原完整教程链接：6.16 An introduction to std::vector

In the previous lesson, we introduced std::array, which provides the functionality of C++’s built-in fixed arrays in a safer and more usable form.

Analogously, the C++ standard library provides functionality that makes working with dynamic arrays safer and easier. This functionality is named std::vector.

Unlike std::array, which closely follows the basic functionality of fixed arrays, std::vector comes with some additional tricks up its sleeves. These help make std::vector one of the most useful and versatile tools to have in your C++ toolkit.

1.Declaring a std::vector is simple:

#include <vector>
 
// no need to specify length at initialization
std::vector<int> array; 
std::vector<int> array2 = { 9, 7, 5, 3, 1 }; // use initializer list to initialize array
std::vector<int> array3 { 9, 7, 5, 3, 1 }; // use uniform initialization to initialize array (C++11 onward)

2.As of C++11, you can also assign values to a std::vector using an initializer-list:

array = { 0, 1, 2, 3, 4 }; // okay, array length is now 5
array = { 9, 8, 7 }; // okay, array length is now 3

3.Vectors remember their length

Unlike built-in dynamic arrays, which don’t know the length of the array they are pointing to, std::vector keeps track of its length. We can ask for the vector’s length via the size() function:

#include <vector>
#include <iostream>
 
int main()
{
    std::vector<int> array { 9, 7, 5, 3, 1 };
    std::cout << "The length is: " << array.size() << '\n';
 
    return 0;
}

4.Resizing an array

Resizing a built-in dynamically allocated array is complicated. Resizing a std::vector is as simple as calling the resize() function:

(1)

#include <vector>
#include <iostream>
 
int main()
{
    std::vector<int> array { 0, 1, 2 };
    array.resize(5); // set size to 5
 
    std::cout << "The length is: " << array.size() << '\n';
 
    for (auto const &element: array)
        std::cout << element << ' ';
 
    return 0;
}

This prints:

The length is: 5
0 1 2 0 0

(2)

#include <vector>
#include <iostream>
 
int main()
{
    std::vector<int> array { 0, 1, 2, 3, 4 };
    array.resize(3); // set length to 3
 
    std::cout << "The length is: " << array.size() << '\n';
 
    for (auto const &element: array)
        std::cout << element << ' ';
 
    return 0;
}

This prints:

The length is: 3
0 1 2