Check for value definedness in C++

Question

I'm working in C++ and I need to know if a scalar value (for instance a double) is "defined" or not. I also need to be able to "undef" it if needed:

class Foo {
public:
    double get_bar();

private:
    double bar;
    void calculate_bar() {
        bar = something();
    }
};

double Foo::get_bar() {
    if ( undefined(bar) )
        calculate_bar();
    return bar;
}

Is it possible in C++?

Thanks

Answer 1

As the other answers says, C++ doesn't have this concept. You can easily work around it though.

Either you can have an undefined value which you initialize bar to in the constructor, typically -1.0 or something similar.

If you know that calculate_bar never returns negative values you can implement the undefined function as a check for < 0.0.

A more general solution is having a bool saying whether bar is defined yet that you initialized to false in the constructor and when you first set it you change it to true. boost::optional does this in an elegant templated way.

This is what the code example you have would look like.

class Foo {
public:
    double get_bar();
    Foo() : barDefined(false) {}
private:
    double bar;
    bool barDefined;
    void calculate_bar() {
        bar = something();
    }
};

double Foo::get_bar() {
    if ( barDefined == false ) {
        calculate_bar();
        barDefined = true;
    }
    return bar;
}

Answer 2

As others pointed out, there is nothing like an "undefined" state. But you may want to look into boost.optional