調整大小後的矢量容量

Question

在下面的程序中，我只創建一個空矢量，然後調整爲5個elelemnts，然後調整爲8個元素。

但是，在將其大小調整爲8個元素後，容量顯示爲10。上一次調整大小後，爲什麼容量10而不是8？

以下是代碼。我已經評論了問題出在哪裏。

//This program has vector method resize() 
//2015-12-05 Sat 12:06 AM 
using namespace std; 
#include<iostream> 
#include<vector> 

int main() 
{ 
    vector<int> vec1; 
    //Initial empty vector 
    cout << "Printing what an empty vector has..." << endl; 
    cout << "Size: " << vec1.size() << endl; 
    cout << "Capacity: " << vec1.capacity() << endl;  
    cout << endl << endl; 

    //Resize to 5, without giving any value. 
    cout << "Printing after resizing to 5 elements..." << endl; 
    vec1.resize(5); 
    cout << "Size: " << vec1.size() << endl; 
    cout << "Capacity: " << vec1.capacity() << endl; 
    cout << endl << endl; 

    //Resize to 8, with value also this time 
    //ISSUE HERE, WHY IS CAPACITY PRINTED AS '10', instead of '8' 
    cout << "Printing after resizing to 8, and also giving values..." << endl; 
    vec1.resize(8, 15); 
    cout << "Size: " << vec1.size() << endl; 
    cout << "Capacity: " << vec1.capacity() << endl; 
    cout << endl; 
    return 0; 
} 

下面是輸出：

user $ ./a.out 
Printing what an empty vector has... 
Size: 0 
Capacity: 0 
Elements: 

Printing after resizing to 5 elements... 
Size: 5 
Capacity: 5 
Elements: 0 0 0 0 0 

Printing after resizing to 8, and also giving values... 
Size: 8 
Capacity: 10 
Elements: 0 0 0 0 0 15 15 15 
user $ 

Answer 1

如果請求的大小比當前的容量越大，容量通常加倍，直到它足夠大。這個規模的翻倍是保持O（1）增長的攤銷成本。