How can I detect the last iteration in a loop over std::map?

Question

I\'m trying to figure out the best way to determine whether I\'m in the last iteration of a loop over a map in order to do something like the following:

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Answer 1

The following code would be optimized by a compiler so that to be the best solution for this task by performance as well as by OOP rules:

if (&*it == &*someMap.rbegin()) {
    //the last iteration
}

This is the best code by OOP rules because std::map has got a special member function rbegin for the code like:

final_iter = someMap.end();
--final_iter;

Answer 2

Since C++11, you can also use std::next()

   for (auto iter = someMap.begin(); iter != someMap.end(); ++iter) { 
        // do something for all iterations
        if (std::next(iter) != someMap.end()) {
            // do something for all but the last iteration
        }
    }

Although the question was asked a while ago, I thought it would be worth sharing.

Answer 3

Surprised no one mentioned it yet, but of course boost has something ;)

Boost.Next (and the equivalent Boost.Prior)

Your example would look like:

for (iter = someMap.begin(); iter != someMap.end(); ++iter) {
    // do something for all iterations
    if (boost::next(iter) != someMap.end()) {
        // do something for all but the last iteration
    }
}

Answer 4

Canonical? I can't claim that, but I'd suggest

final_iter = someMap.end();
--final_iter;
if (iter != final_iter) ...

Edited to correct as suggested by KTC. (Thanks! Sometimes you go too quick and mess up on the simplest things...)

Answer 5

Here's my optimized take:

iter = someMap.begin();

do {
    // Note that curr = iter++ may involve up to three copy operations
    curr = iter;

    // Do stuff with curr

    if (++iter == someMap.end()) {
        // Oh, this was the last iteration
        break;
    }

    // Do more stuff with curr

} while (true);

Answer 6

This seems like the simplest:

bool last_iteration = iter == (--someMap.end());