Boost C++ 库

考虑编写一个泛型工厂函数，它返回一个新构造的泛型类型的对象。 像这样的工厂函数对于封装和本地化资源分配非常有价值。 显然，工厂函数必须接受与所构造的对象类型构造函数完全相同的参数集

template<class T> T* factory_new()
{  return new T();  }

template<class T> T* factory_new(a1)
{  return new T(a1);  }

template<class T> T* factory_new(a1, a2)
{  return new T(a1, a2);  }

不幸的是，在 C++03 中，这种方法更大的问题是 N 个参数的情况需要 2^N 个重载，这立即将其作为通用解决方案排除。 幸运的是，大多数构造函数按值、按 const 引用或按右值引用接受参数。 如果接受这些限制，则 N 个参数情况的转发模拟仅需要 N 个重载。 在 BOOST_FWD_REF 和 boost::forward 的帮助下，此库可以轻松实现此模拟

#include <boost/move/utility_core.hpp>
#include <iostream>

class copyable_only_tester
{
   public:
   copyable_only_tester()
   {  std::cout << "copyable_only_tester()" << std::endl;   }

   copyable_only_tester(const copyable_only_tester&)
   {  std::cout << "copyable_only_tester(const copyable_only_tester&)" << std::endl;   }

   copyable_only_tester(int)
   {  std::cout << "copyable_only_tester(int)" << std::endl;   }

   copyable_only_tester(int, double)
   {  std::cout << "copyable_only_tester(int, double)" << std::endl;   }
};

class copyable_movable_tester
{
   // move semantics
   BOOST_COPYABLE_AND_MOVABLE(copyable_movable_tester)
   public:

   copyable_movable_tester()
   {  std::cout << "copyable_movable_tester()" << std::endl;   }

   copyable_movable_tester(int)
   {  std::cout << "copyable_movable_tester(int)" << std::endl;   }

   copyable_movable_tester(BOOST_RV_REF(copyable_movable_tester))
   {  std::cout << "copyable_movable_tester(BOOST_RV_REF(copyable_movable_tester))" << std::endl;   }

   copyable_movable_tester(const copyable_movable_tester &)
   {  std::cout << "copyable_movable_tester(const copyable_movable_tester &)" << std::endl;   }

   copyable_movable_tester(BOOST_RV_REF(copyable_movable_tester), BOOST_RV_REF(copyable_movable_tester))
   {  std::cout << "copyable_movable_tester(BOOST_RV_REF(copyable_movable_tester), BOOST_RV_REF(copyable_movable_tester))" << std::endl;   }

   copyable_movable_tester &operator=(BOOST_RV_REF(copyable_movable_tester))
   {  std::cout << "copyable_movable_tester & operator=(BOOST_RV_REF(copyable_movable_tester))" << std::endl;
      return *this;  }

   copyable_movable_tester &operator=(BOOST_COPY_ASSIGN_REF(copyable_movable_tester))
   {  std::cout << "copyable_movable_tester & operator=(BOOST_COPY_ASSIGN_REF(copyable_movable_tester))" << std::endl;
      return *this;  }
};

//1 argument
template<class MaybeMovable, class MaybeRv>
void function_construct(BOOST_FWD_REF(MaybeRv) x)
{  MaybeMovable m(boost::forward<MaybeRv>(x));   }

//2 argument
template<class MaybeMovable, class MaybeRv, class MaybeRv2>
void function_construct(BOOST_FWD_REF(MaybeRv) x, BOOST_FWD_REF(MaybeRv2) x2)
{  MaybeMovable m(boost::forward<MaybeRv>(x), boost::forward<MaybeRv2>(x2));  }

int main()
{
   copyable_movable_tester m;
   //move constructor
   function_construct<copyable_movable_tester>(boost::move(m));
   //copy constructor
   function_construct<copyable_movable_tester>(copyable_movable_tester());
   //two rvalue constructor
   function_construct<copyable_movable_tester>(boost::move(m), boost::move(m));

   copyable_only_tester nm;
   //copy constructor (copyable_only_tester has no move ctor.)
   function_construct<copyable_only_tester>(boost::move(nm));
   //copy constructor
   function_construct<copyable_only_tester>(nm);
   //int constructor
   function_construct<copyable_only_tester>(int(0));
   //int, double constructor
   function_construct<copyable_only_tester>(int(0), double(0.0));

   //Output is:
   //copyable_movable_tester()
   //copyable_movable_tester(BOOST_RV_REF(copyable_movable_tester))
   //copyable_movable_tester()
   //copyable_movable_tester(const copyable_movable_tester &)
   //copyable_movable_tester(BOOST_RV_REF(copyable_movable_tester), BOOST_RV_REF(copyable_movable_tester))
   //copyable_only_tester()
   //copyable_only_tester(const copyable_only_tester&)
   //copyable_only_tester(const copyable_only_tester&)
   //copyable_only_tester(int)
   //copyable_only_tester(int, double)
   return 0;
}

如果实现者接受这种类型的转发对于 C++03 编译器的限制，那么构造函数转发可以方便地使用 N 个重载在容器中实现放置插入。 在具有右值引用的编译器中，可以实现完美转发。