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

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...)

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.

This seems like the simplest:

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