取消引用集合迭代器时，将“ string＆”绑定到“ const string”时出错

Question

I'm using C++11 and STL. I have this piece of code:

std::set<std::string>::iterator it;
[...]
std::string &str = *it;

Compiler throws this error:

error: binding reference of type 'std::__cxx11::string& {aka std::__cxx11::basic_string&}' to 'const std::__cxx11::basic_string' discards qualifiers

Why is it happening?

Answer 1

The keys in a set or map are const. If they were not, the set would not be able to provide its guarantee that elements are weakly ordered on the key. ie the would become inexplicably unsorted when users mutated the keys.

The std::set iterator dereferences to a key. Keys are (to reiterate) const.

Mutable references cannot bind to const references (language-imposed rule).

Therefore, either:

std::string const& str = *it;   // refer to the key

or

std::string str = *it;          // take a copy of the key