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 Stride
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:

Boost License 1.0.

Authors:

Walter Bright, David Nadlinger

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.

In previous versions of Phobos, this was known as TypeTuple.

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 an AliasSeq 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)
{
    writeln(a); // 4
    writeln(b); // 16
}

static void testRef(ref int a, ref int b)
{
    writeln(a++); // 16
    writeln(b++); // 256
}

static int a = 2;
static int b = 4;

test(Map!(square, a, b));

test(Map!(refSquare, a, b));
writeln(a); // 4
writeln(b); // 16

testRef(Map!(refRetSquare, a, b));
writeln(a); // 17
writeln(b); // 257

template Alias(alias a)template Alias(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; //OK

See Also:

To alias more than one thing at once, use AliasSeq

Examples:

// 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;
writeln(g); // 7

enum auto staticIndexOf(T, TList...)
enum auto staticIndexOf(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...)template Erase(alias T, TList...)

Returns an AliasSeq 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...)template EraseAll(alias T, TList...)

Returns an AliasSeq 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 an AliasSeq 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)));

template Replace(T, U, TList...)template Replace(alias T, U, TList...)template Replace(T, alias U, TList...)template Replace(alias T, alias U, TList...)

Returns an AliasSeq 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...)template ReplaceAll(alias T, U, TList...)template ReplaceAll(T, alias U, TList...)template ReplaceAll(alias T, alias U, TList...)

Returns an AliasSeq 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 an AliasSeq 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 AliasSeq 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 short-circuited if a true result is encountered; the template predicate must be instantiable with one of 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 are true for T.

The predicates are evaluated from left to right, aborting evaluation in a short-cut manner if a false 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 is true for T.

The predicates are evaluated from left to right, aborting evaluation in a short-cut manner if a true 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...)template ApplyRight(alias Template, args...)

Partially applies Template by binding its first (left) or last (right) arguments to 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 Template

Examples:

// 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 &gt
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]));
writeln(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 sort

Returns:

The sorted alias sequence

Examples:

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 check

Returns:

true if Seq is sorted; otherwise false

Examples:

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));

template Stride(int stepSize, Args...) if (stepSize != 0)

Selects a subset of the argument list by stepping with fixed stepSize over the list. A negative stepSize starts iteration with the last list element.

Parameters:

stepSize	Number of elements to increment on each iteration. Can't be 0.
Args	Template arguments

Returns:

A template argument list filtered by the selected stride.

Examples:

static assert(is(Stride!(1, short, int, long) == AliasSeq!(short, int, long)));
static assert(is(Stride!(2, short, int, long) == AliasSeq!(short, long)));
static assert(is(Stride!(-1, short, int, long) == AliasSeq!(long, int, short)));
static assert(is(Stride!(-2, short, int, long) == AliasSeq!(long, short)));

alias attribs = AliasSeq!(short, int, long, ushort, uint, ulong);
static assert(is(Stride!(3, attribs) == AliasSeq!(short, ushort)));
static assert(is(Stride!(3, attribs[1 .. $]) == AliasSeq!(int, uint)));
static assert(is(Stride!(-3, attribs) == AliasSeq!(ulong, long)));