3.9 KiB
[expr.prim.fold]
7 Expressions [expr]
7.5 Primary expressions [expr.prim]
7.5.7 Fold expressions [expr.prim.fold]
A fold expression performs a fold of a pack ([temp.variadic]) over a binary operator.
fold-expression:
( cast-expression fold-operator ... )
( ... fold-operator cast-expression )
( cast-expression fold-operator ... fold-operator cast-expression )
fold-operator: one of
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- / % ^ & | << >>
+= -= = /= %= ^= &= |= <<= >>= =
== != < > <= >= && || , . ->*
- / % ^ & | << >>
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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]).
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} â end example]
A fold expression is a pack expansion.