cpp11-range

by klmr

Range-based for loops to iterate over a range of numbers or values

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Re-imagining the
for
loop

C++11 now knows two distinct types of

for
loops: the classic loop over an “index” and the range-based
for
loop which vastly simplifies the iteration over a range specified by a pair of iterators.

By contrast, Python knows only one loop type – roughly equivalent to the range-based for loop. In fact, loops over indices are exceedingly rare, but made possible by the use of the

range
method:
for i in range(10):
    print i

Which does what it promises – although Python version < 3.0 does the “wrong” thing and actually instantiates the whole collection in memory at once; a remedy is

xrange
which yields values lazily as they are consumed by the loop.

C++11 effortlessly allows the same but there is no standard library function to provide this. Boost.Range provides part of the functionality via

irange
which only works on integers, and not for unlimited ranges (this will make sense in a second).

The header

range.hpp
provides a very basic implementation for this. It allows running the following code:
for (auto i : range(1, 5))
    cout << i << "\n";

for (auto u : range(0u)) if (u == 3u) break; else cout << u << "\n";

for (auto c : range('a', 'd')) cout << c << "\n";

for (auto i : range(100).step(-3)) if (i < 90) break; else cout << i << "\n";

range
with a single argument deviates from the Python semantic and creates an endless loop, unless it’s interrupted manually. This is an interesting use-case that cannot be modelled in Python using
range
.

Iterating over container indices

In Python, the one-argument version of

range
is often used to iterate over the indices of a container via
range(len(container))
. Because that overload creates an infinite range in our C++ library, we cannot use this idiom.

But we can do better anyway. For those few cases where we actually want to iterate over a container’s indices, we just use the

indices
function:
std::vector x{1, 2, 3};
for (auto i : indices(x))
    cout << i << '\n';

This works as expected for any type which has a member function

size() const
that returns some integral type. It also works with
initializer_list
s and C-style fixed-size arrays.1

Adding

.step(…)
to the end of either
range
or
indices
specifies a step size instead of the default, 1.

The construct works for arbitrary types which fulfil the interface requirements (incrementing, copying, equality comparison, default construction in the case of infinite ranges).

1 This includes string literals, which are C-style strings that include null termination; this may lead to surprising results, because

indices("test")
results in 0, 1, 2, 3, 4, whereas
indices(std::string{"test"})
results in 0, 1, 2, 3.

Performance (the cost of beauty)

When compiling with optimisations enabled (and why wouldn’t you?), using the

range
function yield very similar output compared with a manual
for
loop. In fact, on g++ 4.8 with
-O2
or higher, the following two loops yield identical assembly.
for (int i = 0; i < n; ++i)
    cout << i;

for (int i : range(0, n)) cout << i;

Even though the

range
function creates a proxy container and an iterator wrapper, those are completely elided from the resulting code.

☞ Beauty is free.

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