C++ about dynamic array on stack

Question

So far as I know,C++/C doen't support dynamic array on stack . In the following delcaration :

int data[n] ; // if the n is not decided at compiling time ,this leads to error

But recently , I read some other guys code as following:

//**
It seems the n number can not be decided at compling time,but when I run it , if i fprintf the formation, each time i got the correct array size !!!!!!
the G++ version is 4.7.1 
Is this because the G++ 4.7.1 support C++11 x which allow dynamic array?
 **//

#include <cstdio>
#include <algorithm>
using namespace std;

#include <stdio.h>

char s[31];

int Hash()
{
    int sum=0;
    for(int i=0,k=0;k<7;i++)
    {
        if(s[i]>='0'&&s[i]<='9')
        {
            sum*=10;k++;
            sum+=(s[i]-'0');
        }
        else if(s[i]>='A'&&s[i]<'Z')
        {
            sum*=10;k++;
            sum+=((s[i]-'A'-(s[i]>'Q'))/3+2);
        }
    }
    return sum;
}

int main()
{

    int n;scanf("%d",&n);
    int data[n];getchar();
    //fprintf(stderr,"size is %d\n",sizeof(data)/sizeof(data[0]));
    //**
        It seems the n number can not be decided at compling time,but when I run it , if i fprintf the formation, each time i got the correct array size !!!!!!
     *//
    for(int tmp=0;tmp<n;tmp++)
    {
        gets(s);
        data[tmp]=Hash();
    }
    sort(data,data+n);
    bool p=false;n--;
    for(int i=0,num=1;i<n;i+=num=1)
    {
        while(data[i]==data[i+1])
        {
            num++;
            i++;
        }
        if(num>1)
        {
            printf("%03d-%04d %d\n",data[i]/10000,data[i]%10000,num);
            p=true;
        }
    }
    if(!p)printf("No duplicates.\n");
    return 0;
}

Answer 1

These are called variable length arrays (VLA) and are a g++ compiler extension which is not part of the C++ standard. Do not use it in C++ if you want your code to be portable.

You can make g++ emit a warning with the -Wvla compilation flag, or an error with flag -Werror=vla . I usually compile with -pedantic-errors , which catches this and many other deviations from the standard.

Answer 2

C and C++ are different languages. One example of the differences between the languages is that C does support dynamic arrays on the stack (though a stack isn't really required by either C or C++). They're called VLAs (variable length arrays) : http://en.wikipedia.org/wiki/Variable-length_array

g++ may have an extension that allows this feature in C++, but that extension isn't required for all C++ implementations.