compilers. The Reverse template reverses the names of the data members without physically reversing the data. A helper mutate function is used to easily construct a Reverse<Pair> reference, which can be considered as a view over the original pair object. The output of the above program confirms that the reverse view can be obtained without reorganizing data:

p.first = 1.34, p.second = 5
mutate(p).first = 5, mutate(p).second = 1.34

Known Uses[edit | edit source]

Related Idioms[edit | edit source]

References[edit | edit source]

  1. Capeletto, Matias. "Boost.Bimap". http://www.boost.org/doc/libs/1_43_0/libs/bimap/doc/html/index.html. 
  2. http://beta.boost.org/doc/libs/1_43_0/libs/bimap/test/test_mutant.cpp