initialization new map and set in c++

Question

Hi i am doing my homeworks and i have a problem.

I need diferents maps and set in the application and i want reuse some variables. I have this global variables

map<char,set<char> > IAF; //I Am Father map<char,int> NBM; //Number Before Me set<char> WCR; //Who can run

and every time in the main i need reset this variables.

I have done two things:

IAF = new map<char,set<char> >;

and

IAF = map<char,set<char> >;

But any has run.

Can someone help me?

Answer 1

use

IAF.clear()
NBM.clear()
WCR.clear()

(Edit: references to the spec and caveats)

• map::clear()
• set::clear()

Note that if you're storing pointers, clear() will remove the pointers, but it will not delete the memory pointed to by the pointers.

Answer 2

map<char,set<char> > IAF;

This is a definition of variable. This is not a pointer. If you want to do some kind of initialization you can use one of supported methods, e.g:

std::copy( differentContainer.begin(), differentContainer.end(), IAF.begin());

or

while( ...) {
    IAF.insert( ...);
    // or
    IAF[ key] = value;
}

To delete the content of map you can do (this will not automatically delete memory pointed to by pointer in map - if you store pointers, use a smart pointers then):

IAF.clear();

Answer 3

In addition to the previous answers which are fairly clear. The new keyword, that you used at some point, is to allocate memory for a pointer.

map<char, set<char> > *IAF = new map<char, set<char> >;
//...
// free the memory
delete IAF;

Check the Dynamic memory allocation for more information and to understand when and how to use pointers.

Also, using

IAF =  map<char,set<char> >;

Is incorrect. The map<char, set<char> > is a class name (combined with templates generic programming, see the answer to What is the meaning of "generic programming" in c++? ) and hence you cannot assign it to a variable in this way. What you want to do is to call the constructor that will return an instance of that class:

IAF = map >();

However, doing it this way is not efficient at all. It creates a temporary object, destroys IAF, copies the temporary, and then destroys the temporary (unless you are using C++11 and in this case you use move, but still...). So, it's better to use the clear() as stated by the other answers.