Thread: STL Map Problem with user defined Class

I've a Class called Var which handles multiple basic datatypes like int, float, char, string etc..
that Class works file when I use it like this.

Code:

Var x = (Var)2;
cout<<x;

But When I made a map of string and var its going Crazy.

Code:

typedef vector<string> 			stringList;
typedef map<string, Var> 		paramsMap;

int main(int argc, char *argv[]){
	paramsMap lst;
	lst["x"] = (Var)2;
	lst["y"] = (Var)5;
	
	for(paramsMap::iterator it=lst.begin();it != lst.end();it++){
		cout<<it->first<<" => "<<it->second<<endl;
	}

  return EXIT_SUCCESS;
}

The Above Should Output.

Code:

x => 2
x => 5

But whet its showing is.

Code:

x => y
y => 5

Here is my code for var.h and var.cpp
var.h
http://pastebin.com/m576e6998
var.cpp
http://pastebin.com/m57e02d88
toString()
http://pastebin.com/m788b6711

I'm taking a guess but your errors come from the fact you are building a (string) temporary in your constructors and then taking the pointer value of c_str() without making a copy of it (you are just storing the address). Once the temporary is destroyed, you find yourself with a dangling pointer.

That makes me say, you should use a std::string instead of a const char* to hold the "data". And don't forget to create a constructor for every built-in type or you'll have errors. Because this

Code:

const short s = 0;
Var x(s);

should yield an error.

I understood the problem that you have focused.
But Whats the solution if I store __data in char*

The problem is that you are assigning a pointer. There is one copy of the data in existance, which is deallocated when the std::string destructor is called.
What do you do? You make a copy. I'll leave it to you to figure out what you have to do to make that work.

No.
You are working with pointers and reassigning them. Perhaps a lesson on pointers will help?

Tutorial:
Name: "My data"
Address: XXXX
Data: "My data"

Name: Ptr1
Address: YYYY
Data: XXXX

Name: Ptr2
Address: ZZZZ
Data: XXXX

Name: Ptr3
Address: AAAA
Data: XXXX

Code:
char* Ptr2 = ::toString(val).c_str();
Ptr3 = Ptr2;

Generated Code:
Create Ptr2;
Create temporary std::string.
std::string::string(); // constructor
// Begin std::string::string():
Ptr1 = new char[...];
// End std::string::string():
std::string::c_str(); // Call to get pointer.
Ptr2 = return_value; // Assign returnvalue to Ptr2;
std::string::~string();
// Begin std::string::~string():
delete [] Ptr1;
// End std::string::~string():
Ptr3 = Ptr2;

Ptr1 is used internally by std::string.
Now std::string calls delete on Ptr1.

The result will be this:

Name: "My data"
Address: XXXX
Data: UNDEFINED

Name: Ptr1
Address: YYYY
Data: XXXX

Name: Ptr2
Address: ZZZZ
Data: XXXX

Name: Ptr3
Address: AAAA
Data: XXXX

So now Ptr2 and Ptr3 will all pointer to UNDEFINED, because you copied the address (the pointers) and not the data itself.

See my tutorial above. Perhaps it can shed some light on what's happening.

What foxman tries to say is that the string you generate in toString is a temporary string. It ceases to exist when you leave the function calling toString. It does, however, still point at valid memory, and as long as you don't overwrite the string with something else (string or otherwise), it will still exist in memory.

You need to allocate a separate piece of memory and copy the string into that.

--
Mats

No, you need to look up allocation functions (new/delete) and string copy functions (strcpy).