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std.traits

Templates which extract information about types and symbols at compile time.

License:
Authors:
Walter Bright, Tomasz Stachowiak (isExpressionTuple), Andrei Alexandrescu, Shin Fujishiro, Robert Clipsham, David Nadlinger, Kenji Hara, Shoichi Kato

Source: std/traits.d

template packageName(alias T)
Get the full package name for the given symbol.
Examples:
import std.traits;
static assert(packageName!packageName == "std");
template moduleName(alias T)
Get the module name (including package) for the given symbol.
Examples:
import std.traits;
static assert(moduleName!moduleName == "std.traits");
template fullyQualifiedName(T...) if (T.length == 1)
Get the fully qualified name of a type or a symbol. Can act as an intelligent type/symbol to string converter.

Example:

module myModule;
struct MyStruct {}
static assert(fullyQualifiedName!(const MyStruct[]) == "const(myModule.MyStruct[])");
Examples:
static assert(fullyQualifiedName!fullyQualifiedName == "std.traits.fullyQualifiedName");
template ReturnType(func...) if (func.length == 1 && isCallable!func)
Get the type of the return value from a function, a pointer to function, a delegate, a struct with an opCall, a pointer to a struct with an opCall, or a class with an opCall. Please note that ref is not part of a type, but the attribute of the function (see template functionAttributes).
Examples:
int foo();
ReturnType!foo x;   // x is declared as int
template ParameterTypeTuple(func...) if (func.length == 1 && isCallable!func)
Get, as a tuple, the types of the parameters to a function, a pointer to function, a delegate, a struct with an opCall, a pointer to a struct with an opCall, or a class with an opCall.
Examples:
int foo(int, long);
void bar(ParameterTypeTuple!foo);      // declares void bar(int, long);
void abc(ParameterTypeTuple!foo[1]);   // declares void abc(long);
template arity(alias func) if (isCallable!func && variadicFunctionStyle!func == Variadic.no)
Returns the number of arguments of function func. arity is undefined for variadic functions.
Examples:
void foo(){}
static assert(arity!foo==0);
void bar(uint){}
static assert(arity!bar==1);
void variadicFoo(uint...){}
static assert(__traits(compiles,arity!variadicFoo)==false);
enum ParameterStorageClass: uint;
template ParameterStorageClassTuple(func...) if (func.length == 1 && isCallable!func)
Returns a tuple consisting of the storage classes of the parameters of a function func.
Examples:
alias STC = ParameterStorageClass; // shorten the enum name

void func(ref int ctx, out real result, real param)
{
}
alias pstc = ParameterStorageClassTuple!func;
static assert(pstc.length == 3); // three parameters
static assert(pstc[0] == STC.ref_);
static assert(pstc[1] == STC.out_);
static assert(pstc[2] == STC.none);
none
scope_
out_
ref_
lazy_
return_
These flags can be bitwise OR-ed together to represent complex storage class.
template ParameterIdentifierTuple(func...) if (func.length == 1 && isCallable!func)
Get, as a tuple, the identifiers of the parameters to a function symbol.
Examples:
int foo(int num, string name, int);
static assert([ParameterIdentifierTuple!foo] == ["num", "name", ""]);
template ParameterDefaultValueTuple(func...) if (func.length == 1 && isCallable!func)
Get, as a tuple, the default value of the parameters to a function symbol. If a parameter doesn't have the default value, void is returned instead.
Examples:
int foo(int num, string name = "hello", int[] = [1,2,3]);
static assert(is(ParameterDefaultValueTuple!foo[0] == void));
static assert(   ParameterDefaultValueTuple!foo[1] == "hello");
static assert(   ParameterDefaultValueTuple!foo[2] == [1,2,3]);
enum FunctionAttribute: uint;
template functionAttributes(func...) if (func.length == 1 && isCallable!func)
Returns the attributes attached to a function func.
Examples:
import std.traits : functionAttributes, FunctionAttribute;

alias FA = FunctionAttribute; // shorten the enum name

real func(real x) pure nothrow @safe
{
    return x;
}
static assert(functionAttributes!func & FA.pure_);
static assert(functionAttributes!func & FA.safe);
static assert(!(functionAttributes!func & FA.trusted)); // not @trusted
none
pure_
nothrow_
ref_
property
trusted
safe
nogc
system
const_
immutable_
inout_
shared_
return_
These flags can be bitwise OR-ed together to represent a complex attribute.
template isSafe(alias func) if (isCallable!func)
true if func is @safe or @trusted.
Examples:
@safe    int add(int a, int b) {return a+b;}
@trusted int sub(int a, int b) {return a-b;}
@system  int mul(int a, int b) {return a*b;}

