# 7 Expressions [expr]

## 7.5 Primary expressions [expr.prim]

### 7.5.6 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
+   -   *   /   %   ^   &   |   <<   >>
+=  -=  *=  /=  %=  ^=  &=  |=  <<=  >>=  =
==  !=  <   >   <=  >=  &&  ||  ,    .*   ->*
```
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
:
```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
]