How to make a for loop variable const with the exception of the increment statement?

Solution 1:

From c++20, you can use ranges::views::iota like this:

for (int const i : std::views::iota(0, 10))
{
   std::cout << i << " ";  // ok
   i = 42;                 // error
}

Here's a demo.

From c++11, you can also use the following technique, which uses an IIILE (immediately invoked inline lambda expression):

int x = 0;
for (int i = 0; i < 10; ++i) [&,i] {
    std::cout << i << " ";  // ok, i is readable
    i = 42;                 // error, i is captured by non-mutable copy
    x++;                    // ok, x is captured by mutable reference
}();     // IIILE

Here's a demo.

Note that [&,i] means that i is captured by non-mutable copy, and everything else is captured by mutable reference. The (); at the end of the loop simply means that the lambda is invoked immediately.

Solution 2:

For anyone that likes Cigien's std::views::iota answer but isn't working in C++20 or above, it's rather straightforward to implement a simplified and lightweight version of std::views::iota compatible c++11 or above.

All it requires is:

• A basic "LegacyInputIterator" type (something that defines operator++ and operator*) that wraps an integral value (e.g. an int)
• Some "range"-like class that has begin() and end() that returns the above iterators. This will allow it to work in range-based for loops

A simplified version of this could be:

#include <iterator>

// This is just a class that wraps an 'int' in an iterator abstraction
// Comparisons compare the underlying value, and 'operator++' just
// increments the underlying int
class counting_iterator
{
public:
    // basic iterator boilerplate
    using iterator_category = std::input_iterator_tag;
    using value_type = int;
    using reference  = int;
    using pointer    = int*;
    using difference_type = std::ptrdiff_t;

    // Constructor / assignment
    constexpr explicit counting_iterator(int x) : m_value{x}{}
    constexpr counting_iterator(const counting_iterator&) = default;
    constexpr counting_iterator& operator=(const counting_iterator&) = default;

    // "Dereference" (just returns the underlying value)
    constexpr reference operator*() const { return m_value; }
    constexpr pointer operator->() const { return &m_value; }

    // Advancing iterator (just increments the value)
    constexpr counting_iterator& operator++() {
        m_value++;
        return (*this);
    }
    constexpr counting_iterator operator++(int) {
        const auto copy = (*this);
        ++(*this);
        return copy;
    }

    // Comparison
    constexpr bool operator==(const counting_iterator& other) const noexcept {
        return m_value == other.m_value;
    }
    constexpr bool operator!=(const counting_iterator& other) const noexcept {
        return m_value != other.m_value;
    }
private:
    int m_value;
};

// Just a holder type that defines 'begin' and 'end' for
// range-based iteration. This holds the first and last element
// (start and end of the range)
// The begin iterator is made from the first value, and the
// end iterator is made from the second value.
struct iota_range
{
    int first;
    int last;
    constexpr counting_iterator begin() const { return counting_iterator{first}; }
    constexpr counting_iterator end() const { return counting_iterator{last}; }
};

// A simple helper function to return the range
// This function isn't strictly necessary, you could just construct
// the 'iota_range' directly
constexpr iota_range iota(int first, int last)
{
    return iota_range{first, last};
}

I've defined the above with constexpr where it's supported, but for earlier versions of C++ like C++11/14, you may need to remove constexpr where it is not legal in those versions to do so.

The above boilerplate enables the following code to work in pre-C++20:

for (int const i : iota(0, 10))
{
   std::cout << i << " ";  // ok
   i = 42;                 // error
}

Which will generate the same assembly as the C++20 std::views::iota solution and the classic for-loop solution when optimized.

This works with any C++11-compliant compilers (e.g. compilers like gcc-4.9.4) and still produces nearly identical assembly to a basic for-loop counterpart.

Note: The iota helper function is just for feature-parity with the C++20 std::views::iota solution; but realistically, you could also directly construct an iota_range{...} instead of calling iota(...). The former just presents an easy upgrade path if a user wishes to switch to C++20 in the future.