CODEWITHSUNDEEP
c++stldictionary
alexmherrmann
alexmherrmann

Reputation: 1073

C++ STL map not recognizing key

I have this code, CBString is just a string class I use for some processing

  char * scrummyconfigure::dosub(strtype input)
  {
    CBString tstring;
    tstring = input;
    uint begin;
    uint end;

    begin = tstring.findchr('$');
    end = tstring.findchr('}',begin);        

    CBString k = tstring.midstr(begin+2,end-2); // this is BASE
    strtype vname = (strtype) ((const unsigned char*)k);
    strtype bvar = (strtype) "BASE";
    assert(strcmp(bvar,vname) == 0); // this never fails
    // theconf is just a struct with the map subvars
    // subvars is a map<const char *, const char *>
    out(theconf->subvars[bvar]); // always comes up with the value
    out(theconf->subvars[vname]); // always empty

    uint size = end - begin;
    tstring.remove(begin, size);

    return (const char *)tstring; // it's OKAY! it's got an overload that does things correctly
    //inline operator const char* () const { return (const char *)data; } <-- this is how it is declared in the header of the library
  }

Why is it that the strcmp always says the strings are the same, but only the variable I declared as bvar returns anything?

Upvotes: 4

Views: 298

Answers (3)

Wug
Wug

Reputation: 13196

I'm assuming strtype is defined in the following way:

typedef char * strtype

Your issue is that you're assuming that vname and bvar have the same value, where in reality, they have different values that each point to a block of memory that contains identical data.

std::map is dumbly comparing them with ==, and I bet you'd find that if you compared them with ==, you would get false, as expected. Why exactly arent you using the std::string class?

Edit: I rewrote your method to be less scary:

// implied using namespace std;
string ScrummyConfigure::removeVariableOrSomething(string tstring)
{
    uint begin; // I'll assume uint is a typedef to unsigned int
    uint end;

    begin = tstring.find('$', 0);
    end = tstring.find('}', begin);        

    string vname = tstring.substr(begin + 2, end - 2); // this is supposedly BASE
    assert(vname == "BASE"); // this should be true if vname actually is BASE

    out(this->conf->subvars[bvar]);  // wherever theconf used to be, its now a member
    out(this->conf->subvars[vname]); // of ScrummyConfigure and its been renamed to conf

    uint size = end - begin;
    tstring.erase(begin, size);

    return tstring; // no casting necessary
}

Upvotes: 5

atomicinf
atomicinf

Reputation: 3736

Just because the strings are the same doesn't mean that std::map will treat them as the same key. That depends on the Compare class that is used by the std::map, which defaults to less<KeyType> - which yields the same result as applying the less-than operator.

You can define a class that defines operator() to do a proper comparison on your strtypes and pass that as your third template argument when defining your std::map. Or, as suggested, use std::string as your strtype.

Upvotes: 5

Luchian Grigore
Luchian Grigore

Reputation: 258548

//subvars is a map<const char *, const char *>

The key of this map isn't a string per-say, but a memory address. The corresponding check would be 

assert( bvar == vname);

which will probably fail. You'll need to change the key type to a string class (either std::string or CBString to meaningfully use the map.

Upvotes: 5

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