emplace_back和push_back区别以及移动构造函数

最新推荐文章于 2025-11-09 18:16:26 发布

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本文通过具体的测试代码，详细对比了C++中容器成员函数emplace_back与push_back在不同参数类型下的行为差异，包括左值、右值及参数列表等场景，并分析了构造、移动构造、拷贝构造及析构的过程。

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先说结论：
结论：
1、emplace_back以参数列表的形式传入时，不论是否有移动构造函数，都是原地构造，只会调用一次构造函数
2、emplace_back以左值对象的形式传入时，不论是否有移动构造函数，都是调用一次拷贝构造函数

3、emplace_back以右值对象（例如move（左值对象），或者就是右值）的形式传入时
a、有移动构造函数，调用一次移动构造
b、没有移动构造函数，调用拷贝构造函数
4、emplace_back以Person(“aaa”, “shandong”, 1991)形式传入时
a、有移动构造函数，构造临时文件移动构造临时文件析构
b、没有移动构造函数，构造临时文件拷贝构造临时文件析构

1、push_back以参数列表的形式传入时
a、有移动构造函数，构造临时文件移动构造临时文件析构
b、没有移动构造函数，构造临时文件拷贝构造临时文件析构
2、push_back以左值对象的形式传入时，不论是否有移动构造函数，都是调用一次拷贝构造函数

3、push_back以右值对象（例如move（左值对象），或者就是右值）的形式传入时
a、有移动构造函数，调用一次移动构造
b、没有移动构造函数，调用拷贝构造函数
4、push_back以Person(“aaa”, “shandong”, 1991)形式传入时
a、有移动构造函数，构造临时文件移动构造临时文件析构
b、没有移动构造函数，构造临时文件拷贝构造临时文件析构

测试代码：

#include <vector>  
#include <string>  
#include <iostream>  
using namespace std;
 
struct Person  
{  
    string name;  
    string country;  
    int year;  
 
    Person(string p_name,  string p_country, int p_year)  
        : name(p_name), country(p_country), year(p_year)  
    {  
         cout << "I am being constructed1.\n";  
    }
    
    Person(const char * p_name)  
        : name(p_name), country("defaule"), year(1)  
    {  
         cout << "I am being constructed2.\n";  
    }
    
    Person(const Person& other)
        : name(other.name), country(other.country), year(other.year)
    {
         cout << "I am being copy constructed.\n";
    }

    Person(Person&& other)  
        : name(other.name), country(other.country), year(other.year)  
    {  
         cout << "I am being moved  constructed.\n";  
    }

    ~Person()
    {
        cout << "I am being destructed.\n";
    }
    Person& operator=(const Person& other);  
};  
 
int main()  
{  
    Person p1("aaa", "shandong", 1991);
    Person p2("aaa", "shandong", 1991);
    Person p3("aaa", "shandong", 1991);
    Person p4("aaa", "shandong", 1991);
    
    vector<Person> vec0;  
    cout << "\nemplace_back0:\n";  
    vec0.emplace_back(p1); //复制构造  p1为左值   若将移动构造函数注释 复制构造
    
    vector<Person> vec1;  
    cout << "\nemplace_back1:\n";  
    vec1.emplace_back(move(p2)); //移动构造  move之后p2当做右值  若将移动构造函数注释 复制构造
    
    vector<Person> vec2;  
    cout << "\nemplace_back2:\n";  
    vec2.emplace_back("aaa", "shandong", 1991); //只有原地构造  若将移动构造函数注释 原地构造
    
    vector<Person> vec3;  
    cout << "\nemplace_back3:\n";  
    vec3.emplace_back("aaa"); //只有原地构造  若将移动构造函数注释 原地构造
    
    vector<Person> vec4;  
    cout << "\nemplace_back4:\n";  
    vec4.emplace_back(Person("aaa", "shandong", 1991)); //构造临时文件 移动构造  临时文件析构 若将移动构造函数注释 构造临时文件 复制构造  临时文件析构
    
    vector<Person> Vec0;  
    cout << "\npush_back0:\n";  
    Vec0.push_back(p3); //复制构造  p3为左值  若将移动构造函数注释 复制构造
    
    vector<Person> Vec1; 
    cout << "\npush_back1:\n"; 
    Vec1.push_back(move(p4));//移动构造  move之后p4当做右值  若将移动构造函数注释 复制构造
    
    vector<Person> Vec2;  
    cout << "\npush_back2:\n";  
    Vec2.push_back(Person("bbb", "jaingsu", 1993)); //构造临时文件 移动构造  临时文件析构 若将移动构造函数注释 构造临时文件 复制构造  临时文件析构
    
    vector<Person> Vec3; 
    cout << "\npush_back3:\n"; 
    Vec3.push_back("ccc");//构造临时文件 移动构造  临时文件析构  若将移动构造函数注释 构造临时文件 复制构造  临时文件析构
    
    vector<Person> Vec4; 
    cout << "\npush_back4:\n"; 
    Vec4.push_back(Person("aaa", "shandong", 1991));//构造临时文件 移动构造  临时文件析构  若将移动构造函数注释 构造临时文件 复制构造  临时文件析构
    cout<<endl;
}

输出如下：

I am being constructed1.
I am being constructed1.
I am being constructed1.
I am being constructed1.

emplace_back0:
I am being copy constructed.

emplace_back1:
I am being moved constructed.

emplace_back2:
I am being constructed1.

emplace_back3:
I am being constructed2.

push_back0:
I am being copy constructed.

push_back1:
I am being moved constructed.

push_back2:
I am being constructed1.
I am being moved constructed.
I am being destructed.

push_back3:
I am being constructed2.
I am being moved constructed.
I am being destructed.

I am being destructed.
I am being destructed.
I am being destructed.
I am being destructed.
I am being destructed.
I am being destructed.
I am being destructed.
I am being destructed.
I am being destructed.
I am being destructed.
I am being destructed.
I am being destructed.

若将移动构造函数注释掉输出如下：
I am being constructed1.
I am being constructed1.
I am being constructed1.
I am being constructed1.

emplace_back0:
I am being copy constructed.

emplace_back1:
I am being copy constructed.

emplace_back2:
I am being constructed1.

emplace_back3:
I am being constructed2.

push_back0:
I am being copy constructed.

push_back1:
I am being copy constructed.

push_back2:
I am being constructed1.
I am being copy constructed.
I am being destructed.

push_back3:
I am being constructed2.
I am being copy constructed.
I am being destructed.