如果您创建一个可变长度数组并使用 g++ 编译会发生什么

Question

Following code compiles fine since g++ allows it, but will it cause undefined behavior? Or my code will work fine? what does it mean that c++ standard disallows variable length array if no error is generated on using it?

#include <iostream>

using namespace std;

int main()
{
   int x;
   cin >> x;
   char abc[x];
   cout << sizeof(abc) << "\n";
   
   return 0;
}

Answer 1

GCC documents its support of VLAs here ( emphasis mine )

Variable-length automatic arrays are allowed in ISO C99, and as an extension GCC accepts them in C90 mode and in C++ .

CLANG documentation too follows suit but mentions clearly that the standard doesn't accept ( emphasis mine )

GCC and C99 allow an array's size to be determined at run time. This extension is not permitted in standard C++ . However, Clang supports such variable length arrays for compatibility with GNU C and C99 programs.

If you would prefer not to use this extension, you can always disable it with -Werror=vla to disallow compilation.