Chapter 19 // Exercise 7 - Principles & Practice Using C++

In this exercise I am using Visual Studio 2017 and a modified version of the std_lib_facilities header found here.

Chapter 19 // Exercise 7

Try your solution to exercise 2 with some Numbers. 

Github: https://github.com/l-paz91/principles-practice/tree/master/Chapter%2019/Exercise%207

This one got me until I managed to find this post:

https://stackoverflow.com/questions/5733061/error-c2893-failed-to-specialize-function-template

I then understood that I was trying to return a variable that was a common type between the two given template types. The compiler doesn't know that Number<int> and Number<double> are common types but Number does have ways to add themselves together and return a common type.

There is a way to define a common_type for user defined classes:

https://stackoverflow.com/questions/36523038/stdcommon-type-trait-for-user-defined-types

I used this to create common types for the combinations given in the exercise. It was working...in odd ways. I did some more testing and found my Number class broke spectacularly when doing Number<char> operations with any other type of number. 

It took me a while to realise it was because when assigning, it created a copy of the Number<type> with whatever type was the first operand. So for example:

When using ints and chars, it always produces 970 no matter the order. So I'm guessing that the expected behaviour of int * char is always to treat the char as an int. My Number type was not doing this due to default constructing and returning the wrong type.

I eventually found a fix for this by setting the return type of the operator overload to a common type between the two of them:

It looks hideous. I could get rid of some of the ugliness with a typedef but I left it this way for "clarity". The only problem with this, is that all combinations of common types must be declared for the class.