static assert( isSafe!add);
static assert( isSafe!sub);
static assert(!isSafe!mul);
template isUnsafe(alias func)
true if func is @system.
Examples:
@safe    int add(int a, int b) {return a+b;}
@trusted int sub(int a, int b) {return a-b;}
@system  int mul(int a, int b) {return a*b;}

static assert(!isUnsafe!add);
static assert(!isUnsafe!sub);
static assert( isUnsafe!mul);
template areAllSafe(funcs...) if (funcs.length > 0)
Deprecated. It's badly named and provides redundant functionality. It was also badly broken prior to 2.060 (bug# 8362), so any code which uses it probably needs to be changed anyway. Please use allSatisfy(isSafe, ...) instead. This will be removed in June 2015.
true all functions are isSafe.

Example
@safe    int add(int a, int b) {return a+b;}
@trusted int sub(int a, int b) {return a-b;}
@system  int mul(int a, int b) {return a*b;}

static assert( areAllSafe!(add, add));
static assert( areAllSafe!(add, sub));
static assert(!areAllSafe!(sub, mul));
template functionLinkage(func...) if (func.length == 1 && isCallable!func)
Returns the calling convention of function as a string.
Examples:
import std.stdio : writeln, printf;

string a = functionLinkage!(writeln!(string, int));
assert(a == "D"); // extern(D)

auto fp = &printf;
string b = functionLinkage!fp;
assert(b == "C"); // extern(C)
enum Variadic: int;
template variadicFunctionStyle(func...) if (func.length == 1 && isCallable!func)
Determines what kind of variadic parameters function has.
Examples:
void func() {}
static assert(variadicFunctionStyle!func == Variadic.no);

extern(C) int printf(in char*, ...);
static assert(variadicFunctionStyle!printf == Variadic.c);
no
Function is not variadic.
c
Function is a C-style variadic function.
d
Function is a D-style variadic function, which uses
_argptr and _arguments.
typesafe
Function is a typesafe variadic function.
template FunctionTypeOf(func...) if (func.length == 1 && isCallable!func)
Get the function type from a callable object func.
Using builtin typeof on a property function yields the types of the property value, not of the property function itself. Still, FunctionTypeOf is able to obtain function types of properties.

Note: Do not confuse function types with function pointer types; function types are usually used for compile-time reflection purposes.

Examples:
class C
{
    int value() @property { return 0; }
}
static assert(is( typeof(C.value) == int ));
static assert(is( FunctionTypeOf!(C.value) == function ));
template SetFunctionAttributes(T, string linkage, uint attrs) if (isFunctionPointer!T || isDelegate!T)
template SetFunctionAttributes(T, string linkage, uint attrs) if (is(T == function))
Constructs a new function or delegate type with the same basic signature as the given one, but different attributes (including linkage).
This is especially useful for adding/removing attributes to/from types in generic code, where the actual type name cannot be spelt out.
Parameters:
T The base type.
linkage The desired linkage of the result type.
attrs The desired FunctionAttributes of the result type.
Examples:
alias ExternC(T) = SetFunctionAttributes!(T, "C", functionAttributes!T);

auto assumePure(T)(T t)
    if (isFunctionPointer!T || isDelegate!T)
{
    enum attrs = functionAttributes!T | FunctionAttribute.pure_;
    return cast(SetFunctionAttributes!(T, functionLinkage!T, attrs)) t;
}
template isNested(T) if (is(T == class) || is(T == struct) || is(T == union))
Determines whether T has its own context pointer. T must be either class, struct, or union.
Examples:
static struct S { }
static assert(!isNested!S);

int i;
struct NestedStruct { void f() { ++i; } }
static assert(isNested!NestedStruct);
template hasNested(T)
Determines whether T or any of its representation types have a context pointer.
Examples:
static struct S { }

