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.

It might be worth thinking about an alternative syntax for this, though, since it means that another common use-case – iterating from the start index of a range until its end – is made more complicated: in Python you’d simply write range(len(container)). range.hpp on the other hand requires one to write range(0u, container.size()).

Adding .step(…) to the end 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).

I haven’t done a thorough analysis but the disassembly of a very similar code shows that the above compiles down to identical code to a classical for loop; that is, the first above code produces equivalent machine code to

for (int i = 0; i < 5; ++i)
    cout << i << "\n";