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Enum member std.traits.hasUDA

Determine if a symbol has a given user-defined attribute.

enum hasUDA(alias symbol, alias attribute) = getUDAs!(symbol, attribute).length != 0;

See Also

getUDAs

Example

enum E;
struct S {}

@("alpha") int a;
static assert(hasUDA!(a, "alpha"));
static assert(!hasUDA!(a, S));
static assert(!hasUDA!(a, E));

@(E) int b;
static assert(!hasUDA!(b, "alpha"));
static assert(!hasUDA!(b, S));
static assert(hasUDA!(b, E));

@E int c;
static assert(!hasUDA!(c, "alpha"));
static assert(!hasUDA!(c, S));
static assert(hasUDA!(c, E));

@(S, E) int d;
static assert(!hasUDA!(d, "alpha"));
static assert(hasUDA!(d, S));
static assert(hasUDA!(d, E));

@S int e;
static assert(!hasUDA!(e, "alpha"));
static assert(hasUDA!(e, S));
static assert(!hasUDA!(e, S()));
static assert(!hasUDA!(e, E));

@S() int f;
static assert(!hasUDA!(f, "alpha"));
static assert(hasUDA!(f, S));
static assert(hasUDA!(f, S()));
static assert(!hasUDA!(f, E));

@(S, E, "alpha") int g;
static assert(hasUDA!(g, "alpha"));
static assert(hasUDA!(g, S));
static assert(hasUDA!(g, E));

@(100) int h;
static assert(hasUDA!(h, 100));

struct Named { string name; }

@Named("abc") int i;
static assert(hasUDA!(i, Named));
static assert(hasUDA!(i, Named("abc")));
static assert(!hasUDA!(i, Named("def")));

struct AttrT(T)
{
    string name;
    T value;
}

@AttrT!int("answer", 42) int j;
static assert(hasUDA!(j, AttrT));
static assert(hasUDA!(j, AttrT!int));
static assert(!hasUDA!(j, AttrT!string));

@AttrT!string("hello", "world") int k;
static assert(hasUDA!(k, AttrT));
static assert(!hasUDA!(k, AttrT!int));
static assert(hasUDA!(k, AttrT!string));

struct FuncAttr(alias f) { alias func = f; }
static int fourtyTwo() { return 42; }
static size_t getLen(string s) { return s.length; }

@FuncAttr!getLen int l;
static assert(hasUDA!(l, FuncAttr));
static assert(!hasUDA!(l, FuncAttr!fourtyTwo));
static assert(hasUDA!(l, FuncAttr!getLen));
static assert(!hasUDA!(l, FuncAttr!fourtyTwo()));
static assert(!hasUDA!(l, FuncAttr!getLen()));

@FuncAttr!getLen() int m;
static assert(hasUDA!(m, FuncAttr));
static assert(!hasUDA!(m, FuncAttr!fourtyTwo));
static assert(hasUDA!(m, FuncAttr!getLen));
static assert(!hasUDA!(m, FuncAttr!fourtyTwo()));
static assert(hasUDA!(m, FuncAttr!getLen()));

Authors

Walter Bright, Tomasz Stachowiak (isExpressions), Andrei Alexandrescu, Shin Fujishiro, Robert Clipsham, David Nadlinger, Kenji Hara, Shoichi Kato

License

Boost License 1.0.