int i;
struct NS { void f() { ++i; } }

static assert(!hasNested!(S[2]));
static assert(hasNested!(NS[2]));
template FieldTypeTuple(T)
Get as a typetuple the types of the fields of a struct, class, or union. This consists of the fields that take up memory space, excluding the hidden fields like the virtual function table pointer or a context pointer for nested types. If T isn't a struct, class, or union returns typetuple with one element T.
Examples:
struct S { int x; float y; }
static assert(is(FieldTypeTuple!S == TypeTuple!(int, float)));
template FieldNameTuple(T)
Get as an expression tuple the names of the fields of a struct, class, or union. This consists of the fields that take up memory space, excluding the hidden fields like the virtual function table pointer or a context pointer for nested types. If T isn't a struct, class, or union returns an expression tuple with an empty string.
Examples:
struct S { int x; float y; }
static assert(FieldNameTuple!S == TypeTuple!("x", "y"));
static assert(FieldNameTuple!int == TypeTuple!"");
template RepresentationTypeTuple(T)
Get the primitive types of the fields of a struct or class, in topological order.
Examples:
struct S1 { int a; float b; }
struct S2 { char[] a; union { S1 b; S1 * c; } }
alias R = RepresentationTypeTuple!S2;
assert(R.length == 4
    && is(R[0] == char[]) && is(R[1] == int)
    && is(R[2] == float) && is(R[3] == S1*));
template hasAliasing(T...)
Returns true if and only if T's representation includes at least one of the following:
  1. a raw pointer U* and U is not immutable;
  2. an array U[] and U is not immutable;
  3. a reference to a class or interface type C and C is not immutable.
  4. an associative array that is not immutable.
  5. a delegate.
Examples:
struct S1 { int a; Object b; }
struct S2 { string a; }
struct S3 { int a; immutable Object b; }
struct S4 { float[3] vals; }
static assert( hasAliasing!S1);
static assert(!hasAliasing!S2);
static assert(!hasAliasing!S3);
static assert(!hasAliasing!S4);
template hasIndirections(T)
Returns true if and only if T's representation includes at least one of the following:
  1. a raw pointer U*;
  2. an array U[];
  3. a reference to a class type C.
  4. an associative array.
  5. a delegate.
Examples:
static assert( hasIndirections!(int[string]));
static assert( hasIndirections!(void delegate()));
static assert( hasIndirections!(void delegate() immutable));
static assert( hasIndirections!(immutable(void delegate())));
static assert( hasIndirections!(immutable(void delegate() immutable)));

static assert(!hasIndirections!(void function()));
static assert( hasIndirections!(void*[1]));
static assert(!hasIndirections!(byte[1]));
template hasUnsharedAliasing(T...)
Returns true if and only if T's representation includes at least one of the following:
  1. a raw pointer U* and U is not immutable or shared;
  2. an array U[] and U is not immutable or shared;
  3. a reference to a class type C and C is not immutable or shared.
  4. an associative array that is not immutable or shared.
  5. a delegate that is not shared.
Examples:
struct S1 { int a; Object b; }
struct S2 { string a; }
struct S3 { int a; immutable Object b; }
static assert( hasUnsharedAliasing!S1);
static assert(!hasUnsharedAliasing!S2);
static assert(!hasUnsharedAliasing!S3);

struct S4 { int a; shared Object b; }
struct S5 { char[] a; }
struct S6 { shared char[] b; }
struct S7 { float[3] vals; }
static assert(!hasUnsharedAliasing!S4);
static assert( hasUnsharedAliasing!S5);
static assert(!hasUnsharedAliasing!S6);
static assert(!hasUnsharedAliasing!S7);
template hasElaborateCopyConstructor(S)
True if S or any type embedded directly in the representation of S defines an elaborate copy constructor. Elaborate copy constructors are introduced by defining this(this) for a struct.
Classes and unions never have elaborate copy constructors.
Examples:
static assert(!hasElaborateCopyConstructor!int);

static struct S1 { }
static struct S2 { this(this) {} }
static struct S3 { S2 field; }
static struct S4 { S3[1] field; }
static struct S5 { S3[] field; }
static struct S6 { S3[0] field; }
static struct S7 { @disable this(); S3 field; }
static assert(!hasElaborateCopyConstructor!S1);
static assert( hasElaborateCopyConstructor!S2);
static assert( hasElaborateCopyConstructor!(immutable S2));
static assert( hasElaborateCopyConstructor!S3);
static assert( hasElaborateCopyConstructor!(S3[1]));
static assert(!hasElaborateCopyConstructor!(S3[0]));
static assert( hasElaborateCopyConstructor!S4);
static assert(!hasElaborateCopyConstructor!S5);
static assert(!hasElaborateCopyConstructor!S6);
static assert( hasElaborateCopyConstructor!S7);
template hasElaborateAssign(S)
True if S or any type directly embedded in the representation of S defines an elaborate assignment. Elaborate assignments are introduced by defining opAssign(typeof(this)) or opAssign(ref typeof(this)) for a struct or when there is a compiler-generated opAssign.
A type S gets compiler-generated opAssign in case it has an elaborate copy constructor or elaborate destructor.

Classes and unions never have elaborate assignments.

Note: Structs with (possibly nested) postblit operator(s) will have a hidden yet elaborate compiler generated assignment operator (unless explicitly disabled).

