How to validate the initialization of a const member variable before assigning it in C++

Question

Let's say I have this simple class with a const int member variable:

class MyClass{
    public:
        Myclass(int x, int y);
    private:
        const int importantNumber;
        int anotherNumber;
};

MyClass::MyClass(int x, int y) :importantNumber{x}
{
    this->anotherNumber = y;
}

Since int importantNumber is const, I can only set it during the creation of the object by the constructor (with a member initialization list, as seen above).

Now, the question: how could I possibly add validation for argument x given to the constructor before actually creating importantNumber with that value? Is it possible to create a static int MyClass::validation(int a) and use it on the member initialization list of the constructor like importantNumber{validation(x)}?

Even if it's possible, is there a better way to do it?

Answer 1

You just add it.

MyClass::MyClass(int x, int y) : importantNumber{validate(x)}
{
    this->anotherNumber = y;
}

The int validate(int original) function can now return something other than x or throw an exception or assert or ask the user for confirmation, whichever you deem appropriate.

If it is just a simple check and you don't want to write a validate function you can use a lambda:

MyClass::MyClass(int x, int y) :importantNumber{
    [](int number){
        assert(number > 0);
        return number;
    }(x)}
{
    this->anotherNumber = y;
}

Although this can get a bit convoluted if you overdo it.