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

maxxon profile image

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

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

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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.