C++ array size dependent on function parameter causes compile errors

Question

I have a simple function in which an array is declared with size depending on the parameter which is int.

    void f(int n){
        char a[n];
    };

    i         


        

Answer 1

Typically in C (excepting C99 compilers as others have pointed out) and C++, if you want to allocate memory on the stack, the size of what you want to allocate has to be known at compile time. Local variables are allocated on the stack, so an array whose length depends on a function parameter at run time violates this rule. Klein is correct to point out that using the 'new' operator is one way to solve this problem:

char *a = new char [n];

'a' is still a local variable allocated on the stack, but instead of being the whole array (which has variable length), it's just a pointer to an array (which is always the same size, and thus known at compile time). The array is allocated on the heap, which typically plays the stack's counterpart -- the stack is for things with a size known at compile time, and the heap is for things with a size not known at a compile time.