引擎盖下的矢量擦除和插入方法有哪些？

Question

在很低的层面上,这些方法到底在做什么？

因为我注意到了一些奇怪的行为,例如,如果我有一个带元素的向量...

2, 4, 6, 8

我说myVector.erase(myVector.begin() + 1);,它给了我......

2, 6, 8, 8

但是,如果我说myVector.insert(myVector.begin + 1, 3);,它给了我......

2, 3, 6, 8

我没有得到任何重复的元素.如何在引擎盖下实施这些方法？

编辑:代码.

vector<int> v;

v.push_back(2);
v.push_back(4);
v.push_back(6);
v.push_back(8);

v.erase(v.begin() + 1);

cout << v[0] << v[1] << v[2] << v[3];

这给了我 2688

Answer 1

最初,矢量有4个元素

2, 4, 6, 8

所以它的大小等于4,容量(根据例子)也等于4.

声明后

myVector.erase(myVector.begin() + 1);

擦除元素4之后的所有元素向左移动.

2, 6, 8, 8

但现在大小等于3但容量仍然等于4.向量不会释放分配范围中未使用的内存.

声明后再说

myVector.insert(myVector.begin + 1, 3);

所有元素都向右移动以释放插入的poistion

2, 3, 6, 8

现在大小等于4,容量通常等于4.

您可以通过以下方式想象该过程

#include <iostream>

int main() 
{
    int a[] = { 2, 4, 6, 8 };
    size_t size = 4;
    size_t capacity = 4;

    std::cout << "size = " << size << ", capacity = " << capacity << std::endl;
    std::cout << "current state of the array: ";
    for ( size_t i = 0; i < size; i++ ) std::cout << a[i] << ' ';
    std::cout << std::endl;
    std::cout << "The memory occupied by the array: ";
    for ( int x : a ) std::cout << x << ' ';
    std::cout << std::endl;

    //  removing the element with index 1
    size_t i = 1;

    for ( ; i + 1 < size; i++ ) a[i] = a[i+1];
    --size;

    std::cout << "\nsize = " << size << ", capacity = " << capacity << std::endl;
    std::cout << "current state of the array: ";
    for ( size_t i = 0; i < size; i++ ) std::cout << a[i] << ' ';
    std::cout << std::endl;
    std::cout << "The memory occupied by the array: ";
    for ( int x : a ) std::cout << x << ' ';
    std::cout << std::endl;

    //  inserting the element with index 1 and value 3
    i = 1;
    int value = 3;
    size_t j = size;
    for ( ; j != i; j-- ) a[j] = a[j-1];
    a[j] = value;
    ++size;

    std::cout << "\nsize = " << size << ", capacity = " << capacity << std::endl;
    std::cout << "current state of the array: ";
    for ( size_t i = 0; i < size; i++ ) std::cout << a[i] << ' ';
    std::cout << std::endl;
    std::cout << "The memory occupied by the array: ";
    for ( int x : a ) std::cout << x << ' ';
    std::cout << std::endl;
}

程序输出是

size = 4, capacity = 4
current state of the array: 2 4 6 8 
The memory occupied by the array: 2 4 6 8 

size = 3, capacity = 4
current state of the array: 2 6 8 
The memory occupied by the array: 2 6 8 8 

size = 4, capacity = 4
current state of the array: 2 3 6 8 
The memory occupied by the array: 2 3 6 8