Accessing a std::map with pointer type as key

Question

I'm trying to use a string as key for an std::map because that string can be changed from different parts of the program but I get a problem that I can store data in the map easily but I can't access it because it is asking for the pointer address as the key and I want to access that data from the string's value.

Here is the code

using namespace std;

string *AString = "pointer";

map<string *, bool> AMap; //Declare
AMap[AString] = true; //Insert into map with key AString

cerr << AMap["pointer"]; //Does not work

You can't change the key of a entry inside a std::map. I guess you have to rethink why you wan't to do this. You could store a string as key, and everytime the key is changed you erase the old entry and reinsert it to the map with the updated key. — P3trus
– P3trus, Commented Dec 27, 2011 at 18:50

ChrisWue · Accepted Answer · 2011-12-27 18:44:41Z

3

The solution is to not use a pointer:

using namespace std;

string AString = "pointer";

map<string, bool> AMap;
AMap[AString] = true;

cerr << AMap["pointer"];

You really do not want a map key to be changed from other parts of the program while it's in the map. This just asks for trouble and indicates a design problem.

answered Dec 27, 2011 at 18:44

ChrisWue

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1 Comment

Saif Ur Rehman Over a year ago

Why can't we use shared variable as a key in a map? I want to use a pointer because the value changes and I want the key to stay the same if it changes.

crashmstr · Accepted Answer · 2011-12-27 18:46:41Z

2

Looks like you may be coming from Java or C#?

std::string aString = "Not a Pointer";
std::map<std::string, bool> aMap;
aMap[aString] = true;
std::cerr << aMap["pointer"];

You should not be using pointers to strings in this case. Just use a map of strings instead of your map of string pointers.

In this case, std::cerr << aMap["pointer"]; works because the "pointer" gets converted into a std::string and is then looked up in the map by the string value.

answered Dec 27, 2011 at 18:46

crashmstr

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Comments

Joel · Accepted Answer · 2012-01-03 20:33:28Z

2

"pointer" is a C string that the AString pointer points to in memory. You need to do:

cerr << AMap[AString];

You could also use a a std::map<std::string,bool> which is less error prone and what you probably mean to do. Do you really want to use a pointer for a key? Creating another pointer pointing to a string with value "pointer" would not be the same key for instance.

std::string *BString(new std::string("pointer")); 
AMap[BString] = true; //Not the same key

edited Jan 3, 2012 at 20:33

answered Dec 27, 2011 at 18:45

Joel

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