Question

Whenever I define a variable and give it a value at the same time inside a class, I get an error. What is the reason for this?

As you can see, this doesn’t work…

class myClass {
    private:
        int x = 4; // error
};

But when I keep the variable undefined it does:

class myClass {
    private:
        int x;
};

Answer 1

Since no one else is using member initialization, I’ll introduce you:

class myClass {
    private:
        int x;
    public:
        myClass() : x (4){}
};

It’s always better to use this over assigning in the body of the constructor, since by the time the body begins, all user-defined members will have already been initialized whether you said so or not. Better to do it once and actually initialize the non-user-defined members, and it is the only method that works for both non-static const members, and reference members.

For example, the following will not work because x isn’t being initialized in the body, it’s being assigned to:

class myClass {
    private:
        const int x;
    public:
        myClass() {x = 4;}
};

Using a member initializer, however, will, because you’re initializing it off the bat:

class myClass {
    private:
        const int x;
    public:
        myClass() : x (4){}
};

Note also that your int x = 4; syntax is perfectly valid in C++11, where it subs in for any needed initialization, so you’ll benefit if you start using it.