Dynamic memory allocation in C++?

Question

Why does this work?

    #include <iostream>    

    int main()
    {
        std::cout << "Enter a number: ";
        int arraySize;
        std::cin >> arraySize;

        int array[arraySize];

        for(int element : array)
        {
            element = 42;
            std::cout << element << "\n";
        }

        std::cout << "Array size: " << sizeof(array) << "\n";
        std::cout << "Element count: " << sizeof(array)/sizeof(int);

        return 0;
    }

My understanding of dynamic memory allocation in C++ tells me that one case where it is needed is when you do NOT know the amount of memory you need to allocate at compile-time. In this program, clearly the array size is not known when the program is compiled, but is dynamic, as it can change with the value the user enters.

Here is a program run after it is successfully compiled(warning and error free) with:
g++ program.cpp -std=c++11 -o program.exe

Enter a number: 12
42
42
42
42
42
42
42
42
42
42
42
42
Array size: 48
Element count: 12

As you can see, an array was created with a user-defined amount of elements, each was successfully assigned to 42, and proven to exist with the sizeof() operator.

Why does this not count as dynamic memory allocation and compile?

This is not dynamic memory allocation. This is an extension to the C++ language by your compiler. — Sam Varshavchik, Aug 11 '17 at 00:46
*int array[arraySize];* -- This is not valid C++, thus it may or may not be "dynamic allocation". It is whatever the compiler vendor who supports this syntax decides it is. — PaulMcKenzie, Aug 11 '17 at 00:52
*Why does this work?* -- [No, it doesn't work](http://rextester.com/BMOLTH93698) — PaulMcKenzie, Aug 11 '17 at 00:54
You are not printing out the array elements. Just a variable you keep setting to the same value. — Galik, Aug 11 '17 at 01:08
Possible duplicate of [Local variable length array](https://stackoverflow.com/questions/30694199/local-variable-length-array) — Swift - Friday Pie, Aug 11 '17 at 04:22

score 4 · Accepted Answer · answered Aug 11 '17 at 00:50

This declaration of variable-length array (VLA) is not standard:

int array[arraySize];

The reason this line compiles with g++ is that the compiler has provided this functionality as an extension:

Variable-length automatic arrays are allowed in ISO C99, and as an extension GCC accepts them in C90 mode and in C++. These arrays are declared like any other automatic arrays, but with a length that is not a constant expression. The storage is allocated at the point of declaration and deallocated when the block scope containing the declaration exits.

score 3 · Answer 2 · answered Aug 11 '17 at 00:49

The variable you've declared in this line is called a variable-length array:

int array[arraySize];

Variable-length arrays are a feature of the C programming language, but they're not actually a part of C++. Many compilers support variable-length arrays as a compiler extension (here's the documentation for the gcc version of variable-length arrays, for example), but it's not guaranteed that it will work on all compilers.

Dynamic memory allocation in C++?

2 Answers2