C++ memory allocation in arrays

Question

    int m,n;
    cin>>m>>n;
    int A[m][n];

Question is: Will array A get memory on stack or heap in C++ ?

Edit: I know using new is a better route. This technique works in my mingw g++ compiler. I am just curious.

Answer 1

This behaviour depends on the particular compiler and is not part of the standard .

In gcc , which mingw is a port of, the memory for automatic variables as such, including variable lengths arrays is allocated on the stack.

According to the gcc manual:

6.19 Arrays of Variable Length

[...] 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. [...] You can use the function alloca to get an effect much like variable-length arrays.

Ref: https://gcc.gnu.org/onlinedocs/gcc/Variable-Length.html

According to man 3 alloca :

The space allocated by alloca() is allocated within the stack frame

Please keep in mind that:

ISO C++ forbids variable length arrays

Alternatively you can allocate your array dynamically (with new ) or preferably use the C++ containers anyway where possible.

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Edit: Added note on variable behaviour between compilers, based on Paul's comment.