Clear-and-minimize

More C++ Idioms Clear-and-minimize

11.0.2 Also Known As

This is sometimes called the swap with temporary idiom.

NOTE:

1、显然，它依赖于temporary、swap

2、显然，"swap with temporary idiom"是更加形象的

11.0.4 Solution and Sample Code

#include <vector>
#include <iostream>
int main()
{
    std::vector <int> v;
    for(int i=0;i<100;i++)
    {
        v.push_back(i);
    }
    v.clear();
    std::cout<<"size of v:"<<v.size()<<std::endl;
    std::cout<<"capacity of v:"<<v.capacity()<<std::endl;
    std::vector<int>().swap (v);
    std::cout<<"size of v:"<<v.size()<<std::endl;
    std::cout<<"capacity of v:"<<v.capacity()<<std::endl;
    return 0;
}

NOTE:

1、输出如下：

size of v:0
capacity of v:128
size of v:0
capacity of v:0

2、下面是精髓所在:

std::vector<int>().swap (v);

11.0.5 Solution in C++14

Since C++11, some containers declare the function shrink_to_fit(), e.g. vector , deque, basic_string. shrink_to_fit() which is a non-binding request to reduce capacity() to size(). Thus, using clear() and shrink_to_fit() is a non-binding request to clear-and-minimize.

NOTE: vector在C++11中已经支持shrink_to_fit()

stackoverflow How do I clear the std::queue efficiently?

A

A common idiom for clearing standard containers is swapping with an empty version of the container:

void clear( std::queue<int> &q )
{
   std::queue<int> empty;
   std::swap( q, empty );
}

It is also the only way of actually clearing the memory held inside some containers (std::vector)