tmp/tmpo7o0lm5v/{from.md → to.md}
RENAMED
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| 1 |
+
## Deduction guides <a id="temp.deduct.guide">[[temp.deduct.guide]]</a>
|
| 2 |
+
|
| 3 |
+
Deduction guides are used when a *template-name* appears as a type
|
| 4 |
+
specifier for a deduced class type ([[dcl.type.class.deduct]]).
|
| 5 |
+
Deduction guides are not found by name lookup. Instead, when performing
|
| 6 |
+
class template argument deduction ([[over.match.class.deduct]]), any
|
| 7 |
+
deduction guides declared for the class template are considered.
|
| 8 |
+
|
| 9 |
+
``` bnf
|
| 10 |
+
deduction-guide:
|
| 11 |
+
'explicit'ₒₚₜ template-name '(' parameter-declaration-clause ') ->' simple-template-id ';'
|
| 12 |
+
```
|
| 13 |
+
|
| 14 |
+
[*Example 1*:
|
| 15 |
+
|
| 16 |
+
``` cpp
|
| 17 |
+
template<class T, class D = int>
|
| 18 |
+
struct S {
|
| 19 |
+
T data;
|
| 20 |
+
};
|
| 21 |
+
template<class U>
|
| 22 |
+
S(U) -> S<typename U::type>;
|
| 23 |
+
|
| 24 |
+
struct A {
|
| 25 |
+
using type = short;
|
| 26 |
+
operator type();
|
| 27 |
+
};
|
| 28 |
+
S x{A()}; // x is of type S<short, int>
|
| 29 |
+
```
|
| 30 |
+
|
| 31 |
+
— *end example*]
|
| 32 |
+
|
| 33 |
+
The same restrictions apply to the *parameter-declaration-clause* of a
|
| 34 |
+
deduction guide as in a function declaration ([[dcl.fct]]). The
|
| 35 |
+
*simple-template-id* shall name a class template specialization. The
|
| 36 |
+
*template-name* shall be the same *identifier* as the *template-name* of
|
| 37 |
+
the *simple-template-id*. A *deduction-guide* shall be declared in the
|
| 38 |
+
same scope as the corresponding class template and, for a member class
|
| 39 |
+
template, with the same access. Two deduction guide declarations in the
|
| 40 |
+
same translation unit for the same class template shall not have
|
| 41 |
+
equivalent *parameter-declaration-clause*s.
|
| 42 |
+
|
| 43 |
+
<!-- Link reference definitions -->
|
| 44 |
+
[basic.def]: basic.md#basic.def
|
| 45 |
+
[basic.def.odr]: basic.md#basic.def.odr
|
| 46 |
+
[basic.link]: basic.md#basic.link
|
| 47 |
+
[basic.lookup]: basic.md#basic.lookup
|
| 48 |
+
[basic.lookup.argdep]: basic.md#basic.lookup.argdep
|
| 49 |
+
[basic.lookup.classref]: basic.md#basic.lookup.classref
|
| 50 |
+
[basic.lookup.qual]: basic.md#basic.lookup.qual
|
| 51 |
+
[basic.lookup.unqual]: basic.md#basic.lookup.unqual
|
| 52 |
+
[basic.scope]: basic.md#basic.scope
|
| 53 |
+
[basic.scope.hiding]: basic.md#basic.scope.hiding
|
| 54 |
+
[basic.stc.dynamic.deallocation]: basic.md#basic.stc.dynamic.deallocation
|
| 55 |
+
[basic.types]: basic.md#basic.types
|
| 56 |
+
[class]: class.md#class
|
| 57 |
+
[class.abstract]: class.md#class.abstract
|
| 58 |
+
[class.access]: class.md#class.access
|
| 59 |
+
[class.base.init]: special.md#class.base.init
|
| 60 |
+
[class.derived]: class.md#class.derived
|
| 61 |
+
[class.dtor]: special.md#class.dtor
|
| 62 |
+
[class.friend]: class.md#class.friend
|
| 63 |
+
[class.local]: class.md#class.local
|
| 64 |
+
[class.mem]: class.md#class.mem
|
| 65 |
+
[class.member.lookup]: class.md#class.member.lookup
|
| 66 |
+
[class.qual]: basic.md#class.qual
|
| 67 |
+
[class.temporary]: special.md#class.temporary
|
| 68 |
+
[conv]: conv.md#conv
|
| 69 |
+
[conv.array]: conv.md#conv.array
|
| 70 |
+
[conv.fctptr]: conv.md#conv.fctptr
|
| 71 |
+
[conv.func]: conv.md#conv.func
|
| 72 |
+
[conv.qual]: conv.md#conv.qual
|
| 73 |
+
[dcl.align]: dcl.md#dcl.align
|
| 74 |
+
