Question

Previously I had:

MyClass::MyClass() : myMember(VALUE,this,5,"etc.") { ... }

but I need to do some calculation of the values passed to myMember’s constructor. How do I initialise myMember within the constructor body instead? I’m not sure if I should do it like this:

MyMember myMember(calculatedValue,this,5,"etc.");

Answer 1

All member initialisation occurs before the constructor body runs. So if you did this:

MyClass::MyClass()
{
    myMember = MyMember(calculatedValue,this,5,"etc.");
}

it would be assignment, not initialisation. So it won’t work if MyMembers are not assignable (e.g. they have a private assignment operator). It also won’t work if myMember is declared as const.

You could, however, do something like this:

class MyClass {
public:
    MyClass() : myMember(calculate(VALUE),this,5,"etc.") {}

private:
    static int calculate(int x) { return x*3; }
};

or even something like this:

class MyClass {
public:
    MyClass() : myMember(createMember()) {}

private:
    static MyMember createMember(int x) {
        MyMember m;
        m.foo = 5;
        ...
        return m;
    }
};

Note that I’ve used static helper functions here, because using non-static member functions before the constructor has completed gets a little weird, in the general case.