Return Type Resolver

1、典型的template user define conversion function

2、使用案例 microsoft/GSL/pointers class not_null

More C++ Idioms/Return Type Resolver

NOTE: 提供**templatized conversion operator function**，充分运用compiler自动地调用**conversion operator function**来讲将**temporary object**转换为目标变量（即等号左侧的变量）从而达到简化代码的目的

Motivation

#include<list>
#include<vector>
#include <stdlib.h>     /* srand, rand */
template <class Container>
Container getRandomN(std::size_t n) 
{
  Container c;
  for(size_t i = 0;i < n; ++i)
    c.insert(c.end(), rand());
  return c;
}

int main (void)
{
   std::list<int> l = getRandomN<std::list<int> > (10);
   std::vector<long> v = getRandomN<std::vector<long> > (100);
}

Solution and Sample Code

Return type resolver idiom makes use of a proxy class and templatized conversion operator functions in the class. getRandomN function above can be implemented with a class and a member conversion function as follows.

#include<list>
#include<vector>
#include<set>
#include <stdlib.h>     /* srand, rand */

class getRandomN
{
    std::size_t count;

public:
    getRandomN(int n = 1) :
                    count(n)
    {
    }

    template<class Container>
    operator Container()
    {
        Container c;
        for (size_t i = 0; i < count; ++i)
            c.insert(c.end(), rand()); // push_back is not supported by all standard containers.
        return c;
    }
};

int main()
{
    std::set<int> random_s = getRandomN(10);
    std::vector<int> random_v = getRandomN(10);
    std::list<int> random_l = getRandomN(10);
}
// g++ test.cpp --std=c++03

getRandomN class has a constructor and a templatized conversion operator function. For initialization, a temporary object of getRandomN class is created and assigned to the desired container class. C++ compiler attempts to convert the temporary object into the container class object. The only way to do that is via the conversion operator. The conversion operator is instantiated with the type of the container that is being populated. Due to automatic resolution of the return type, the user does not have to spell it out again. Note that insert member function has been used instead of push_back because std::set does not support push_back.

Known Uses

The nullptr idiom makes use of the return type resolver idiom to automatically deduce a null pointer of the correct type depending upon the pointer variable it is assigning to.

补充样例

1、microsoft/GSL/pointers not_null