How to enable automatic type deduction for class specific typedefs (c++17)?

user_3906918 0
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I would like to deduce a class' (with default template parameter) typedef with c++17 automatically. Does somebody know if this is possible? The following code tries to illustrate this:

#include <vector>
template <typename T = int>
struct A{
    using Vec = std::vector<T>;
};

int main() {
    A a{}; // works with c++ 17
    A<int>::Vec vec2{}; //works
    A::Vec vec{}; //does not seem to work with c++ 17. Is that possible somehow?
}

In the code above, a is instantiated without problems, and the deduced template type is int as per default. Nevertheless, I have to pass the parameter type to use the typedef Vec. I would be very grateful if somebody knew an answer to the two following questions:

  • What is the paper number/title of the ISO mailing list for the feature allowing automatic type deduction for A a{}? I am curious to learn more about this feature.
  • Is it possible to infer the type of Vec automatically without specifying the template type of A explicitly?

I am grateful for any hints or suggestions!

c++17 templates c++
2 Answers

Is it possible to infer the type of Vec automatically without specifying the template type of A explicitly?

Yes. You can leave the <> empty:

A<>::Vec vec{};

Otherwise, A without <> or <int> etc., names a template with an unknown template type parameter, and you cannot use that to access nested identifier.

What is the paper number/title of the ISO mailing list for the feature allowing automatic type deduction for A a{}? I am curious to learn more about this feature.

P1814 is the main paper on class template argument deduction ("CTAD"). Here is the page on cpprefernece. Note that A<>::Vec vec{} does not rely on CTAD, as there is no contstructor involved that performs an actual deduction.

user_95935960

Is it possible to infer the type of Vec automatically without specifying the template type of A explicitly?

Yes:

A<>::Vec
user_101473990
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