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std.meta
Templates to manipulate template argument lists (also known as type lists).
Some operations on alias sequences are built in to the language,
such as TL[n] which gets the nth type from the
alias sequence. TL[lwr .. upr] returns a new type
list that is a slice of the old one.
Several templates in this module use or operate on eponymous templates that
take a single argument and evaluate to a boolean constant. Such templates
are referred to as template predicates.
Category | Templates |
---|---|
Building blocks | Alias AliasSeq aliasSeqOf |
Alias sequence filtering | Erase EraseAll Filter NoDuplicates |
Alias sequence type hierarchy | DerivedToFront MostDerived |
Alias sequence transformation | Repeat Replace ReplaceAll Reverse staticMap staticSort |
Alias sequence searching | allSatisfy anySatisfy staticIndexOf |
Template predicates | templateAnd templateNot templateOr staticIsSorted |
Template instantiation | ApplyLeft ApplyRight |
References: Based on ideas in Table 3.1 from Modern C++ Design, Andrei Alexandrescu (Addison-Wesley Professional, 2001)
License:
Authors:
Source: std/meta.d
- template
AliasSeq
(TList...) - Creates a sequence of zero or more aliases. This is most commonly used as template parameters or arguments.Examples:
import std.meta; alias TL = AliasSeq!(int, double); int foo(TL td) // same as int foo(int, double); { return td[0] + cast(int)td[1]; }
Examples:alias TL = AliasSeq!(int, double); alias Types = AliasSeq!(TL, char); static assert(is(Types == AliasSeq!(int, double, char)));
Examples:Returns anAliasSeq
expression of Func being applied to every variadic template argument.auto ref ArgCall(alias Func, alias arg)() { return Func(arg); } template Map(alias Func, args...) { static if (args.length > 1) { alias Map = AliasSeq!(ArgCall!(Func, args[0]), Map!(Func, args[1 .. $])); } else { alias Map = ArgCall!(Func, args[0]); } } static int square(int arg) { return arg * arg; } static int refSquare(ref int arg) { arg *= arg; return arg; } static ref int refRetSquare(ref int arg) { arg *= arg; return arg; } static void test(int a, int b) { assert(a == 4); assert(b == 16); } static void testRef(ref int a, ref int b) { assert(a++ == 16); assert(b++ == 256); } static int a = 2; static int b = 4; test(Map!(square, a, b)); test(Map!(refSquare, a, b)); assert(a == 4); assert(b == 16); testRef(Map!(refRetSquare, a, b)); assert(a == 17); assert(b == 257);
- template
Alias
(alias a)
templateAlias
(T) - Allows aliasing of any single symbol, type or compile-time expression.Not everything can be directly aliased. An alias cannot be declared of - for example - a literal: alias a = 4; //Error With this template any single entity can be aliased: alias b =
Alias
!4; //OKSee Also:To alias more than one thing at once, use AliasSeqExamples:// Without Alias this would fail if Args[0] was e.g. a value and // some logic would be needed to detect when to use enum instead alias Head(Args ...) = Alias!(Args[0]); alias Tail(Args ...) = Args[1 .. $]; alias Blah = AliasSeq!(3, int, "hello"); static assert(Head!Blah == 3); static assert(is(Head!(Tail!Blah) == int)); static assert((Tail!Blah)[1] == "hello");
Examples:alias a = Alias!(123); static assert(a == 123); enum abc = 1; alias b = Alias!(abc); static assert(b == 1); alias c = Alias!(3 + 4); static assert(c == 7); alias concat = (s0, s1) => s0 ~ s1; alias d = Alias!(concat("Hello", " World!")); static assert(d == "Hello World!"); alias e = Alias!(int); static assert(is(e == int)); alias f = Alias!(AliasSeq!(int)); static assert(!is(typeof(f[0]))); //not an AliasSeq static assert(is(f == int)); auto g = 6; alias h = Alias!g; ++h; assert(g == 7);
- enum auto
staticIndexOf
(T, TList...);
enum autostaticIndexOf
(alias T, TList...); - Returns the index of the first occurrence of type T in the sequence of zero or more types TList. If not found, -1 is returned.Examples:
import std.stdio; void foo() { writefln("The index of long is %s", staticIndexOf!(long, AliasSeq!(int, long, double))); // prints: The index of long is 1 }
- template
Erase
(T, TList...)
