C++ 为什么我不能创建一个大小为 n 的数组？

Why can't I create an array of size n?

提问by Mihran Hovsepyan

Possible Duplicate:
Why can't I create an array with size determined by a global variable?

可能的重复：
为什么我不能创建一个大小由全局变量决定的数组？

This is definition of simple array with constant size 4, which is stored in stack memory:

这是常量大小为 4 的简单数组的定义，它存储在堆栈内存中：

int array[4];

Now If I want to declare array of dynamic size in stack it seems that I should write this code:

现在，如果我想在堆栈中声明动态大小的数组，似乎应该编写以下代码：

int n;
cin >> n;
int array[n];

But as we know this is not allowed in C++ and instead we can write this one, which will create the array in dynamic memory (i.e. heap):

但是我们知道这在 C++ 中是不允许的，我们可以编写这个，它将在动态内存（即堆）中创建数组：

int n;
cin >> n;
int *array = new int[n];

But this is more slower and (because of using new operator) and requires to call delete [] operator after we finish our work with array.

但这更慢（因为使用 new 运算符）并且需要在我们完成数组工作后调用 delete [] 运算符。

So my question is here:

所以我的问题在这里：

• Why is it that C++ don't allow you to create array of dynamic length in stack memory?

• 为什么 C++ 不允许您在堆栈内存中创建动态长度数组？

回答by Prasoon Saurav

int n;
cin >> n;
int array[n];

This will work if use g++. g++ support VLAs as an extension. However ISO C++ mandates size of an array to be a constant expression i.e the size must be known at compile time.

如果使用 g++，这将起作用。g++ 支持 VLA 作为扩展。然而，ISO C++ 要求数组的大小是一个常量表达式，即必须在编译时知道大小。

Why is it that C++ don't allow you to create array of dynamic length in stack memory?

为什么 C++ 不允许您在堆栈内存中创建动态长度数组？

Simple answer "Because the standard says so". Even the upcoming C++ Standard (C++0x) is not going to allow Variable Length Arrays.

简单的回答“因为标准是这样说的”。即使是即将到来的 C++ 标准 (C++0x) 也不会允许可变长度数组。

BTW we always have std::vectorin C++. So there's no reason to worry. :)

顺便说一句，我们总是std::vector在 C++ 中使用。所以没有理由担心。:)

回答by Jonathan Leffler

C99 does allow variable length arrays (VLAs); C89 did not.

C99 确实允许可变长度数组 (VLA)；C89没有。

void function(int n)
{
    int array[n];
    ...
}

C++ (98, 03) does not allow VLAs in the same way that C99 does, but it has vectors and related types which are better in many respects.

C++ (98, 03) 不像 C99 那样允许 VLA，但它有向量和相关类型，它们在许多方面都更好。

回答by Nawaz

int n;
cin >> n;
int array[n];

In C++, the size of the array must be known at compile time. But in your code the size will be known at runtime. That is not allowed by the language!

在 C++ 中，必须在编译时知道数组的大小。但是在您的代码中，大小将在运行时已知。这是语言所不允许的！

回答by qrdl

There is no language called C/C++. There is C, which allows variable-length arrays (VLAs) since 1999, and there is C++, which doesn't allow.

没有称为 C/C++ 的语言。有 C，它允许自 1999 年以来的可变长度数组 (VLA)，还有 C++，它不允许。

To answer your question "why" - C++ standard body for whatever reason didn't include VLAs into C++ standard.

回答你的问题“为什么”——C++ 标准体无论出于何种原因都没有将 VLA 包含到 C++ 标准中。

回答by Vlad

In addition to other answers:
My speculation is that the C++ standard board thinks that the C-style arrays are obsolete and evil, and promotes using of std::vector(which has this feature by design) this way.

除了其他答案：
我的猜测是 C++ 标准委员会认为 C 风格的数组是过时和邪恶的，并std::vector以这种方式促进使用 of （设计上具有此功能）。