7 Expressions [expr]

7.5 Primary expressions [expr.prim]

7.5.6 Fold expressions [expr.prim.fold]

1
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A fold expression performs a fold of a pack ([temp.variadic]) over a binary operator.
fold-operator: one of
+   -   *   /   %   ^   &   |   <<   >>
+=  -=  *=  /=  %=  ^=  &=  |=  <<=  >>=  =
==  !=  <   >   <=  >=  &&  ||  ,   .*   ->*
2
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An expression of the form (... op e) where op is a fold-operator is called a unary left fold.
An expression of the form (e op ...) where op is a fold-operator is called a unary right fold.
Unary left folds and unary right folds are collectively called unary folds.
In a unary fold, the cast-expression shall contain an unexpanded pack ([temp.variadic]).
3
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An expression of the form (e1 op1 ... op2 e2) where op1 and op2 are fold-operators is called a binary fold.
In a binary fold, op1 and op2 shall be the same fold-operator, and either e1 shall contain an unexpanded pack or e2 shall contain an unexpanded pack, but not both.
If e2 contains an unexpanded pack, the expression is called a binary left fold.
If e1 contains an unexpanded pack, the expression is called a binary right fold.
[Example 1: template<typename ...Args> bool f(Args ...args) { return (true && ... && args); // OK } template<typename ...Args> bool f(Args ...args) { return (args + ... + args); // error: both operands contain unexpanded packs } — end example]