c++ - 理解c++中动态内存分配的使用

Question

考虑这个程序:

#include <iostream>
using namespace std;

int main ()
{
    int i;
    cout << "How many numbers would you like to type? ";
    cin >> i;

    int * p;
    p= new int[i];

    cout << "Enter the numbers: \n";

    for (int n=0; n<i; n++)
      cin >> p[n];

    cout << "You have entered: ";

    for (int n=0; n<i; n++)
      cout << p[n] << ", ";

    delete[] p;

    return 0;
}  

还有这个:

#include <iostream>

using namespace std;

int main()
{
    int num;

    cout << "How many numbers would you like to type? ";
    cin >> num;

    int arr[num];

    cout << "Enter the numbers: \n";

    for (int a = 0; a <num; ++a)
        cin >>arr[a];

    cout << "You have entered: ";  

    for (int a = 0; a <num; ++a)
        cout <<arr[a]<< ", ";
    return 0;
}

两个程序都在完成相同的任务，而且 - 对我来说 - 后者很多
比前者更容易理解。现在我的问题是，为什么我们仍然需要动态内存分配？

Answer 1

当 num 不是编译器时间常量时，int arr[num]; 是 VLA(可变长度数组)，这不是标准 C++。它是一些编译器提供的语言扩展(我假设您使用的是 G++)。

同样易于使用且不需要您使用原始指针和处理手动动态分配的东西是 std::vector :

int i;
cout << "How many numbers would you like to type? ";
if (!(cin >> i)) {   // error checking is important
    cout << "Not a number, abort!\n";
    return 1;
}

std::vector<int> numbers(i);

cout << "Enter the numbers: \n";

for (int a = 0; a < i; ++a)
    cin >> numbers[a];