C++笔记——emplace_back的用法

概要：

emplace内部实现是多参数模板，保证在插入对象的时候，直接插入构造函数的参数列表。如下例子中，自定义类C的插入

#include <iostream>
#include <thread>
#include <unistd.h> //休眠时间
#include <vector>
using namespace std;

void hello(int n){
    sleep(1);
    cout << "hello concurrent thread " << n << endl;
}

class B;
class A{
public:
    A(){
        cout << "A is construction. " << endl;
    }

    ~A(){
        cout << "A is deconstruction. " << endl;
    }

    //shared_ptr<B> pa;
    weak_ptr<B> pa;
};


class C{
public:
    //构造函数带参数列表
    C(int a, int b, int c): name(a), habby(b), age(c){
        cout << "a = " << a << " b = " << b << " c = " << c << endl;
    }

    //构造函数内为成员参数赋值
/*    C(int a, int b, int c) {
        cout << "a = " << a << " b = " << b << " c = " << c << endl;
        name = a;
        habby = b;
        age = c;
    }*/

    int name;
    int habby;
    int age;
};




class B{
public:
    B(){
        cout << "B is construction. " << endl;
    }

    ~B(){
        cout << "B is deconstruction. " << endl;
    }

    //shared_ptr<A> pb;
    weak_ptr<A> pb;

};

int main(){

/*
 * 、//多线程对象的使用
    thread t(hello, 3);
    cout << "main thread 1. " << endl;


    //sleep(1);
   // t.detach();
      t.join();
  cout << "main thread 2 " << endl;
*/

/*    //shared_ptr的循环引用
    std::shared_ptr<A> sa = std::make_shared<A>();
    std::shared_ptr<B> sb = std::make_shared<B>();
    sa->pa = sb;
    sb->pb = sa;
    std::cout << "B expired = " << sa->pa.expired() << std::endl;
    std::cout << "A expired = " << sb->pb.expired() << std::endl;*/

    //测试emplace_back和push_back
    vector<C> vec;
    vec.emplace_back(1,2,3);//正确
    //vec.push_back(1,3,2);  //报错
    vec.push_back(C(12,13,15));//正确
    vec.emplace_back(C(6,7,8));//正确
    return 0;
}

而push_back 不能直接将多个构造参数加进去。

直接插入对象C(6,7,8)，这个操作调用了拷贝构造函数，整个过程是先生成C(6,7,8)临时对象，然后拷贝到构造函数的形参中，最后释放临时对象。效率上emplace_back更高。

 

注：在内置类型下，两者都是一样的效率。都是直接插入，没有拷贝。