If a foreach is encountered by the compiler

foreach (element; range)
    // Loop body...

it's internally rewritten similar to the following:

for (auto __rangeCopy = range;
    auto element = __rangeCopy.front;
    // Loop body...

Any object which fulfills the following interface is called a range (or more specific InputRange) and is thus a type that can be iterated over:

    interface InputRange(E)
        bool empty();
        E front();
        void popFront();

Have a look at the example on the right to inspect the implementation and usage of an input range closer.


Ranges are lazy. They won't be evaluated until requested. Hence, a range from an infinite range can be taken:

42.repeat.take(3).writeln; // [42, 42, 42]

Value vs. Reference types

If the range object is a value type, then range will be copied and only the copy will be consumed:

auto r = 5.iota;
r.drop(5).writeln; // []
r.writeln; // [0, 1, 2, 3, 4]

If the range object is a reference type (e.g. class or std.range.refRange), then the range will be consumed and won't be reset:

auto r = 5.iota;
auto r2 = refRange(&r);
r2.drop(5).writeln; // []
r2.writeln; // []

Copyable InputRanges are ForwardRanges

Most of the ranges in the standard library are structs and so foreach iteration is usually non-destructive, though not guaranteed. If this guarantee is important, a specialization of an InputRange can be used— forward ranges with a .save method:

interface ForwardRange(E) : InputRange!E
    typeof(this) save();
// by value (Structs)
auto r = 5.iota;
auto r2 = refRange(&r);
r2.save.drop(5).writeln; // []
r2.writeln; // [0, 1, 2, 3, 4]

ForwardRanges can be extended to Bidirectional ranges + random access ranges

There are two extensions of the copyable ForwardRange: (1) a bidirectional range and (2) a random access range. A bidirectional range allows iteration from the back:

interface BidirectionalRange(E) : ForwardRange!E
     E back();
     void popBack();
5.iota.retro.writeln; // [4, 3, 2, 1, 0]

A random access range has a known length and each element can be directly accessed.

interface RandomAccessRange(E) : ForwardRange!E
     E opIndex(size_t i);
     size_t length();

The best known random access range is D's array:

auto r = [4, 5, 6];
r[1].writeln; // 5

Lazy range algorithms

The functions in std.range and std.algorithm provide building blocks that make use of this interface. Ranges enable the composition of complex algorithms behind an object that can be iterated with ease. Furthermore, ranges enable the creation of lazy objects that only perform a calculation when it's really needed in an iteration e.g. when the next range's element is accessed. Special range algorithms will be presented later in the D's Gems section.


rdmd playground.d