Why not modify key of associative container?
c++, c++11, language-lawyer, std
Solution
This injects Undefined Behavior in your program if you modify the values in a way that the comparison of any two keys is different after the change according to the comparator you specified.
Per Paragraph 23.2.4/3 of the C++11 Standard ([associative.reqmts]):
The phrase “equivalence of keys” means the equivalence relation imposed by the comparison and not the `operator==` on keys. That is, two keys `k1` and `k2` are considered to be equivalent if for the comparison object `comp`, `comp(k1, k2) == false && comp(k2, k1) == false`. For any two keys `k1` and `k2` in the same container, calling `comp(k1, k2)` shall always return the same value.
Problem
I know that it's a terrible idea to change the key of an object in an associative container, but I wonder where exactly the standard forbids me to do so. Consider: ``` #include <map> #include <memory> struct X { int i; }; struct lt { bool operator()( const std::shared_ptr< X >& lhs, const std::shared_ptr< X >& rhs ) const { return lhs->i < rhs->i; } }; int main() { std::map< std::shared_ptr< X >, int, lt > m; auto x = std::make_shared< X >(); x->i = 1; m.insert( std::make_pair( x, 2 ) ); x->i = 42; // change key wrt the container! } ``` I assume that the above should be illegal, but I was reading the standard for some time now and I can't find anything that actually makes it illegal. Where is it? Or is it hiding in a future defect report?