Examples:
static assert(!hasElaborateAssign!int);

static struct S  { void opAssign(S) {} }
static assert( hasElaborateAssign!S);
static assert(!hasElaborateAssign!(const(S)));

static struct S1 { void opAssign(ref S1) {} }
static struct S2 { void opAssign(int) {} }
static struct S3 { S s; }
static assert( hasElaborateAssign!S1);
static assert(!hasElaborateAssign!S2);
static assert( hasElaborateAssign!S3);
static assert( hasElaborateAssign!(S3[1]));
static assert(!hasElaborateAssign!(S3[0]));
template hasElaborateDestructor(S)
True if S or any type directly embedded in the representation of S defines an elaborate destructor. Elaborate destructors are introduced by defining ~this() for a struct.
Classes and unions never have elaborate destructors, even though classes may define ~this().
Examples:
static assert(!hasElaborateDestructor!int);

static struct S1 { }
static struct S2 { ~this() {} }
static struct S3 { S2 field; }
static struct S4 { S3[1] field; }
static struct S5 { S3[] field; }
static struct S6 { S3[0] field; }
static struct S7 { @disable this(); S3 field; }
static assert(!hasElaborateDestructor!S1);
static assert( hasElaborateDestructor!S2);
static assert( hasElaborateDestructor!(immutable S2));
static assert( hasElaborateDestructor!S3);
static assert( hasElaborateDestructor!(S3[1]));
static assert(!hasElaborateDestructor!(S3[0]));
static assert( hasElaborateDestructor!S4);
static assert(!hasElaborateDestructor!S5);
static assert(!hasElaborateDestructor!S6);
static assert( hasElaborateDestructor!S7);
template hasMember(T, string name)
Yields true if and only if T is an aggregate that defines a symbol called name.
Examples:
static assert(!hasMember!(int, "blah"));
struct S1 { int blah; }
struct S2 { int blah(){ return 0; } }
class C1 { int blah; }
class C2 { int blah(){ return 0; } }
static assert(hasMember!(S1, "blah"));
static assert(hasMember!(S2, "blah"));
static assert(hasMember!(C1, "blah"));
static assert(hasMember!(C2, "blah"));
template EnumMembers(E) if (is(E == enum))
Retrieves the members of an enumerated type enum E.
Parameters:
E An enumerated type. E may have duplicated values.
Returns:
Static tuple composed of the members of the enumerated type E. The members are arranged in the same order as declared in E.

Note: An enum can have multiple members which have the same value. If you want to use EnumMembers to e.g. generate switch cases at compile-time, you should use the std.typetuple.NoDuplicates template to avoid generating duplicate switch cases.

Note: Returned values are strictly typed with E. Thus, the following code does not work without the explicit cast:

enum E : int { a, b, c }
int[] abc = cast(int[]) [ EnumMembers!E ];
Cast is not necessary if the type of the variable is inferred. See the example below.
Examples:
Creating an array of enumerated values:
enum Sqrts : real
{
    one   = 1,
    two   = 1.41421,
    three = 1.73205,
}
auto sqrts = [ EnumMembers!Sqrts ];
assert(sqrts == [ Sqrts.one, Sqrts.two, Sqrts.three ]);

A generic function rank(v) in the following example uses this template for finding a member e in an enumerated type E.
// Returns i if e is the i-th enumerator of E.
size_t rank(E)(E e)
    if (is(E == enum))
{
    foreach (i, member; EnumMembers!E)
    {
        if (e == member)
            return i;
    }
    assert(0, "Not an enum member");
}

enum Mode
{
    read  = 1,
    write = 2,
    map   = 4,
}
assert(rank(Mode.read ) == 0);
assert(rank(Mode.write) == 1);
assert(rank(Mode.map  ) == 2);
template BaseTypeTuple(A)
Get a TypeTuple of the base class and base interfaces of this class or interface. BaseTypeTuple!Object returns the empty type tuple.
Examples:
interface I1 { }
interface I2 { }
interface I12 : I1, I2 { }
static assert(is(BaseTypeTuple!I12 == TypeTuple!(I1, I2)));

