c++c++17language-lawyertemplate-argument-deductionctad

When does type information flow backwards in C++?


I just watched Stephan T. Lavavej talk at CppCon 2018 on "Class Template Argument Deduction", where at some point he incidentally says:

In C++ type information almost never flows backwards ... I had to say "almost" because there's one or two cases, possibly more but very few.

Despite trying to figure out which cases he might be referring to, I couldn't come up with anything. Hence the question:

In which cases the C++17 standard mandates that type information propagate backwards?


Solution

  • Here is at least one case:

    struct foo {
      template<class T>
      operator T() const {
        std::cout << sizeof(T) << "\n";
        return {};
      }
    };
    

    if you do foo f; int x = f; double y = f;, type information will flow "backwards" to figure out what T is in operator T.

    You can use this in a more advanced way:

    template<class T>
    struct tag_t {using type=T;};
    
    template<class F>
    struct deduce_return_t {
      F f;
      template<class T>
      operator T()&&{ return std::forward<F>(f)(tag_t<T>{}); }
    };
    template<class F>
    deduce_return_t(F&&)->deduce_return_t<F>;
    
    template<class...Args>
    auto construct_from( Args&&... args ) {
      return deduce_return_t{ [&](auto ret){
        using R=typename decltype(ret)::type;
        return R{ std::forward<Args>(args)... };
      }};
    }
    

    so now I can do

    std::vector<int> v = construct_from( 1, 2, 3 );
    

    and it works.

    Of course, why not just do {1,2,3}? Well, {1,2,3} isn't an expression.

    std::vector<std::vector<int>> v;
    v.emplace_back( construct_from(1,2,3) );
    

    which, admittedly, require a bit more wizardry: Live example. (I have to make the deduce return do a SFINAE check of F, then make the F be SFINAE friendly, and I have to block std::initializer_list in deduce_return_t operator T.)