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Subject: Re: The standard does not specify when to supply default arguments as template arguments
From: Peter Sommerlad (C++) (peter.cpp_at_[hidden])
Date: 2020-07-17 05:40:38


First, if the standard were wrong in that area, multiple implementors
would have complained decades ago and filed defect reports. They do, for
example, with the text of the C++20 standard when implementing new
features uncovers holes in the specification

Second, in your example below, func takes one template argument, that
could be deduced from a single parameter template T. However, you are
using a two parameter template as its argument when calling func.
Therefore, the deduction of funs's T does not work and you get an error
message. Calling func(Test<int>{}) should compile imho (not checked),
because func's T get deduced as int and Test's U get defaulted to int as
well. It is like you want to fit a square peg in a round hole that would
not fit.

I suggest not using the ISO standard text as a basis for learning C++
templates. For templates, the excellent book by Daveed Vandervorde and
Nico Josuttis (2nd edition) is what I would look at first.


jim x via Std-Discussion wrote on 17.07.20 05:06:
> template<typename T, typename U = T>
> struct Test{};
> template<typename T>
> void func(Test<T> /*#1*/){
> }
> int main(){
> func(Test<int,char>{});
> }
> For this case, the report error is "deduced conflicting types for
> parameter 'T' ('int' and 'char')", please see
> https://godbolt.org/z/MnWGYq, It sounds like the function parameter
> would be the form as Test<T,T>, then deduced Test<T,T> from Test<int,
> char>, then T will have two different type values. So, I wonder that
> Does the compiler view Test<T> at #1 as Test<T,T>. At this point,
> there's no instantiation happening, hence , how does the rule "When a
> simple-template-id does not name a function, a default
> template-argument is implicitly instantiated when the value of that
> default argument is needed." applies to this case. In my mind, the
> default argument is required at #1, but there shall be no
> instantiation.

Peter Sommerlad
Better Software: Consulting, Training, Reviews
Modern, Safe & Agile C++
+41 79 432 23 32

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