这个C ++代码是如何在GCC中运行而不是在Visual C ++中运行的？

Question

#include<iostream>
using namespace std;
main()
{
  int m;
  cin>>m;
  int re[m];
  cout<<sizeof(re);
} 

This code is running perfectly in codeforces GNU C++ 4.7(But not in my Microsoft Visual C++). But why? shouldn't the array size be a constant?

Answer 1

As you mentioned, C++ array sizes must be constant expressions.
- With VS, you will get: error C2057: expected constant expression
- GCC has an extension to the standard, which allows your code to compile.

Answer 2

Variable length arrays are introduced in C99. Standard C++ doesn't support it, but GCC supports it as an extension:

See GCC extension: Arrays of Variable Length for detail.

Answer 3

For stack allocation, array size needs to be determined at compile time. But the array size is determined at runtime, so it must go on the heap.

Use

int *re = new int[m];
cout << m << endl;
delete[] re;

Answer 4

As others already mentioned, your code is non-standard C++ as you have a variable length array :

  int m;
  cin>>m;
  int re[m]; // m is not a compile time constant!

GCC allows this as language extension . You get a warning if you enable -Wvla . Afaik, this code gets rejected if you enfore gcc to use a specific c++ standard, such aus -std=c++11 .

Now you _could do this instead (as Paul Draper already wrote)

int *re = new int[m]; // dynamic array allocation
delete[] re;          // giving the memory back to the operating system

However, C++ provides an easy to use wrapper for this:

std::vector<int> re(m);

For most things, vector behaves just like a dynamically allocated array, but prevents you from accidentally forgetting to delete or double delete and makes passing the data to functions a bit easier. Learn more about vector on cppreference.com .