Class template argument deduction before C++17

Question

I was reading standard library documentation and found out that there were some functions like make_tuple. As far as I can see, these functions serve only one purpose: to construct class instance using automatic template argument deduction. Since CTAD was not available before C++17, associated make-functions had to be called. This gap was filled only in C++17. Now I wonder, why it couldn't be implemented earlier "the same way" as function template argument deduction? There should be an example that leads to ambiguity that cannot be resolved before C++17, I guess. Can someone confirm it? What is the difference between functions and classes in that regard? Thank you!

Answer 1

As mentioned in the comments, standardization of a feature takes a long time.
If you wish to know in detail about it, you can check out the section on Standardization from https://isocpp.org , particularly "The life of an ISO proposal: From “cool idea” to “international standard”" .

The paper that eventually made CTAD possible in C++17 is titled Template Tidbits and was first published on 2012-09-22.

It underwent many revisions and the paper "Template argument deduction for class templates" , which is the sixth revision of the paper was published on 2016-06-24.

So it took about five years for CTAD to go from “cool idea” to “international standard”.

Also as @StoryTeller notes, the idea itself has been around for a much longer time and is presented in the paper Argument Deduction for Constructors which was published in 2007.