Chapter 28. Boost.CompressedPair

Boost.CompressedPair provides boost::compressed_pair, a class that behaves like std::pair. However, if one or both template parameters are empty classes, boost::compressed_pair consumes less memory. boost::compressed_pair uses a technique known as empty base class optimization.

To use boost::compressed_pair, include the header file boost/compressed_pair.hpp.

Example 28.1. Reduced memory requirements with boost::compressed_pair

  1. #include <boost/compressed_pair.hpp>
  2. #include <utility>
  3. #include <iostream>
  5. struct empty {};
  7. int main()
  8. {
  9.   std::pair<int, empty> p;
  10.   std::cout << sizeof(p) << '\n';
  12.   boost::compressed_pair<int, empty> cp;
  13.   std::cout << sizeof(cp) << '\n';
  14. }

Example 28.1 illustrates this by using boost::compressedpair for _cp and std::pair for p. When compiled using Visual C++ 2013 and run on a 64-bit Windows 7 system, the example returns 4 for sizeof(cp) and 8 for sizeof(p).

Please note that there is another difference between boost::compressed_pair and std::pair: the values stored in boost::compressed_pair are accessed through the member functions first() and second(). std::pair uses two identically named member variables instead.