templateErase
(alias T, TList...) - Returns a typetuple created from TList with the first occurrence, if any, of T removed.Examples:
alias Types = AliasSeq!(int, long, double, char); alias TL = Erase!(long, Types); static assert(is(TL == AliasSeq!(int, double, char)));
- template
EraseAll
(T, TList...)
templateEraseAll
(alias T, TList...) - Returns a typetuple created from TList with the all occurrences, if any, of T removed.Examples:
alias Types = AliasSeq!(int, long, long, int); alias TL = EraseAll!(long, Types); static assert(is(TL == AliasSeq!(int, int)));
- template
NoDuplicates
(TList...) - Returns a typetuple created from TList with the all duplicate types removed.Examples:
alias Types = AliasSeq!(int, long, long, int, float); alias TL = NoDuplicates!(Types); static assert(is(TL == AliasSeq!(int, long, float))); // Bugzilla 14561: huge enums alias LongList = Repeat!(1500, int); static assert(NoDuplicates!LongList.length == 1);
- template
Replace
(T, U, TList...)
templateReplace
(alias T, U, TList...)
templateReplace
(T, alias U, TList...)
templateReplace
(alias T, alias U, TList...) - Returns a typetuple created from TList with the first occurrence of type T, if found, replaced with type U.Examples:
alias Types = AliasSeq!(int, long, long, int, float); alias TL = Replace!(long, char, Types); static assert(is(TL == AliasSeq!(int, char, long, int, float)));
- template
ReplaceAll
(T, U, TList...)
templateReplaceAll
(alias T, U, TList...)
templateReplaceAll
(T, alias U, TList...)
templateReplaceAll
(alias T, alias U, TList...) - Returns a typetuple created from TList with all occurrences of type T, if found, replaced with type U.Examples:
alias Types = AliasSeq!(int, long, long, int, float); alias TL = ReplaceAll!(long, char, Types); static assert(is(TL == AliasSeq!(int, char, char, int, float)));
- template
Reverse
(TList...) - Returns a typetuple created from TList with the order reversed.Examples:
alias Types = AliasSeq!(int, long, long, int, float); alias TL = Reverse!(Types); static assert(is(TL == AliasSeq!(float, int, long, long, int)));
- template
MostDerived
(T, TList...) - Returns the type from TList that is the most derived from type T. If none are found, T is returned.Examples:
class A { } class B : A { } class C : B { } alias Types = AliasSeq!(A, C, B); MostDerived!(Object, Types) x; // x is declared as type C static assert(is(typeof(x) == C));
- template
DerivedToFront
(TList...) - Returns the typetuple TList with the types sorted so that the most derived types come first.Examples:
class A { } class B : A { } class C : B { } alias Types = AliasSeq!(A, C, B); alias TL = DerivedToFront!(Types); static assert(is(TL == AliasSeq!(C, B, A)));
- template
staticMap
(alias F, T...) - Evaluates to AliasSeq!(F!(T[0]), F!(T[1]), ..., F!(T[$ - 1])).Examples:
import std.traits : Unqual; alias TL = staticMap!(Unqual, int, const int, immutable int); static assert(is(TL == AliasSeq!(int, int, int)));
- template
allSatisfy
(alias F, T...) - Tests whether all given items satisfy a template predicate, i.e. evaluates to F!(T[0]) && F!(T[1]) && ... && F!(T[$ - 1]).Evaluation is not short-circuited if a
false
result is encountered; the template predicate must be instantiable with all the given items.Examples:import std.traits : isIntegral; static assert(!allSatisfy!(isIntegral, int, double)); static assert( allSatisfy!(isIntegral, int, long));
- template
anySatisfy
