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Discussion on: That `overloaded` Trick: Overloading Lambdas in C++17

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mmatrosov profile image
Mikhail Matrosov

It would be nice to mention why we changed () to {} when we switched to deduction guide. E.g. if you add constructor

overloaded(Ts... ts) : Ts(ts)... {}

then you can use () again. At least I was curious about this.

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tmr232 profile image
Tamir Bahar Author

I did mention that we need to use aggregate initialization. Though I now think my reasoning for it is wrong. Also - I never tried making it work with ().

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misiaszek profile image
Marcin Misiaszek

This also works without deduction guide:

template<class... Ts> struct overload : Ts... {
  overload(Ts...) = delete;
  using Ts::operator()...;
};
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maxxon profile image
Ma-XX-oN

I'm confused with this part:

        Print{ // (2)
            [](const char* str) { puts(str); },
            [](int i) { printf("%d\n", i); }
        }

The braces indicate a constructor call, but there is none. There is a non-member helper template though, but it has no body. So, what's happening here?

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maxxon profile image
Ma-XX-oN

It's not a non-member helper template, it's a user defined deduction guide. See this article:

arne-mertz.de/2017/06/class-templa...

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misiaszek profile image
Marcin Misiaszek

It works also without deduction guide and the 'using' fix is for g++. 'operator()' cannot be ambiguous in visitors but gcc is very careful with overloading derived functions. In clang we can easily have the same results with minimal code (checked output assembler with Compiler Explorer):

template<class... Ts> struct overload : Ts... {
  overload(Ts...) = delete;
};

There is needed aggregate Initialisation + variadic templates.