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Generic Programming Templates in C++

Fold Expressions

Explore fold expressions introduced in C++17 to efficiently process variadic template parameter packs using binary operators. Understand the syntax, variations, supported operators, and how fold expressions perform compile-time evaluation for more flexible and concise template code.

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Fold Expressions (C++17)

Fold expressions is a nice syntax to evaluate binary operators at compile-time. Fold expressions reduce parameter packs on binary operators.

C++11 provides support for parameter packs:

C++
bool all_14(){
  return true;
}
template<typename T, typename ...Ts>
bool all_14(T t, Ts ... ts){
  return t && all_14(ts...);
}

C++17 provides support for fold expressions:

C++
template<typename ... Args>
bool all_17(Args ... args){
  return ( ... && args);
}
bool val == all_14(true, true, true, false)
         == all_17(true, true, true, false
         == ((true && true)&& true)&& false
         == false;

Two variations

  • The fold expression either has or does not have an initial value
  • The parameter pack will be processed from left or right

C++17 supports the following 32 operators in fold expressions:

+ - * / % ^ & | = < > << >> ...