# I’ve finally figured out, why I was having so many problems, with complex numbers, in a C++ program.

In This earlier posting, I had written that many things could go wrong, with the way C++ templates define complex numbers. Well, after studying the programming exercise which I was basing that posting on, I think I’ve finally found out, what the single problem in fact was.

In my programming exercise, I had defined the data-type ‘complex<double_t>‘. This in itself caused a lot of problems, without being obvious to me as the culprit. The way C++ templates define complex numbers, will often derive the base-type of one complex number, from the base-type of another, preexisting one, just by transferring a template parameter. However, there are two situations where the templates, defined in the headers, can run into trouble with this:

1. They can try to convert a ‘real number’ to a complex number, where the base-type of the derived complex number was never declared,
2. They can try to mix mathematical operations between complex numbers and ‘real numbers’, in such a way that the type of the real number must match the base-type of the complex number exactly.

Specifically in situation (1) above, the templates will try the specializations of ‘float‘, ‘double‘, and ‘long double‘, as educated guesses, for what type of complex number is required. And the problem may well be, that only these 3 template-specializations are attempted, not, ‘double_t’.

And in situation (2) above, I was not taking into consideration that my code was often providing literal numbers, that were of type ‘double‘ by nature, not of type ‘double_t‘. This created a mismatch.

In any case, now that I realize what my mistake was, as well as having removed all the ‘mixed computations, between complex and real numbers’, resulting in code that no longer generates errors, I am more confident that I only was days ago, in the C++ template-definitions, for complex numbers.

(Update 9/18/2019, 12h55 : )

An added observation would be, that when my code tried to find the absolute, of the imaginary component, of a complex number, that component was also a ‘double_t‘ (deterministically), but the absolute function that’s predefined, again, only recognizes parameter-types ‘int‘, ‘float‘, ‘double‘, and ‘long double‘, in certain versions of the GCC compiler, which can again result in an incorrect match, with a working version of the absolute function.

Dirk