Question

My question is simple. When I use STL containers, do they copy the value I store there (by using copy constructor) or not? What if I give them array of characters (char *) instead of string instance? How do they behave? Is guaranteed that information will be stored in heap instead of system stack?

Thanks for answers.

Answer 1

Values in STL containers are stored by-value. If you have a vector like this:

class BigObject
{
...
};

vector<BigObject> myObjs;
myObjs.push_back(obj1);
myObjs.push_back(obj2);
...

The vector will make a copy of the object you’re pushing in. Also in the case of a vector, it may make new copies later when it has to reallocate the underlying memory, so keep that in mind.

The same thing is true when you have a vector of pointers, like vector<char*> — but the difference here is that the value that is copies is the pointer, not the string it points to. So if you have:

vector<char*> myStrings;
char* str = new char[256];     // suppose str points to mem location 0x1234 here
sprintf(str, "Hello, buffer");
myStrings.push_back(str);
delete [] str;

…the vector will get a copy of the pointer. The pointer it gets will have the same value (0x1234), and since you deleted that pointer after pushing in the pointer, your vector contains a wild pointer and your code will eventually crash (sooner than later, hopefully).

Which, by the way, could have been avoided if instead of using char*s you used strings:

typedef vector<string> strings;
strings myStrings;
myStrings.push_back("Hello, buffer");