interface I3 : I1 { }
interface I123 : I1, I2, I3 { }
static assert(is(BaseTypeTuple!I123 == TypeTuple!(I1, I2, I3)));
template BaseClassesTuple(T) if (is(T == class))
Get a TypeTuple of all base classes of this class, in decreasing order. Interfaces are not included. BaseClassesTuple!Object yields the empty type tuple.
Examples:
class C1 { }
class C2 : C1 { }
class C3 : C2 { }
static assert(!BaseClassesTuple!Object.length);
static assert(is(BaseClassesTuple!C1 == TypeTuple!(Object)));
static assert(is(BaseClassesTuple!C2 == TypeTuple!(C1, Object)));
static assert(is(BaseClassesTuple!C3 == TypeTuple!(C2, C1, Object)));
static assert(!BaseClassesTuple!Object.length);
template InterfacesTuple(T)
Get a TypeTuple of all interfaces directly or indirectly inherited by this class or interface. Interfaces do not repeat if multiply implemented. InterfacesTuple!Object yields the empty type tuple.
template TransitiveBaseTypeTuple(T)
Get a TypeTuple of all base classes of T, in decreasing order, followed by T's interfaces. TransitiveBaseTypeTuple!Object yields the empty type tuple.
Examples:
interface J1 {}
interface J2 {}
class B1 {}
class B2 : B1, J1, J2 {}
class B3 : B2, J1 {}
alias TL = TransitiveBaseTypeTuple!B3;
assert(TL.length == 5);
assert(is (TL[0] == B2));
assert(is (TL[1] == B1));
assert(is (TL[2] == Object));
assert(is (TL[3] == J1));
assert(is (TL[4] == J2));

assert(TransitiveBaseTypeTuple!Object.length == 0);
template MemberFunctionsTuple(C, string name) if (is(C == class) || is(C == interface))
Returns a tuple of non-static functions with the name name declared in the class or interface C. Covariant duplicates are shrunk into the most derived one.
Examples:
interface I { I foo(); }
class B
{
    real foo(real v) { return v; }
}
class C : B, I
{
    override C foo() { return this; } // covariant overriding of I.foo()
}
alias MemberFunctionsTuple!(C, "foo") foos;
static assert(foos.length == 2);
static assert(__traits(isSame, foos[0], C.foo));
static assert(__traits(isSame, foos[1], B.foo));
template TemplateOf(alias T : Base!Args, alias Base, Args...)
Returns an alias to the template that T is an instance of.
template TemplateArgsOf(alias T : Base!Args, alias Base, Args...)
template TemplateArgsOf(T : Base!Args, alias Base, Args...)
Returns a TypeTuple of the template arguments used to instantiate T.
Examples:
struct Foo(T, U) {}
static assert(is(TemplateArgsOf!(Foo!(int, real)) == TypeTuple!(int, real)));
template classInstanceAlignment(T) if (is(T == class))
Returns class instance alignment.
Examples:
class A { byte b; }
class B { long l; }

// As class instance always has a hidden pointer
static assert(classInstanceAlignment!A == (void*).alignof);
static assert(classInstanceAlignment!B == long.alignof);
template CommonType(T...)
Get the type that all types can be implicitly converted to. Useful e.g. in figuring out an array type from a bunch of initializing values. Returns void if passed an empty list, or if the types have no common type.
Examples:
alias X = CommonType!(int, long, short);
assert(is(X == long));
alias Y = CommonType!(int, char[], short);
assert(is(Y == void));
template ImplicitConversionTargets(T)
Returns a tuple with all possible target types of an implicit conversion of a value of type T.
Important note:

The possible targets are computed more conservatively than the D 2.005 compiler does, eliminating all dangerous conversions. For example, ImplicitConversionTargets!double does not include float.
template isImplicitlyConvertible(From, To)
Is From implicitly convertible to To?
template isAssignable(Lhs, Rhs = Lhs)
Returns true iff a value of type Rhs can be assigned to a variable of type Lhs.
isAssignable returns whether both an lvalue and rvalue can be assigned.

If you omit Rhs, isAssignable will check identity assignable of Lhs.
Examples:
static assert( isAssignable!(long, int));
static assert(!isAssignable!(int, long));
static assert( isAssignable!(const(char)[], string));
static assert(!isAssignable!(string, char[]));

// int is assignable to int
static assert( isAssignable!int);

// immutable int is not assignable to immutable int
static assert(!isAssignable!(immutable int));
template isCovariantWith(F, G) if (is(F == function) && is(G == function))
Determines whether the function type F is covariant with G, i.e., functions of the type F can override ones of the type G.
Examples:
interface I { I clone(); }
interface J { J clone(); }
class C : I
{
    override C clone()   // covariant overriding of I.clone()
    {
        return new C;
    }
}

// C.clone() can override I.clone(), indeed.
static assert(isCovariantWith!(typeof(C.clone), typeof(I.clone)));

// C.clone() can't override J.clone(); the return type C is not implicitly
// convertible to J.
static assert(!isCovariantWith!(typeof(C.clone), typeof(J.clone)));
@property T rvalueOf(T)(inout __InoutWorkaroundStruct = __InoutWorkaroundStruct.init);
@property ref T lvalueOf(T)(inout __InoutWorkaroundStruct = __InoutWorkaroundStruct.init);
Creates an lvalue or rvalue of type T for typeof(...) and _traits(compiles, ...) purposes. No actual value is returned.

