vector 中的匿名构造函数和析构函数调用

Question

我对下面的程序及其输出有点困惑。

有人可以澄清一下吗？

#include <iostream>
#include <vector>

 using namespace std;
class sample
{
   public:
    int a;
    sample( ){ cout<<" Am in Default Cons "<< a << "\n"; }
    sample( int b ){ a=b; cout<<" Am in Cons "<< a << "\n"; }
    sample(const sample& s){ a=s.a; cout<<" Am in copy " <<s.a<<"\n"; }
    ~sample(){ cout<<" Am in Des "<<a <<"\n"; }

};

int main()
{
    vector < sample > Object;
    {
     Object.push_back(  sample( 1 ));
     Object.push_back(  sample( 2 ));
     cout<<" End of loop \n";
    }
    cout<<" \n End of Program \n";
    return 0; 
}

输出为

 Am in Cons 1 // Understood this is for object 1 creation                                                                                                                                         
 Am in copy 1 // Understood this is for object 1 copy to vector 
 Am in Des 1 // Understood this is for object 1 destruction.
 Am in Cons 2  // Understood this is for object 2 creation                                                                                                                                                 
 Am in copy 2 // Understood this is for object 2 copy to vector                                                                                                                                              
 Am in copy 1 // **Don't understand this COPY CALL**                                                                                                                                              
 Am in Des 1  // **Don't understand this DESTRUCTOR CALL**                                                                                                                                               
 Am in Des 2 // Understood this is for object 2 destruction.                                                                                                                                            
 End of loop                                                                                                                                               
 End of Program                                                                                                                                            
 Am in Des 1 //// Understood this is for object 1 destruction in vector .                                                                                                                                         
 Am in Des 2 //// Understood this is for object 2 destruction in vector .

Answer 1

std::vector 是一个动态数组。当您添加一个元素并且其内部缓冲区的容量已满时，vector 需要：

分配新的内存缓冲区，
copy/move 个元素从旧缓冲区到新缓冲区，
销毁原始缓冲区中的元素，
取消分配原始缓冲区，
copy/move 在新缓冲区的末尾添加了元素。

（不一定是这个顺序。）

Step 2.涉及到你不懂的copy constructor。第3步涉及你不懂的析构函数

为防止这种（低效的）行为，请在插入元素之前使用 std::vector::reserve（如果可以的话）。然后不会重新分配，也不会隐藏 copy/move 元素。

vector 中的匿名构造函数和析构函数调用

Anonymous constructor and destructor call in vector

c++

stdvector