Re: STL Map questions for a nooob...
tanis1@gmail.com wrote:
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.
Once a char* value has been used as a key, is the pointer the key or is
the value the key?
A char* is just a pointer. Anything it points to is up to yourself and
std::map doesn't even look at what it points to, it only looks at the
pointer.
Same questions on the value.... is the value of the char* copied or is
the value just a pointer to the value.
The value is copied. Since the value is a char pointer, the pointer is
copied. Whatever this pointer points to is not copied or even looked at,
just as the key.
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?
I think that you are confusing a pointer and what a pointer points to. Even
if your local copy (which would be rather the original) of the pointer went
out of scope, it wouldn't affect the copy in the map. A totally different
thing is again what these pointers point to, but that is outside the scope
of the map.
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?
Well, assuming you want to store strings, use a map<string,string>. Unless
you have very good reasons, you should do this.
If you really can't put a proper object into the map, e.g. because the
object can't be copied, then you need to use pointers. However, you should
consider using so-called smart pointers, which are classes that behave
almost like pointers but care for proper deallocation of the memory. If in
doubt, use boost::shared_ptr, which has semantics close to those used by
Java references.
HTH
Uli
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