This warning appears in a range-based loop with a loop variable of const ref type where the type of the variable does not match the one returned by the iterator. This means that an implicit conversion happens, which can for example result in expensive deep copies.


map<int, vector<string>> my_map;
for (const pair<int, vector<string>>& p : my_map) {}
// The iterator type is in fact pair<const int, vector<string>>, which means
// that the compiler added a conversion, resulting in a copy of the vectors.

The easiest solution is usually to use const auto& instead of writing the type manually.