Home » Simple integer range for C++11 range-based for loops

Simple integer range for C++11 range-based for loops


Why write it yourself if you can use Boost.Range‘s irange. You can even adapt this to set the starting index to 0 and get std::iota type behavior (called iota_n here).

#include <boost/range/irange.hpp>
#include <iostream>

template<class Integer>
decltype(auto) iota_n(Integer last)
    return boost::irange(0, last);    

template<class Integer, class StepSize>
decltype(auto) iota_n(Integer last, StepSize step_size)
    return boost::irange(0, last, step_size);    

int main()
    for (auto x : iota_n(5)) // 01234
        std::cout << x;

Live Example, using Clang 3.4 return-type-deduction in C++1y mode (also supported by gcc 4.9, and other compilers soon (use trailing -> decltype(/*statement inside function*/) return types for C++11 compilers)

You can make the class template with trivial changes (add template<typename T> and change int by T in your classes), then make a construction function that deduces integer types:

template<typename T>
LoopRange<T> range(T from, T to)
                  "range only accepts integral values");

    return { from, to };

That will even allow you to explicitly tell what kind of integer you want to loop with if needed:

for (auto i: range<unsigned>(0, 5))
    std::cout << i << " ";

If you need to generate indices to iterate through a std::vector, this can be useful since std::vector<T>::size_type is probably bigger than int. While the static_assert avoids some potential problems with floating point values, it also inhibits the use of integer-like classes (for example, a hypothetical BigNum class).

You can simplify some of your functions thanks to list initialization. For example, used in a return statement, it sallows you not to explicitly repeat the return type (unless the return type’s constructor is explicit):

LoopRangeIterator begin() const
    return { from };

LoopRangeIterator end() const
    return { to };

On a side note, such a range utility would also be interesting if it worked with floating point numbers, and maybe decimal numbers in the future (akin to Python’s numpy.arange). However, you would have to special-case the class for those types if you want to avoid problems: if you repeatedly add the same floating point (say 0.01), you will accumulate rounding errors. Computing every value from the base value with a multiplication could be away to circumvent such a problem.

If you have operator!=, you should also have operator== for symmetry:

bool operator==(LoopRangeIterator const& other) const
    return value == other.value;

In addition, it’s more common to overload operator!= in terms of ==:

bool operator!=(LoopRangeIterator const& other) const
    return !(value == other.value);

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