(alias F, T...) - Tests whether any given items satisfy a template predicate, i.e. evaluates to F!(T[0]) || F!(T[1]) || ... || F!(T[$ - 1]).Evaluation is not short-circuited if a
true
result is encountered; the template predicate must be instantiable with all the given items.Examples:import std.traits : isIntegral; static assert(!anySatisfy!(isIntegral, string, double)); static assert( anySatisfy!(isIntegral, int, double));
- template
Filter
(alias pred, TList...) - Filters an AliasSeq using a template predicate. Returns a AliasSeq of the elements which satisfy the predicate.Examples:
import std.traits : isNarrowString, isUnsigned; alias Types1 = AliasSeq!(string, wstring, dchar[], char[], dstring, int); alias TL1 = Filter!(isNarrowString, Types1); static assert(is(TL1 == AliasSeq!(string, wstring, char[]))); alias Types2 = AliasSeq!(int, byte, ubyte, dstring, dchar, uint, ulong); alias TL2 = Filter!(isUnsigned, Types2); static assert(is(TL2 == AliasSeq!(ubyte, uint, ulong)));
- template
templateNot
(alias pred) - Negates the passed template predicate.Examples:
import std.traits : isPointer; alias isNoPointer = templateNot!isPointer; static assert(!isNoPointer!(int*)); static assert(allSatisfy!(isNoPointer, string, char, float));
- template
templateAnd
(Preds...) - Combines several template predicates using logical AND, i.e. constructs a new predicate which evaluates to
true
for a given input T if and only if all of the passed predicates aretrue
for T.The predicates are evaluated from left to right, aborting evaluation in a short-cut manner if afalse
result is encountered, in which case the latter instantiations do not need to compile.Examples:import std.traits : isNumeric, isUnsigned; alias storesNegativeNumbers = templateAnd!(isNumeric, templateNot!isUnsigned); static assert(storesNegativeNumbers!int); static assert(!storesNegativeNumbers!string && !storesNegativeNumbers!uint); // An empty list of predicates always yields true. alias alwaysTrue = templateAnd!(); static assert(alwaysTrue!int);
- template
templateOr
(Preds...) - Combines several template predicates using logical OR, i.e. constructs a new predicate which evaluates to
true
for a given input T if and only at least one of the passed predicates istrue
for T.The predicates are evaluated from left to right, aborting evaluation in a short-cut manner if atrue
result is encountered, in which case the latter instantiations do not need to compile.Examples:import std.traits : isPointer, isUnsigned; alias isPtrOrUnsigned = templateOr!(isPointer, isUnsigned); static assert( isPtrOrUnsigned!uint && isPtrOrUnsigned!(short*)); static assert(!isPtrOrUnsigned!int && !isPtrOrUnsigned!(string)); // An empty list of predicates never yields true. alias alwaysFalse = templateOr!(); static assert(!alwaysFalse!int);
- template
aliasSeqOf
(alias range) - Converts an input range range to an alias sequence.Examples:
import std.algorithm.iteration : map; import std.algorithm.sorting : sort; import std.string : capitalize; struct S { int a; int c; int b; } alias capMembers = aliasSeqOf!([__traits(allMembers, S)].sort().map!capitalize()); static assert(capMembers[0] == "A"); static assert(capMembers[1] == "B"); static assert(capMembers[2] == "C");
Examples:static immutable REF = [0, 1, 2, 3]; foreach (I, V; aliasSeqOf!([0, 1, 2, 3])) { static assert(V == I); static assert(V == REF[I]); }
- template
ApplyLeft
(alias Template, args...)
