As far as I know we should never compare two const character strings using relational operators <>... because the fact that it compares the addresses rather than the values:
const char* sz1 = "Hello";
const char* sz2 = "hello";
if(sz1 < sz2);// not valid. So use strcmp instead.
map, multimap, set, multiset impose a restriction on their key so that the key should some how be compared to order the elements in the container. The default operator for the key is < operator.Everything is clear until I've created a map, set of const char* then I get the results incorrect:
std::set<const char*> scp{ "Hello", "World", "C++", "Programming" };
std::set<std::string> sstr{ "Hello", "World", "C++", "Programming" };
// error
std::copy(scp.cbegin(), scp.cend(), std::ostream_iterator<const char*>(std::cout, " "));
std::cout << std::endl;
// Ok
std::copy(sstr.cbegin(), sstr.cend(), std::ostream_iterator<std::string>(std::cout, " "));
std::cout << std::endl;
It is apparent that scp compares pointers to character strings while sstr is Ok as long as class string has defined < to work properly.
Why STL allows this? (creating associative containers whose key element type is a char*) and why here's even no warning?
The default operator for the key is
<operator.
This is not true. The default comparison operator for the non-hashed associative containers is std::less. std::less uses operator < for comparisons, but with one key difference. Unlike the pointers built in operator < where
neither pointer is required to compare greater than the other.
std::less has that the
specializations for any pointer type yield a strict total order that is consistent among those specializations and is also consistent with the partial order imposed by the built-in operators
<,>,<=,>=.
So this is a safe operation and we can reliably store pointers in the map.