The only way I have found to check for duplicates is by inserting and checking the std::pair.second for false, but the problem is that this still inserts something if the key is unused, whereas what I want is a map.contains(key); function.
Use my_map.count( key ); it can only return 0 or 1, which is essentially the Boolean result you want.
Alternately my_map.find( key ) != my_map.end() works too.
Potatoswatter's answer is all right, but I prefer to use find or lower_bound instead. lower_bound is especially useful because the iterator returned can subsequently be used for a hinted insertion, should you wish to insert something with the same key.
map<K, V>::iterator iter(my_map.lower_bound(key));
if (iter == my_map.end() || key < iter->first) { // not found
// ...
my_map.insert(iter, make_pair(key, value)); // hinted insertion
} else {
// ... use iter->second here
}
value may be skipped if insertion is unnecessary.
lower_bound-based solution is overkill. I kind of just mentioned my answer "for completeness"; like I said, yours is perfectly adequate. :-)
insert a priori. Actually, there is another difference if using a multimap, the lower_bound method inserts at the beginning of the equivalent range whereas the plain insert method adds to the end of the range.
Your desideratum,map.contains(key), was scheduled for the draft standard C++2a and implemented in C++20. In 2017 it was implemented by gcc 9.2. It's also in clang.
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map::countis implemented asfind(__x) == end() ? 0 : 1;. Formultimapthe you may have a performance argument, but that's not OP's question and I still prefer elegance.has(k)/contains(k)like every other sane map class on the planet. Poor interface design. The find() approach is too verbose and thecount(k)approach is definitely not at semantic parity withhas(k). For that matter neither isfind(k). Check out the view count on this question.