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];     };      int main() {         return 0;     } 

This piece of code compiles fine on GNU C++, but not on MSVC 2005.

I get the following compilation errors:

    .\main.cpp(4) : error C2057: expected constant expression     .\main.cpp(4) : error C2466: cannot allocate an array of constant size 0     .\main.cpp(4) : error C2133: 'a' : unknown size 

What can I do to correct this?

(I’m interested in making this work with MSVC,without using new/delete)

Answer 1

What you have found it one of the Gnu compiler’s extensions to the C++ language. In this case, Visual C++ is completely correct. Arrays in C++ must be defined with a size that is a compile-time constant expression.

There was a feature added to C in the 1999 update to that language called variable length arrays, where this is legal. If you can find a C compiler that supports C99, which is not easy. But this feature is not part of standard C++, not is it going to be added in the next update to the C++ standard.

There are two solutions in C++. The first is to use a std::vector, the second is just to use operator new []:

char *a = new char [n]; 

While I was writing my answer, another one posted a suggestion to use _alloca. I would strongly recommend against that. You would just be exchanging one non-standard, non-portable method for another one just as compiler-specific.