CODEWITHSUNDEEP
c++visual-c++stl
user1799537
user1799537

Reputation:

std::map::find doesn't work

I'm trying to keep information in std::map. But I have a problem with find:

typedef map<string, string, equal_to<string>> MAP_STRING2STRING;
....
MAP_STRING2STRING map;
MAP_STRING2STRING::const_iterator iter;

When I try to find key, I get the following error:

iter = map.find(key);

Error from Visual C++
What am I doing wrong?
This error appears only when I have something in map.

Upvotes: 2

Views: 1627

Answers (2)

juanchopanza
juanchopanza

Reputation: 227370

Your map has the wrong kind of comparison functor. You need strict weak ordering (less-than or greater-than type comparison), not equality. You can just omit the comparison functor parameter and use a less-than comparison for std::string. This implements strict weak ordering via a lexicographical comparison of strings:

typedef map<string, string> MAP_STRING2STRING;

This is equivalent to

typedef map<string, string, less<string> > MAP_STRING2STRING;

Internally, the map uses the strict weak ordering comparison to both order itself, and determine whether two keys are the equal.

The third template parameter allows you to instantiate a map with a custom ordering criterion. For example, this would create a map with the reverse ordering of the one above:

typedef map<string, string, greater<string> > MAP_STRING2STRING;

Upvotes: 9

Mr.C64
Mr.C64

Reputation: 42924

If you want a simple map from std::string to std::string, just use map<string, string> (drop that equal_to<string> bogus "comparator").

Moreover, since you have a variable named "map", this can cause conflict with the STL map class. Either change the variable name (e.g. call it myMap), or use the std:: namespace prefix for std::map class:

typedef map<string, string> MAP_STRING2STRING;
....
MAP_STRING2STRING myMap;

In addition, since from the error message you are using VS2010, you can use the convenient auto C++11 keyword, to avoid the "clutter" of MAP_STRING2STRING::const_iterator, and just use:

auto iter = myMap.find(someKey);

Upvotes: 1

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