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Merge 2014-11 CWG Motion 13
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‎source/expressions.tex‎

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\terminal{this}\br
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\terminal{(} expression \terminal{)}\br
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id-expression\br
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lambda-expression
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lambda-expression\br
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fold-expression
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\end{bnf}
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\begin{bnf}
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\indextext{expression!lambda|)}%
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\indextext{expression!primary|)}
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\rSec2[expr.prim.fold]{Fold expressions}%
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\indextext{expression!fold|(}
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\pnum
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A fold expression performs a fold of a template parameter
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pack~(\ref{temp.variadic}) over a binary operator.
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\begin{bnf}
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\nontermdef{fold-expression}\br
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\terminal{(} cast-expression fold-operator \terminal{...} \terminal{)}\br
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\terminal{(} \terminal{...} fold-operator cast-expression \terminal{)}\br
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\terminal{(} cast-expression fold-operator \terminal{...} fold-operator cast-expression \terminal{)}
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\end{bnf}
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\begin{bnf}
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\nontermdef{fold-operator} \textnormal{one of}\br
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\terminal{+ }\quad\terminal{- }\quad\terminal{* }\quad\terminal{/ }\quad\terminal{\% }\quad\terminal{\^{} }\quad\terminal{\& }\quad\terminal{| }\quad\terminal{\shl\ }\quad\terminal{\shr }\br
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\terminal{+=}\quad\terminal{-=}\quad\terminal{*=}\quad\terminal{/=}\quad\terminal{\%=}\quad\terminal{\^{}=}\quad\terminal{\&=}\quad\terminal{|=}\quad\terminal{\shl=}\quad\terminal{\shr=}\quad\terminal{=}\br
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\terminal{==}\quad\terminal{!=}\quad\terminal{< }\quad\terminal{> }\quad\terminal{<=}\quad\terminal{>=}\quad\terminal{\&\&}\quad\terminal{||}\quad\terminal{, }\quad\terminal{.* }\quad\terminal{->*}
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\end{bnf}
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\pnum
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\indextext{fold!unary}%
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An expression of the form
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\tcode{(...} \placeholder{op} \tcode{e)}
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where \placeholder{op} is a \grammarterm{fold-operator}
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is called a \defn{unary left fold}.
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An expression of the form
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\tcode{(e} \placeholder{op} \tcode{...)}
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where \placeholder{op} is a \grammarterm{fold-operator}
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is called a \defn{unary right fold}.
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Unary left folds and unary right folds
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are collectively called \defnx{unary folds}{unary fold}.
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In a unary fold,
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the \grammarterm{cast-expression}
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shall contain an unexpanded parameter pack~(\ref{temp.variadic}).
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\pnum
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\indextext{fold!binary}%
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An expression of the form
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\tcode{(e1} \placeholder{op1} \tcode{...} \placeholder{op2} \tcode{e2)}
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where \placeholder{op1} and \placeholder{op2} are \grammarterm{fold-operator}{s}
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is called a \defn{binary fold}.
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In a binary fold,
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\placeholder{op1} and \placeholder{op2}
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shall be the same \grammarterm{fold-operator},
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and either \tcode{e1}
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shall contain an unexpanded parameter pack
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or \tcode{e2}
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shall contain an unexpanded parameter pack,
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but not both.
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If \tcode{e2} contains an unexpanded parameter pack,
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the expression is called a \defn{binary left fold}.
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If \tcode{e1} contains an unexpanded parameter pack,
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the expression is called a \defn{binary right fold}.
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\enterexample
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\begin{codeblock}
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template<typename ...Args>
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bool f(Args ...args) {
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return (true && ... && args); // OK
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}
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template<typename ...Args>
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bool f(Args ...args) {
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return (args + ... + args); // error: both operands contain unexpanded parameter packs
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}
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\end{codeblock}
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\exitexample
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\indextext{expression!fold|)}
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\rSec1[expr.post]{Postfix expressions}%
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\indextext{expression!postfix|(}
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‎source/templates.tex‎

