Where is the fold expression ()?

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Reference fluentCPP article

The below code explanation says that this structure inherits from several lambdas, can be constructed from those lambdas, and folds over the using expression.

template<typename... Lambdas>
struct overloaded : public Lambdas...
{
    explicit overloaded(Lambdas... lambdas) : Lambdas(lambdas)... {}

    using Lambdas::operator()...;
};

My doubt is that parentheses i.e () indicate a c++17 fold expression but I don't see any enclosing parentheses around the using statement. How will it fold?

fold-expression c++17 c++
1 Answers

This isn't a fold expression. You cannot have any expression statements in class scope. And as you point out, there are no parentheses that are part of the fold expression syntax.

This is a declaration with a parameter pack expansion. Pack expansions can be used in many more context than just fold expressions.

what will this statement do?

It will declare

using L::operator();

for each type L in the parameter pack Lambdas.

Usually using is used same as typedef

That's not the only use case of that keyword. In this context, it is used to introduce a member (function) of a base class into the derived class.

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