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; }