View source code
Display the source code in std/range/package.d from which this page was generated on github.
Report a bug
If you spot a problem with this page, click here to create a Bugzilla issue.
Improve this page
Quickly fork, edit online, and submit a pull request for this page. Requires a signed-in GitHub account. This works well for small changes. If you'd like to make larger changes you may want to consider using local clone.

Function std.range.tee

Implements a "tee" style pipe, wrapping an input range so that elements of the range can be passed to a provided function or OutputRange as they are iterated over. This is useful for printing out intermediate values in a long chain of range code, performing some operation with side-effects on each call to front or popFront, or diverting the elements of a range into an auxiliary OutputRange.

auto tee(Flag!("pipeOnPop") pipeOnPop = Yes.pipeOnPop, R1, R2) (
  R1 inputRange,
  R2 outputRange
if (isInputRange!R1 && isOutputRange!(R2, ElementType!R1));

auto tee(alias fun, Flag!("pipeOnPop") pipeOnPop = Yes.pipeOnPop, R1) (
  R1 inputRange
if (is(typeof(fun) == void) || isSomeFunction!fun);

It is important to note that as the resultant range is evaluated lazily, in the case of the version of tee that takes a function, the function will not actually be executed until the range is "walked" using functions that evaluate ranges, such as array or fold.


pipeOnPop If Yes.pipeOnPop, simply iterating the range without ever calling front is enough to have tee mirror elements to outputRange (or, respectively, fun). Note that each popFront() call will mirror the old front value, not the new one. This means that the last value will not be forwarded if the range isn't iterated until empty. If No.pipeOnPop, only elements for which front does get called will be also sent to outputRange/fun. If front is called twice for the same element, it will still be sent only once. If this caching is undesired, consider using map instead.
inputRange The input range being passed through.
outputRange This range will receive elements of inputRange progressively as iteration proceeds.
fun This function will be called with elements of inputRange progressively as iteration proceeds.


An input range that offers the elements of inputRange. Regardless of whether inputRange is a more powerful range (forward, bidirectional etc), the result is always an input range. Reading this causes inputRange to be iterated and returns its elements in turn. In addition, the same elements will be passed to outputRange or fun as well.

See Also



import std.algorithm.comparison : equal;
import std.algorithm.iteration : filter, map;

// Sum values while copying
int[] values = [1, 4, 9, 16, 25];
int sum = 0;
auto newValues = values.tee!(a => sum += a).array;
assert(equal(newValues, values));
writeln(sum); // 1 + 4 + 9 + 16 + 25

// Count values that pass the first filter
int count = 0;
auto newValues4 = values.filter!(a => a < 10)
                        .tee!(a => count++)
                        .map!(a => a + 1)
                        .filter!(a => a < 10);

//Fine, equal also evaluates any lazy ranges passed to it.
//count is not 3 until equal evaluates newValues4
assert(equal(newValues4, [2, 5]));
writeln(count); // 3


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


Boost License 1.0.