c++ - C++ vector 的 size() 和 capacity()

Question

我刚刚开始使用 C++ 的标准库，我首先开始使用的是 std::vector。我对 vector 中的 capacity() 有点困惑。我知道在每次 push_back() 之后， vector 的 capacity 以指数幂变化，但在下面的输出中 capacity 保持不变有时值，即使在插入之后。有人可以向我解释一下内部工作吗？

#include<iostream>
#include<vector>

using namespace std;

int main(){
    vector<int> v;
    int capacity=v.capacity();
    cout<<"Capacity before push_back(): "<<capacity<<endl;
    for(int i=0;i<10;i++){
        v.push_back(i);
        cout<<"Capacity: "<<v.capacity()<<endl;
        
    }
    for(auto j=v.begin();j!=v.end();j++){
        cout<<*j<<endl;
    }
     
    cout<<"Size of vector: "<<v.size()<<endl;
    cout<<"Final Capacity of vector: "<<v.capacity()<<endl;
    
    return 0;
}

输出:

Capacity before push_back(): 0
Capacity: 1
Capacity: 2
Capacity: 4
Capacity: 4
Capacity: 8
Capacity: 8
Capacity: 8
Capacity: 8
Capacity: 16
Capacity: 16
0
1
2
3
4
5
6
7
8
9
Size of vector: 10
Final Capacity of vector: 16

Answer 1

I know that after each push_back() the capacity of the vector changes in exponential powers but in the above OUTPUT the capacity is still remaining same sometimes even after insertion.

当capacity大于插入后的size时，capacity不需要，保证不变。

此策略允许顺序推回具有恒定的复杂性(摊销)。