[dcl.attr.grammar]: dcl.md#dcl.attr.grammar
|
| 75 |
+
[dcl.dcl]: dcl.md#dcl.dcl
|
| 76 |
+
[dcl.enum]: dcl.md#dcl.enum
|
| 77 |
+
[dcl.fct]: dcl.md#dcl.fct
|
| 78 |
+
[dcl.fct.def.general]: dcl.md#dcl.fct.def.general
|
| 79 |
+
[dcl.fct.default]: dcl.md#dcl.fct.default
|
| 80 |
+
[dcl.init]: dcl.md#dcl.init
|
| 81 |
+
[dcl.init.list]: dcl.md#dcl.init.list
|
| 82 |
+
[dcl.meaning]: dcl.md#dcl.meaning
|
| 83 |
+
[dcl.spec.auto]: dcl.md#dcl.spec.auto
|
| 84 |
+
[dcl.struct.bind]: dcl.md#dcl.struct.bind
|
| 85 |
+
[dcl.type.auto.deduct]: dcl.md#dcl.type.auto.deduct
|
| 86 |
+
[dcl.type.class.deduct]: dcl.md#dcl.type.class.deduct
|
| 87 |
+
[dcl.type.elab]: dcl.md#dcl.type.elab
|
| 88 |
+
[except.spec]: except.md#except.spec
|
| 89 |
+
[expr.const]: expr.md#expr.const
|
| 90 |
+
[expr.new]: expr.md#expr.new
|
| 91 |
+
[expr.prim.fold]: expr.md#expr.prim.fold
|
| 92 |
+
[expr.prim.lambda]: expr.md#expr.prim.lambda
|
| 93 |
+
[expr.prim.lambda.closure]: expr.md#expr.prim.lambda.closure
|
| 94 |
+
[expr.ref]: expr.md#expr.ref
|
| 95 |
+
[expr.sizeof]: expr.md#expr.sizeof
|
| 96 |
+
[expr.typeid]: expr.md#expr.typeid
|
| 97 |
+
[implimits]: limits.md#implimits
|
| 98 |
+
[intro.defs]: intro.md#intro.defs
|
| 99 |
+
[intro.object]: intro.md#intro.object
|
| 100 |
+
[lex.string]: lex.md#lex.string
|
| 101 |
+
[namespace.def]: dcl.md#namespace.def
|
| 102 |
+
[namespace.memdef]: dcl.md#namespace.memdef
|
| 103 |
+
[namespace.udecl]: dcl.md#namespace.udecl
|
| 104 |
+
[over.ics.rank]: over.md#over.ics.rank
|
| 105 |
+
[over.match]: over.md#over.match
|
| 106 |
+
[over.match.best]: over.md#over.match.best
|
| 107 |
+
[over.match.class.deduct]: over.md#over.match.class.deduct
|
| 108 |
+
[over.match.conv]: over.md#over.match.conv
|
| 109 |
+
[over.match.ref]: over.md#over.match.ref
|
| 110 |
+
[over.over]: over.md#over.over
|
| 111 |
+
[special]: special.md#special
|
| 112 |
+
[stmt.if]: stmt.md#stmt.if
|
| 113 |
+
[support.types]: language.md#support.types
|
| 114 |
+
[tab:fold.empty]: #tab:fold.empty
|
| 115 |
+
[temp]: #temp
|
| 116 |
+
[temp.alias]: #temp.alias
|
| 117 |
+
[temp.arg]: #temp.arg
|
| 118 |
+
[temp.arg.explicit]: #temp.arg.explicit
|
| 119 |
+
[temp.arg.nontype]: #temp.arg.nontype
|
| 120 |
+
[temp.arg.template]: #temp.arg.template
|
| 121 |
+
[temp.arg.type]: #temp.arg.type
|
| 122 |
+
[temp.class]: #temp.class
|
| 123 |
+
[temp.class.order]: #temp.class.order
|
| 124 |
+
[temp.class.spec]: #temp.class.spec
|
| 125 |
+
[temp.class.spec.match]: #temp.class.spec.match
|
| 126 |
+
[temp.class.spec.mfunc]: #temp.class.spec.mfunc
|
| 127 |
+
[temp.decls]: #temp.decls
|
| 128 |
+
[temp.deduct]: #temp.deduct
|
| 129 |
+
[temp.deduct.call]: #temp.deduct.call
|
| 130 |
+
[temp.deduct.conv]: #temp.deduct.conv
|
| 131 |
+
[temp.deduct.decl]: #temp.deduct.decl
|
| 132 |
+
[temp.deduct.funcaddr]: #temp.deduct.funcaddr
|
| 133 |
+
[temp.deduct.guide]: #temp.deduct.guide
|
| 134 |
+
[temp.deduct.partial]: #temp.deduct.partial
|
| 135 |
+
[temp.deduct.type]: #temp.deduct.type
|
| 136 |
+
[temp.dep]: #temp.dep
|
| 137 |
+
[temp.dep.candidate]: #temp.dep.candidate
|
| 138 |
+
[temp.dep.constexpr]: #temp.dep.constexpr
|
| 139 |
+
[temp.dep.expr]: #temp.dep.expr
|
| 140 |
+
[temp.dep.res]: #temp.dep.res
|
| 141 |
+
[temp.dep.temp]: #temp.dep.temp
|
| 142 |
+
[temp.dep.type]: #temp.dep.type
|
| 143 |
+
[temp.expl.spec]: #temp.expl.spec
|
| 144 |
+
[temp.explicit]: #temp.explicit
|
| 145 |
+
[temp.fct]: #temp.fct
|
| 146 |
+
[temp.fct.spec]: #temp.fct.spec
|
