Dynamic array in Stack?

Is this correct ? This is compiled with g++ (3.4) sucessfully.

int main()
{
    int x = 12;
    char pz[x]; 
}

Solution 1:

Here's your combination answer of all these other ones:

Your code right now is not standard C++. It is standard C99. This is because C99 allows you to declare arrays dynamically that way. To clarify, this is also standard C99:

#include <stdio.h>

int main()
{
    int x = 0;

    scanf("%d", &x);

    char pz[x]; 
}

This is not standard anything:

#include <iostream>

int main()
{
    int x = 0;
    std::cin >> x;
    char pz[x]; 
}

It cannot be standard C++ because that required constant array sizes, and it cannot be standard C because C does not have std::cin (or namespaces, or classes, etc...)

To make it standard C++, do this:

int main()
{
    const int x = 12; // x is 12 now and forever...
    char pz[x]; // ...therefore it can be used here
}

If you want a dynamic array, you can do this:

#include <iostream>

int main()
{
    int x = 0;
    std::cin >> x;

    char *pz = new char[x];

    delete [] pz;
}

But you should do this:

#include <iostream>
#include <vector>

int main()
{
    int x = 0;
    std::cin >> x;

    std::vector<char> pz(x);
}

Solution 2:

Technically, this isn't part of C++. You can do variable length arrays in C99 (ISO/IEC 9899:1999) but they are not part of C++. As you've discovered, they are supported as an extension by some compilers.

Solution 3:

G++ supports a C99 feature that allows dynamically sized arrays. It is not standard C++. G++ has the -ansi option that turns off some features that aren't in C++, but this isn't one of them. To make G++ reject that code, use the -pedantic option:

$ g++ -pedantic junk.cpp
junk.cpp: In function ‘int main()’:
junk.cpp:4: error: ISO C++ forbids variable-size array ‘pz’

Solution 4:

If you want a dynamic array on the stack:

void dynArray(int x)
{
    int *array = (int *)alloca(sizeof(*array)*x);

    // blah blah blah..
}