templateApplyRight
(alias Template, args...) -
Behaves like the identity function when args is empty.Parameters:
Template template to partially apply args arguments to bind Returns:Template with arity smaller than or equal to TemplateExamples:// enum bool isImplicitlyConvertible(From, To) import std.traits : isImplicitlyConvertible; static assert(allSatisfy!( ApplyLeft!(isImplicitlyConvertible, ubyte), short, ushort, int, uint, long, ulong)); static assert(is(Filter!(ApplyRight!(isImplicitlyConvertible, short), ubyte, string, short, float, int) == AliasSeq!(ubyte, short)));
Examples:import std.traits : hasMember, ifTestable; struct T1 { bool foo; } struct T2 { struct Test { bool opCast(T : bool)() { return true; } } Test foo; } static assert(allSatisfy!(ApplyRight!(hasMember, "foo"), T1, T2)); static assert(allSatisfy!(ApplyRight!(ifTestable, a => a.foo), T1, T2));
Examples:import std.traits : Largest; alias Types = AliasSeq!(byte, short, int, long); static assert(is(staticMap!(ApplyLeft!(Largest, short), Types) == AliasSeq!(short, short, int, long))); static assert(is(staticMap!(ApplyLeft!(Largest, int), Types) == AliasSeq!(int, int, int, long)));
Examples:import std.traits : FunctionAttribute, SetFunctionAttributes; static void foo() @system; static int bar(int) @system; alias SafeFunctions = AliasSeq!( void function() @safe, int function(int) @safe); static assert(is(staticMap!(ApplyRight!( SetFunctionAttributes, "D", FunctionAttribute.safe), typeof(&foo), typeof(&bar)) == SafeFunctions));
- template
Repeat
(size_t n, TList...) if (n > 0) - Creates an AliasSeq which repeats a type or an AliasSeq exactly n times.Examples:
alias ImInt1 = Repeat!(1, immutable(int)); static assert(is(ImInt1 == AliasSeq!(immutable(int)))); alias Real3 = Repeat!(3, real); static assert(is(Real3 == AliasSeq!(real, real, real))); alias Real12 = Repeat!(4, Real3); static assert(is(Real12 == AliasSeq!(real, real, real, real, real, real, real, real, real, real, real, real))); alias Composite = AliasSeq!(uint, int); alias Composite2 = Repeat!(2, Composite); static assert(is(Composite2 == AliasSeq!(uint, int, uint, int)));
Examples:auto staticArray(T, size_t n)(Repeat!(n, T) elems) { T[n] a = [elems]; return a; } auto a = staticArray!(long, 3)(3, 1, 4); assert(is(typeof(a) == long[3])); assert(a == [3, 1, 4]);
- template
staticSort
(alias cmp, Seq...) - Sorts a AliasSeq using cmp.
Parameters: cmp = A template that returns a bool (if its first argument is less than the second one) or an int (-1 means less than, 0 means equal, 1 means greater than)
Seq = The AliasSeq to sortReturns:The sorted alias sequenceExamples:alias Nums = AliasSeq!(7, 2, 3, 23); enum Comp(int N1, int N2) = N1 < N2; static assert(AliasSeq!(2, 3, 7, 23) == staticSort!(Comp, Nums));
Examples:alias Types = AliasSeq!(uint, short, ubyte, long, ulong); enum Comp(T1, T2) = __traits(isUnsigned, T2) - __traits(isUnsigned, T1); static assert(is(AliasSeq!(uint, ubyte, ulong, short, long) == staticSort!(Comp, Types)));
- template
staticIsSorted
(alias cmp, Seq...) - Checks if an AliasSeq is sorted according to cmp.
Parameters: cmp = A template that returns a bool (if its first argument is less than the second one) or an int (-1 means less than, 0 means equal, 1 means greater than)
Seq = The AliasSeq to checkReturns:true if Seq is sorted; otherwise falseExamples:enum Comp(int N1, int N2) = N1 < N2; static assert( staticIsSorted!(Comp, 2, 2)); static assert( staticIsSorted!(Comp, 2, 3, 7, 23)); static assert(!staticIsSorted!(Comp, 7, 2, 3, 23));
Examples:enum Comp(T1, T2) = __traits(isUnsigned, T2) - __traits(isUnsigned, T1); static assert( staticIsSorted!(Comp, uint, ubyte, ulong, short, long)); static assert(!staticIsSorted!(Comp, uint, short, ubyte, long, ulong));
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