| 147 |
+
[temp.friend]: #temp.friend
|
| 148 |
+
[temp.func.order]: #temp.func.order
|
| 149 |
+
[temp.inject]: #temp.inject
|
| 150 |
+
[temp.inst]: #temp.inst
|
| 151 |
+
[temp.local]: #temp.local
|
| 152 |
+
[temp.mem]: #temp.mem
|
| 153 |
+
[temp.mem.class]: #temp.mem.class
|
| 154 |
+
[temp.mem.enum]: #temp.mem.enum
|
| 155 |
+
[temp.mem.func]: #temp.mem.func
|
| 156 |
+
[temp.names]: #temp.names
|
| 157 |
+
[temp.nondep]: #temp.nondep
|
| 158 |
+
[temp.over]: #temp.over
|
| 159 |
+
[temp.over.link]: #temp.over.link
|
| 160 |
+
[temp.param]: #temp.param
|
| 161 |
+
[temp.point]: #temp.point
|
| 162 |
+
[temp.res]: #temp.res
|
| 163 |
+
[temp.spec]: #temp.spec
|
| 164 |
+
[temp.static]: #temp.static
|
| 165 |
+
[temp.type]: #temp.type
|
| 166 |
+
[temp.variadic]: #temp.variadic
|
| 167 |
+
|
| 168 |
+
[^1]: Since template *template-parameter*s and template
|
| 169 |
+
*template-argument*s are treated as types for descriptive purposes,
|
| 170 |
+
the terms *non-type parameter* and *non-type argument* are used to
|
| 171 |
+
refer to non-type, non-template parameters and arguments.
|
| 172 |
+
|
| 173 |
+
[^2]: A `>` that encloses the *type-id* of a `dynamic_cast`,
|
| 174 |
+
`static_cast`, `reinterpret_cast` or `const_cast`, or which encloses
|
| 175 |
+
the *template-argument*s of a subsequent *template-id*, is
|
| 176 |
+
considered nested for the purpose of this description.
|
| 177 |
+
|
| 178 |
+
[^3]: There is no such ambiguity in a default *template-argument*
|
| 179 |
+
because the form of the *template-parameter* determines the
|
| 180 |
+
allowable forms of the *template-argument*.
|
| 181 |
+
|
| 182 |
+
[^4]: There is no way in which they could be used.
|
| 183 |
+
|
| 184 |
+
[^5]: That is, declarations of non-template functions do not merely
|
| 185 |
+
guide overload resolution of function template specializations with
|
| 186 |
+
the same name. If such a non-template function is odr-used (
|
| 187 |
+
[[basic.def.odr]]) in a program, it must be defined; it will not be
|
| 188 |
+
implicitly instantiated using the function template definition.
|
| 189 |
+
|
| 190 |
+
[^6]: Friend declarations do not introduce new names into any scope,
|
| 191 |
+
either when the template is declared or when it is instantiated.
|
| 192 |
+
|
| 193 |
+
[^7]: Default arguments are not considered to be arguments in this
|
| 194 |
+
context; they only become arguments after a function has been
|
| 195 |
+
selected.
|
| 196 |
+
|
| 197 |
+
[^8]: Although the *template-argument* corresponding to a
|
| 198 |
+
*template-parameter* of type `bool` may be deduced from an array
|
| 199 |
+
bound, the resulting value will always be `true` because the array
|
| 200 |
+
bound will be nonzero.
|
| 201 |
+
|
| 202 |
+
[^9]: The parameters of function template specializations contain no
|
| 203 |
+
template parameter types. The set of conversions allowed on deduced
|
| 204 |
+
arguments is limited, because the argument deduction process
|
| 205 |
+
produces function templates with parameters that either match the
|
| 206 |
+
call arguments exactly or differ only in ways that can be bridged by
|
| 207 |
+
the allowed limited conversions. Non-deduced arguments allow the
|
| 208 |
+
full range of conversions. Note also that [[over.match.best]]
|
| 209 |
+
specifies that a non-template function will be given preference over
|
| 210 |
+
a template specialization if the two functions are otherwise equally
|
| 211 |
+
good candidates for an overload match.
|