How to store some kind of "null" in C++ double or int variable?

Question

I have a class that looks like this

struct A {
     double a1;
     int b1;
     double a2;
     int b2;
};

I have to read off of a file values for a1, b1, a2, and b2. Most of the time all four numbers are on the file, but sometimes there are only two numbers.

When there are two numbers, I want to store the values in a1, and b1 and I want to store "nothing" in a2 and b2. If a2 and b2 were pointers, I could just assign them to be nullptr, but they are not pointers.

Is there something I can store in double and int variables to indicate that 'nothing' is stored?

I know Boost.Optional is available, but I'm trying to avoid that library.

Answer 1

You could assign NAN to the double a2, which would also indicate that the int b2 is invalid.

This page for NAN usage.

Answer 2

You cannot. I can think of two alternative ways:

1. use int *; or
2. Use a value that for sure invalid in your context. For example, if it can never be negative, then use -1 to indicate null. But I still prefer the first way, since its correctness is not depended on requirement or context.