定义使用数组变量的大小

问题描述

在C语言中，这合法吗？

Is this valid in C language?

#include <stdio.h>
int main()
{
  int i = 5;
  int a[i];     // Compiler doesn't give error here. Why?
  printf("%d",sizeof(a));  //prints 5 * 4 =20. 4 is the size of integer datatype.
  return 0;
}

编译器不会在声明中给出错误 int类型的[I]; 。我是不是一个常量，那么它怎么能编译成功？难道是因为我使用gcc编译器？它是允许C ++？

Compiler doesn't give error at the statement int a[i];. i isn't a constant then how can it compile successfully? Is it because I am using gcc compiler? Is it allowed in C++?

推荐答案

是的，这是因为有效，和被称为（VLA）。换句话说，它已经在官方语言标准，大约14年。

Yes, this is valid as of C99, and is called a variable-length array (VLA). In other words, it has been in an official language standard for around 14 years.

不，这不是在C ++无效，了解详情。

No, it's not valid in C++, see this question for details.

另外请注意，的sizeof 是不是一个功能，这样就可以的printf写成（％祖\\ n，sizeof的一）; 也使用了为size_t 值正确的格式说明。

Also note that sizeof is not a function, so that can be written as printf("%zu\n", sizeof a); which also uses the proper format specifier for a size_t value.

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