如何在运行时定义成员数组大小

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.

Answer 2

Use a vector.

class myClass {
    private:
        std::vector<int> myArray;
    public:
        myClass();
        someOtherMethod();
};

myClass::myClass (int size)
    : myArray (size)
{ 
    ...
}

Then, you can fill in the vector as you would an array. Alternatively, as Nawaz points out, use reserve() , which reserves space for new elements, and/or push_back() , which adds elements onto the back, one at a time.

Answer 3

The class template std::vector is designed for this purpose.

class myClass {
    private:
        std::vector<int> myArray;
    public:
        myClass(int size);
        someOtherMethod();
};

myClass::myClass(int size) : myArray(size)
{
}