What is const_cast and when is it (rarely) appropriate?

const_cast adds or removes const/volatile qualifiers from a pointer or reference. Removing const to modify an object that is genuinely const is undefined behavior; const_cast is only safe to remove const you know was added artificially (e.g., interfacing with a legacy non-const API on an object that is actually mutable). It's a code smell to be used sparingly.

void legacy(char* s);
void wrap(const char* s) { legacy(const_cast<char*>(s)); } // only if legacy won't modify