Implementation principle of function emplace_back in vector

vector中函数emplace_back的实现原理

在vector中的emplace_back函数, 其效率比push_back高很多!

/*例子中使用的Student类的声明*/
class Student
{
    
private:
    int age;
public:
    Student();
    explicit Student(int age);
    ~Student();
    int getAge();
};

原理分析

push_back函数

vector<Student> team;
team.push(Student(24));

代码运行过程中, 首先是执行Student()创建了一个临时的Student对象, Then use the copy constructor to copy the value of the member variable of this temporary object toteamin the space.

There are two reasons for the slow efficiency:

1. 创建临时Student对象时,需要申请内存空间,Allocating memory space has always been a time-consuming operation
2. The copy operation of the copy constructor is also requiredCPU时间的

emplace_back函数

  vector<Student> team;
  team.emplace_back(24);  

• This code achieves the same result as above,都在team里添加了一个24岁的Student对象.
• 但在执行效率上,emplace_back函数很快

其原理就是emplace_backThe function is directly inteamon the existing space,调用了Student类的构造函数,Saves the memory space application for temporary objects and the copy operation of the copy constructor.

emplace_back实现原理

void* ptr = malloc(sizeof(Student));
new (ptr)Student(100);
cout << ((Student*)ptr)->getAge() << endl;

第1行：主要是分配一个Student对象所需的内存空间,但在vector里,这步不需要考虑,内部会在实现;

第2行：这才是重点,通过这样的语法,就可以对已在的内存空间,调用相应的Student类构造函数进行初始化;

第3行：输出验证结果.