Note: Trying to use returned value will result in a "Symbol Undefined" error at link time.

Examples:
// Note that `f` doesn't have to be implemented
// as is isn't called.
int f(int);
bool f(ref int);
static assert(is(typeof(f(rvalueOf!int)) == int));
static assert(is(typeof(f(lvalueOf!int)) == bool));

int i = rvalueOf!int; // error, no actual value is returned
template isBoolean(T)
Detect whether T is a built-in boolean type.
Examples:
static assert( isBoolean!bool);
enum EB : bool { a = true }
static assert( isBoolean!EB);
static assert(!isBoolean!(SubTypeOf!bool));
template isIntegral(T)
Detect whether T is a built-in integral type. Types bool, char, wchar, and dchar are not considered integral.
template isFloatingPoint(T)
Detect whether T is a built-in floating point type.
template isNumeric(T)
Detect whether T is a built-in numeric type (integral or floating point).
template isScalarType(T)
Detect whether T is a scalar type (a built-in numeric, character or boolean type).
Examples:
static assert(!isScalarType!void);
static assert( isScalarType!(immutable(int)));
static assert( isScalarType!(shared(float)));
static assert( isScalarType!(shared(const bool)));
static assert( isScalarType!(const(dchar)));
template isBasicType(T)
Detect whether T is a basic type (scalar type or void).
Examples:
static assert(isBasicType!void);
static assert(isBasicType!(immutable(int)));
static assert(isBasicType!(shared(float)));
static assert(isBasicType!(shared(const bool)));
static assert(isBasicType!(const(dchar)));
template isUnsigned(T)
Detect whether T is a built-in unsigned numeric type.
template isSigned(T)
Detect whether T is a built-in signed numeric type.
template isSomeChar(T)
Detect whether T is one of the built-in character types.
Examples:
static assert(!isSomeChar!int);
static assert(!isSomeChar!byte);
static assert(!isSomeChar!string);
static assert(!isSomeChar!wstring);
static assert(!isSomeChar!dstring);
static assert(!isSomeChar!(char[4]));
template isSomeString(T)
Detect whether T is one of the built-in string types.
The built-in string types are Char[], where Char is any of char, wchar or dchar, with or without qualifiers.

Static arrays of characters (like char[80]) are not considered built-in string types.
Examples:
static assert(!isSomeString!int);
static assert(!isSomeString!(int[]));
static assert(!isSomeString!(byte[]));
static assert(!isSomeString!(typeof(null)));
static assert(!isSomeString!(char[4]));

enum ES : string { a = "aaa", b = "bbb" }
static assert( isSomeString!ES);
template isStaticArray(T)
Detect whether type T is a static array.
Examples:
static assert(!isStaticArray!(const(int)[]));
static assert(!isStaticArray!(immutable(int)[]));
static assert(!isStaticArray!(const(int)[4][]));
static assert(!isStaticArray!(int[]));
static assert(!isStaticArray!(int[char]));
static assert(!isStaticArray!(int[1][]));
static assert(!isStaticArray!(int[int]));
static assert(!isStaticArray!int);
template isDynamicArray(T)
Detect whether type T is a dynamic array.
template isArray(T)
Detect whether type T is an array (static or dynamic; for associative arrays see isAssociativeArray).
template isAssociativeArray(T)
Detect whether T is an associative array type
template isBuiltinType(T)
Detect whether type T is a builtin type.
template isSIMDVector(T)
Detect whether type T is a SIMD vector type.
template isPointer(T)
Detect whether type T is a pointer.
template PointerTarget(T : T*)
Returns the target type of a pointer.
deprecated alias pointerTarget = PointerTarget(T : T*);
Deprecated. Please use PointerTarget instead. This will be removed in June 2015.
template isAggregateType(T)
Detect whether type T is an aggregate type.
template isIterable(T)
Returns true if T can be iterated over using a foreach loop with a single loop variable of automatically inferred type, regardless of how the foreach loop is implemented. This includes ranges, structs/classes that define opApply with a single loop variable, and builtin dynamic, static and associative arrays.
Examples:
struct OpApply
{
    int opApply(int delegate(ref uint) dg) { assert(0); }
}

struct Range
{
    @property uint front() { assert(0); }
    void popFront() { assert(0); }
    enum bool empty = false;
}

