std.meta

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)));

enum staticIndexOf(T, TList...);
enum 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
}

alias IndexOf = staticIndexOf(T, TList...);

Kept for backwards compatibility

template Erase(T, TList...)
template Erase(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...)
template EraseAll(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)));

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 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...)
template ReplaceAll(alias T, U, TList...)
template ReplaceAll(T, alias U, TList...)
template ReplaceAll(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 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);