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

Question

Consider a standard for loop:

for (int i = 0; i < 10; ++i)  {    // do something with i } 

I want to prevent the variable i from being modified in the body of the for loop.

However, I cannot declare i as const as this makes the increment statement invalid. Is there a way to make i a const variable outside of the increment statement?

Answer 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.

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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.