如果我只能创建一个数组，为什么还需要动态内存分配？

Question

我正在阅读有关动态内存分配和静态内存分配的内容，发现以下有关动态内存分配的内容：

In the programs seen in previous chapters, all memory needs were determined before program execution by defining the variables needed. But there may be cases where the memory needs of a program can only be determined during runtime. For example, when the memory needed depends on user input.

所以我用C++写了下面的程序：

#include <iostream>

int main()
{
  int n = 0;
  int i = 0;

  std::cout << "Enter size: ";
  std::cin >> n;
  int vector[n];

  for (i=0; i<n; i++)
  {
    vector[i] = i;
  }

  return 0;
}

这个程序有效。我不明白它是如何工作的。这里的大小是什么时候确定的？在这种情况下如何分配向量？

Answer 1

根据this（强调我的）：

Variable-length automatic arrays are allowed in ISO C99, and as an extension GCC accepts them in C90 mode and in C++. These arrays are declared like any other automatic arrays, but with a length that is not a constant expression. The storage is allocated at the point of declaration and deallocated when the block scope containing the declaration exits.

请注意，这只是一个扩展，并不适用于所有编译器，例如它不适用于我的 MSVC（我收到错误 "expression must have a constant value"）。

Answer 2

以上代码将在最新版本的 compiler.This 中生成错误代码将在旧版本的 DOSBOX 中运行。

数组的大小必须是常量整数。

所以你可以用两种方式定义它

1.#define Macron

#include<iostream>
#define n 5

main() {
   ...
      ...
      int array[n];
}

2.const 关键词

#include<iostream>
....
main() {
   int x;
   cout << "Enter Size Of Array";
   cin >> x;

   const int n = x;

   int array[n];
   ...
      ...
}

如果我只能创建一个数组，为什么还需要动态内存分配？

Why do I need dynamic memory allocation if I can just create an array?

c++

variable-length-array

static-memory-allocation