static assert( isIterable!(uint[]));
static assert( isIterable!OpApply);
static assert( isIterable!(uint[string]));
static assert( isIterable!Range);

static assert(!isIterable!uint);
template isMutable(T)
Returns true if T is not const or immutable. Note that isMutable is true for string, or immutable(char)[], because the 'head' is mutable.
Examples:
static assert( isMutable!int);
static assert( isMutable!string);
static assert( isMutable!(shared int));
static assert( isMutable!(shared const(int)[]));

static assert(!isMutable!(const int));
static assert(!isMutable!(inout int));
static assert(!isMutable!(shared(const int)));
static assert(!isMutable!(shared(inout int)));
static assert(!isMutable!(immutable string));
template isInstanceOf(alias S, T)
Returns true if T is an instance of the template S.
Examples:
static struct Foo(T...) { }
static struct Bar(T...) { }
static struct Doo(T) { }
static struct ABC(int x) { }
static assert(isInstanceOf!(Foo, Foo!int));
static assert(!isInstanceOf!(Foo, Bar!int));
static assert(!isInstanceOf!(Foo, int));
static assert(isInstanceOf!(Doo, Doo!int));
static assert(isInstanceOf!(ABC, ABC!1));
static assert(!__traits(compiles, isInstanceOf!(Foo, Foo)));
template isExpressionTuple(T...)
Check whether the tuple T is an expression tuple. An expression tuple only contains expressions.
See Also:
Examples:
static assert(isExpressionTuple!(1, 2.0, "a"));
static assert(!isExpressionTuple!(int, double, string));
static assert(!isExpressionTuple!(int, 2.0, "a"));
template isTypeTuple(T...)
Check whether the tuple T is a type tuple. A type tuple only contains types.
Examples:
static assert(isTypeTuple!(int, float, string));
static assert(!isTypeTuple!(1, 2.0, "a"));
static assert(!isTypeTuple!(1, double, string));
template isFunctionPointer(T...) if (T.length == 1)
Detect whether symbol or type T is a function pointer.
Examples:
static void foo() {}
void bar() {}

auto fpfoo = &foo;
static assert( isFunctionPointer!fpfoo);
static assert( isFunctionPointer!(void function()));

auto dgbar = &bar;
static assert(!isFunctionPointer!dgbar);
static assert(!isFunctionPointer!(void delegate()));
static assert(!isFunctionPointer!foo);
static assert(!isFunctionPointer!bar);

static assert( isFunctionPointer!((int a) {}));
template isDelegate(T...) if (T.length == 1)
Detect whether symbol or type T is a delegate.
Examples:
static void sfunc() { }
int x;
void func() { x++; }

int delegate() dg;
assert(isDelegate!dg);
assert(isDelegate!(int delegate()));
assert(isDelegate!(typeof(&func)));

int function() fp;
assert(!isDelegate!fp);
assert(!isDelegate!(int function()));
assert(!isDelegate!(typeof(&sfunc)));
template isSomeFunction(T...) if (T.length == 1)
Detect whether symbol or type T is a function, a function pointer or a delegate.
template isCallable(T...) if (T.length == 1)
Detect whether T is a callable object, which can be called with the function call operator (...).
Examples:
interface I { real value() @property; }
struct S { static int opCall(int) { return 0; } }
class C { int opCall(int) { return 0; } }
auto c = new C;

static assert( isCallable!c);
static assert( isCallable!S);
static assert( isCallable!(c.opCall));
static assert( isCallable!(I.value));
static assert( isCallable!((int a) { return a; }));