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\item In a \tcode{sizeof...} expression~(\ref{expr.sizeof}); the pattern is an
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\grammarterm{identifier}.
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\item In a \grammarterm{fold-expression} (\ref{expr.prim.fold});
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the pattern is the \grammarterm{cast-expression}
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that contains an unexpanded parameter pack.
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\end{itemize}
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\pnum
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\pnum
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The instantiation of a pack expansion
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that is not a \tcode{sizeof...} expression
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that is neither a \tcode{sizeof...} expression
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nor a \grammarterm{fold-expression}
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produces a
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list
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$\mathtt{E}_1, \mathtt{E}_2, ..., \mathtt{E}_N$,
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an integral constant containing the number of elements in the parameter pack
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it expands.
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\pnum
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The instantiation of a \grammarterm{fold-expression} produces:
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\begin{itemize}
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\item
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\tcode{((}$\mathtt{E}_1$
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\placeholder{op} $\mathtt{E}_2$\tcode{)}
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\placeholder{op} $\cdots$\tcode{)}
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\placeholder{op} $\mathtt{E}_N$
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for a unary left fold,
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\item
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$\mathtt{E}_1$ \placeholder{op}
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\tcode{(}$\cdots$ \placeholder{op}
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\tcode{(}$\mathtt{E}_{N-1}$ \placeholder{op}
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$\mathtt{E}_N$\tcode{))}
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for a unary right fold,
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\item
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\tcode{(((}$\mathtt{E}$
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\placeholder{op} $\mathtt{E}_1$\tcode{)}
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\placeholder{op} $\mathtt{E}_2$\tcode{)}
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\placeholder{op} $\cdots$\tcode{)}
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\placeholder{op} $\mathtt{E}_N$
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for a binary left fold, and
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\item
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$\mathtt{E}_1$ \placeholder{op}
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\tcode{(}$\cdots$ \placeholder{op}
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\tcode{(}$\mathtt{E}_{N-1}$ \placeholder{op}
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\tcode{(}$\mathtt{E}_{N}$ \placeholder{op}
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$\mathtt{E}$\tcode{)))}
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for a binary right fold.
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\end{itemize}
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In each case,
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\placeholder{op} is the \grammarterm{fold-operator},
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$N$ is the number of elements in the pack expansion parameters,
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and each $\mathtt{E}_i$ is generated by instantiating the pattern
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and replacing each pack expansion parameter with its $i$th element.
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For a binary fold-expression,
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$\mathtt{E}$ is generated
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by instantiating the \grammarterm{cast-expression}
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that did not contain an unexpanded parameter pack.
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\enterexample
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\begin{codeblock}
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template<typename ...Args>
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bool all(Args ...args) { return (... && args); }
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bool b = all(true, true, true, false);
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\end{codeblock}
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Within the instantiation of \tcode{all},
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the returned expression expands to
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\tcode{((true \&\& true) \&\& true) \&\& false},
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which evalutes to \tcode{false}.
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\exitexample
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If $N$ is zero for a unary fold-expression,
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the value of the expression is shown in Table~\ref{tab:fold.empty};
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if the operator is not listed in Table~\ref{tab:fold.empty},
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the instantiation is ill-formed.
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\begin{floattable}{Value of folding empty sequences}{tab:fold.empty}
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{ll}
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\topline
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\lhdr{Operator} & \rhdr{Value when parameter pack is empty} \\
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\capsep
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\tcode{*} & \tcode{1} \\
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\tcode{+} & \tcode{int()} \\
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\tcode{\&} & \tcode{-1} \\
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\tcode{|} & \tcode{int()} \\
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\tcode{\&\&} & \tcode{true} \\
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\tcode{||} & \tcode{false} \\
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\tcode{,} & \tcode{void()} \\
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\end{floattable}
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\rSec2[temp.friend]{Friends}
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\pnum
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A \grammarterm{braced-init-list} is type-dependent if any element is
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type-dependent or is a pack expansion.
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\pnum
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A \grammarterm{fold-expression} is type-dependent.
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\rSec3[temp.dep.constexpr]{Value-dependent expressions}
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\pnum
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Expressions of the following form are value-dependent:
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\begin{ncbnftab}
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\terminal{sizeof} \terminal{...} \terminal{(} identifier \terminal{)}
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\terminal{sizeof} \terminal{...} \terminal{(} identifier \terminal{)}\br
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fold-expression
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\end{ncbnftab}
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\pnum

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