c++ - How to define member array size at runtime

Question

Let's say I have a class that has a member which is an array. Is it possible to define its size upon construction/at run-time, in the following way:

class myClass {
    private:
        int myArray[n]
    public:
        myClass();
        someOtherMethod();
};

Where n is a variable that is defined based on user input. If not, what would be the best alternative?

Answer 1

It depends.

Semantically, there are 3 types of arrays:

• arrays with a size fixed at compile time
• arrays with a size fixed at runtime
• arrays with a dynamic size

C++ directly supports the first and third cases, respectively with regular arrays and the std::vector class.

C also supports the second type with two constructs:

• variable length arrays (on the stack)
• the oldie struct hack or tail-padding

I would advise, in C++, using the std::vector class in your case. It provides more than what you need, but is simpler to use.

On the other hand, you can still use tail-padding, even in C++. It does require careful engineering though.