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

You can use new/delete to allocate/free memory on the heap. This is slower and possibly more error prone than using char[n], but it's not part of the C++ standard yet, sadly.

You can used boost's scoped array class for an exception-safe method for using new[]. delete[] is automatically called on a when it goes out of scope.

void f(int n) {
    boost::scoped_array a(new char[n]);

    /* Code here. */
}

You can also use std::vector, and reserve() some bytes:

void f(int n) {
    std::vector a;
    a.resize(n);

    /* Code here. */
}

If you do want to use char[n], compile as C99 code instead of C++ code.

If you absolutely must allocate data on the stack for some reason, use _alloca or _malloca/_freea, which are extensions provided by MSVC libs and such.