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Enum member std.range.primitives.isBidirectionalRange

Returns true if R is a bidirectional range. A bidirectional range is a forward range that also offers the primitives back and popBack. The following code should compile for any bidirectional range.

enum isBidirectionalRange(R) = isForwardRange!R && is(typeof((R r) => r.popBack)) && (is(typeof((return ref R r) => r.back)) || is(typeof((return ref R r) ref => r.back))) && is(typeof(R.init.back.init) == ElementType!R);
enum isBidirectionalRange(R, E) = .isBidirectionalRange!R && isQualifierConvertible!(ElementType!R, E);

The semantics of a bidirectional range (not checkable during compilation) are assumed to be the following (r is an object of type R):

  • r.back returns (possibly a reference to) the last element in the range. Calling r.back is allowed only if calling r.empty has, or would have, returned false.

See Also

The header of std.range for tutorials on ranges.


R type to be tested
E if present, the elements of the range must be qualifier-convertible to this type


true if R is a bidirectional range (possibly with element type E), false if not


alias R = int[];
R r = [0,1];
static assert(isForwardRange!R);           // is forward range
r.popBack();                               // can invoke popBack
auto t = r.back;                           // can get the back of the range
auto w = r.front;
static assert(is(typeof(t) == typeof(w))); // same type for front and back

// Checking the element type
static assert( isBidirectionalRange!(int[], const int));
static assert(!isBidirectionalRange!(int[], immutable int));

static assert(!isBidirectionalRange!(const(int)[], int));
static assert( isBidirectionalRange!(const(int)[], const int));
static assert(!isBidirectionalRange!(const(int)[], immutable int));

static assert(!isBidirectionalRange!(immutable(int)[], int));
static assert( isBidirectionalRange!(immutable(int)[], const int));
static assert( isBidirectionalRange!(immutable(int)[], immutable int));


Andrei Alexandrescu, David Simcha, and Jonathan M Davis. Credit for some of the ideas in building this module goes to Leonardo Maffi.


Boost License 1.0.