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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(ReturnType!((R r) => r.back) == ElementType!R);

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.


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


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.