Modeling infinity for the largest double value

Question

The question is about modeling infinity in C++ for the double data type. I need it in a header file, so we cannot use functions like numeric_limits

Answer 1

numeric_limits functions are all constexpr so they work just fine as compile time constants (assuming you're using the current version of C++). So std::numeric_limits::infinity() ought to work in any context.

Even if you're using an older version, this will still work anywhere that you don't require a compile time constant. It's not clear from your question if your use really needs a compile time constant or not; just being in a header doesn't necessarily require it.

If you are using an older version, and you really do need a compile time constant, the macro INFINITY in cmath should work for you. It's actually the float value for infinity, but it can be converted to a double.