static assert(!isCallable!I);
template isAbstractFunction(T...) if (T.length == 1)
Detect whether T is a an abstract function.
template isFinalFunction(T...) if (T.length == 1)
Detect whether T is a a final function.
Examples:
struct S { void bar() { } }
final class FC { void foo(); }
class C
{
    void bar() { }
    final void foo();
}
static assert(!isFinalFunction!(S.bar));
static assert( isFinalFunction!(FC.foo));
static assert(!isFinalFunction!(C.bar));
static assert( isFinalFunction!(C.foo));
template isNestedFunction(alias f)
Determines whether function f requires a context pointer.
template isAbstractClass(T...) if (T.length == 1)
Detect whether T is a an abstract class.
Examples:
struct S { }
class C { }
abstract class AC { }
static assert(!isAbstractClass!S);
static assert(!isAbstractClass!C);
static assert( isAbstractClass!AC);
template isFinalClass(T...) if (T.length == 1)
Detect whether T is a a final class.
Examples:
class C { }
abstract class AC { }
final class FC1 : C { }
final class FC2 { }
static assert(!isFinalClass!C);
static assert(!isFinalClass!AC);
static assert( isFinalClass!FC1);
static assert( isFinalClass!FC2);
template Unqual(T)
Removes all qualifiers, if any, from type T.
Examples:
static assert(is(Unqual!int == int));
static assert(is(Unqual!(const int) == int));
static assert(is(Unqual!(immutable int) == int));
static assert(is(Unqual!(shared int) == int));
static assert(is(Unqual!(shared(const int)) == int));
template ForeachType(T)
Returns the inferred type of the loop variable when a variable of type T is iterated over using a foreach loop with a single loop variable and automatically inferred return type. Note that this may not be the same as std.range.ElementType!Range in the case of narrow strings, or if T has both opApply and a range interface.
Examples:
static assert(is(ForeachType!(uint[]) == uint));
static assert(is(ForeachType!string == immutable(char)));
static assert(is(ForeachType!(string[string]) == string));
static assert(is(ForeachType!(inout(int)[]) == inout(int)));
template OriginalType(T)
Strips off all enums from type T.
Examples:
enum E : real { a }
enum F : E    { a = E.a }
alias G = const(F);
static assert(is(OriginalType!E == real));
static assert(is(OriginalType!F == real));
static assert(is(OriginalType!G == const real));
template KeyType(V : V[K], K)
Get the Key type of an Associative Array.
Examples:
import std.traits;
alias Hash = int[string];
static assert(is(KeyType!Hash == string));
static assert(is(ValueType!Hash == int));
KeyType!Hash str = "a"; // str is declared as string
ValueType!Hash num = 1; // num is declared as int
template ValueType(V : V[K], K)
Get the Value type of an Associative Array.
Examples:
import std.traits;
alias Hash = int[string];
static assert(is(KeyType!Hash == string));
static assert(is(ValueType!Hash == int));
KeyType!Hash str = "a"; // str is declared as string
ValueType!Hash num = 1; // num is declared as int
template Unsigned(T)
Returns the corresponding unsigned type for T. T must be a numeric integral type, otherwise a compile-time error occurs.
template Largest(T...) if (T.length >= 1)
Returns the largest type, i.e. T such that T.sizeof is the largest. If more than one type is of the same size, the leftmost argument of these in will be returned.
Examples:
static assert(is(Largest!(uint, ubyte, ushort, real) == real));
static assert(is(Largest!(ulong, double) == ulong));
static assert(is(Largest!(double, ulong) == double));
static assert(is(Largest!(uint, byte, double, short) == double));
template Signed(T)
Returns the corresponding signed type for T. T must be a numeric integral type, otherwise a compile-time error occurs.
Examples:
alias S1 = Signed!uint;
static assert(is(S1 == int));
alias S2 = Signed!(const(uint));
static assert(is(S2 == const(int)));
alias S3 = Signed!(immutable(uint));
static assert(is(S3 == immutable(int)));
template mostNegative(T) if (isNumeric!T || isSomeChar!T || isBoolean!T)
Returns the most negative value of the numeric type T.
Examples:
static assert(mostNegative!float == -float.max);
static assert(mostNegative!double == -double.max);
static assert(mostNegative!real == -real.max);
static assert(mostNegative!bool == false);
Examples:
foreach(T; TypeTuple!(bool, byte, short, int, long))
    static assert(mostNegative!T == T.min);

foreach(T; TypeTuple!(ubyte, ushort, uint, ulong, char, wchar, dchar))
    static assert(mostNegative!T == 0);
template mangledName(sth...) if (sth.length == 1)
Returns the mangled name of symbol or type sth.
mangledName is the same as builtin .mangleof property, except that the correct names of property functions are obtained.
module test;
import std.traits : mangledName;

class C
{
    int value() @property;
}
pragma(msg, C.value.mangleof);      // prints "i"
pragma(msg, mangledName!(C.value)); // prints "_D4test1C5valueMFNdZi"
template Select(bool condition, T...) if (T.length == 2)
Aliases itself to T[0] if the boolean condition is true and to T[1] otherwise.
Examples:
// can select types
static assert(is(Select!(true, int, long) == int));
static assert(is(Select!(false, int, long) == long));

// can select symbols
int a = 1;
int b = 2;
alias selA = Select!(true, a, b);
alias selB = Select!(false, a, b);
assert(selA == 1);
assert(selB == 2);
A select(bool cond : true, A, B)(A a, lazy B b);
B select(bool cond : false, A, B)(lazy A a, B b);
If cond is true, returns a without evaluating b. Otherwise, returns b without evaluating a.