Re: STL Map questions for a nooob...
tanis1@gmail.com wrote:
Alright.... I'll admit it. I've been in the Java world for a long time
and still struggle with C++ memory issues. So, I will just get that
out there. Now with the questions...
Basically, I want to store values in an stl map... specifically in a
map that has a 'const char *' as the key and a 'const char*' as a
value. I'm having a scope issue in that I'm not sure how to keep the
key valid if the char* I declared goes out of scope after it has been
assigned as a key.
Why do you want to use const char * as the key. When you were in the
Java world did you use the String class or did you mess around with
char arrays there too? My guess is that you used the String class. So
when in C++ use the C++ string class.
Once a char* value has been used as a key, is the pointer the key or is
the value the key?
The pointer, unless you have specified a predicate to the contrary. You
would rarely want to have a pointer as a key. Not necessarily "never"
because if you are writing an allocating tool you might want to keep a
store of all the pointers you have allocated.
Same questions on the value.... is the value of the char* copied or is
the value just a pointer to the value.
Pointers themselves are variables that contain values. The pointer
itself is copied. What it points to is not copied.
And if it is a pointer to the
value... if that pointer I declared goes out of scope... is the map now
containing a pointer that points to dangerous nothingness?
No, when a pointer goes out of scope, what it points to is not deleted.
If there is automatic garbage collection, as some would like, then when
all the pointers are out of scope then the memory they point to would
be reclaimed. But C++ does not have garbage collection as a standard
language feature.
I can see how one could just create a char* on the heap and then delete
the value after the map is done with it by iterating along and using
'delete'. But how would one deal with this for the keys used?
The keys are the "first" item of the iterator so in pseudo-lambda-code:
for_each( map.begin(), map.end(), delete _1.first );
But you're not going to worry about this anyway because you're going to
use std::string as your key, not char *.
Thanks for any help. I've searched newsgroups and googled this issue
to death. I can't seem to get my head around it.
You've also posted the same question on codeguru.com
The books I have
read all contain examples that keep things in scope.
The books you have read probably use std::string as the key in their
examples. They don't bother to show you how to do something inferior.
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
[ comp.lang.c++.moderated. First time posters: Do this! ]