google abseil c++ Tip of the Week #65: Putting Things in their Place 把对象放入容器的方式

传统方法

push_back

1. 参数是内含类的构造方法的参数时,用这些参数调用那个构造方法构造一个临时对象,调用拷贝构造函数,参数是临时对象
2. 参数是内含类的临时对象时(一个在push_back参数里构造的对象),对象自己会调用一次构造,一次析构,函数会调用一次拷贝构造;

开销

1.一个临时对象的构造
2.一个临时对象的析构
3.一个容器内对象的拷贝构造

新方法

emplace``emplace_back

1. 参数是内含类的构造方法的参数时,用这些参数在容器内部应该存放的位置直接且只调用一次构造.
2. 参数是内含类的临时对象时(一个在push_back参数里构造的对象),对象自己会调用一次构造,一次析构,函数会调用一次拷贝构造;

总结

emplace emplace_back 在比较新的c++已经会判断是否去调用push_back了,所以基本上都用emplace即可.

测试代码

#include 
#include 
#include 
#include 
#include 

using namespace std;

class Point2D{
public:
    Point2D(int a,int b){
        std::cout<<"construct"<<std::endl;
        x=a;
        y=b;
    }
    Point2D(const Point2D& another){
        std::cout<<"copy construct"<<std::endl;
        x=another.x;
        y=another.y;
    }
    ~Point2D(){
        std::cout<<"destruct"<<std::endl;
    }
    int x;
    int y;
};


int main()
{
    std::vector<Point2D> vec;
    vec.reserve(5);
    std::cout<<"________________________vec.push_back({1,2});_________________________"<<std::endl;
    vec.push_back({1,2});
    std::cout<<"________________________vec.push_back(Point2D(1,2));_________________________"<<std::endl;
    vec.push_back(Point2D(1,2));
    std::cout<<"________________________vec.emplace_back(1,2);_________________________"<<std::endl;
    vec.emplace_back(1,2);
    std::cout<<"________________________vec.emplace_back(Point2D(1,2));_________________________"<<std::endl;
    vec.emplace_back(Point2D(1,2));
    std::cout<<"vec.size="<<vec.size()<<std::endl;
    return 0;
}
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输出

________________________vec.push_back({1,2});_________________________
construct
copy construct
destruct
________________________vec.push_back(Point2D(1,2));_________________________
construct
copy construct
destruct
________________________vec.emplace_back(1,2);_________________________
construct
________________________vec.emplace_back(Point2D(1,2));_________________________
construct
copy construct
destruct
vec.size=4
destruct
destruct
destruct
destruct

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