C++, how to correctly copy std::vector<Class *> in copy constructor?
The data()
is not necessary because that will be done automatically to the vector before the constructor is entered. You only need to initialise members that are POD (plain old data) types or types which have no default constructor (or references, constants, etc).
You can initialise the vector with the number of elements that the other one has, so that the vector doesn't have to resize itself as it grows. If you don't, you're starting with a small vector and making it incrementally reach the destination size via allocations and reallocations. This will make the vector the correct size from the very beginning:
B::B(const B& orig) : data(orig.data.size()) {
for (std::size_t i = 0; i < orig.data.size(); ++i)
data[i] = new A(*orig.data[i]);
}
Notice that you are not using push_back
any more because the vector is already full of orig.data.size()
number of elements that are default constructed (which is NULL
in the case of pointers).
This also trims down the code because you can use an integer to iterate it instead of an iterator.
If you really want to use iterators, you can do
B::B(const B& orig) : data(orig.data.size()) {
// auto is preferable here but I don't know if your compiler supports it
vector<A*>::iterator thisit = data.begin();
vector<A*>::const_iterator thatit = orig.data.cbegin();
for (; thatit != orig.data.cend(); ++thisit, ++thatit)
*thisit = new A(**thatit);
}
The advantage of this is that it will work with other container types (like list
) by just changing the types of the iterators (but of course that would go away if you have auto
).
If you want to add exception-safety, you need a try/catch
block:
B::B(const B& orig) : data(orig.data.size()) {
try {
// auto is preferable here but I don't know if your compiler supports it
vector<A*>::iterator thisit = data.begin();
vector<A*>::const_iterator thatit = orig.data.cbegin();
for (; thatit != orig.data.cend(); ++thisit, ++thatit)
*thisit = new A(**thatit);
} catch (...) {
for (vector<A*>::iterator i = data.begin(); i != data.end(); ++i)
if (!*i)
break;
else
delete *i;
throw;
}
}
This way you will not have a memory leak if one of the new
calls throws an exception. Of course you can use the try/catch
along with the way without iterators if you'd rather do it that way.
Vyktor
"I've been using VIM for 2 years now; mostly because I can't figure out how to turn it off." Started with html, php and C++. Now working with C, C#, python and doing some reverse engineering.
Updated on July 19, 2022Comments
-
Vyktor almost 2 years
I'm using this two classes
// This is generic data structure containing some binary data class A { public: A(); A(const A&); ~A(); } // Main data container class B { public: B(); B( const B&); ~B(); protected: std::vector<A *> data; } // Copy constructor for class b B::B( const B& orig):data() { for( std::vector<A *>::const_iterator it = orig.data.begin(); it < orig.data.end(); ++it){ data.push_back( new A( *(*it))); } }
I guess this class would do its job, but I'm looking for a way to reach total perfection.
First,
:data()
- is this initialization required to initialize empty vector correctly (is it clean code)?How is
vector::iterator
used in copy constructor? The only way I found is the one I've written into code (const should be mandatory for copy constructor).Does copying the vector copy pointer values and not whole objects?
And finally new data initialization... Is there any way to replace the whole loop with less code and/or is there any standard how to write copy constructor for std::containers which contain object pointers?
Sub question: I'm assuming using
vector<A *>
is much more suitable and effective for various reasons than justvector<A>
(not copying every time, power to decide whether (not) to copy objects...) Is this assumption correct? -
someguy over 12 yearsYou need to dereference
orig.data[i]
. -
Emil Styrke over 12 yearsOne more thing to take into consideration is error handling: what happens if one of the
new
s fail in the middle of the loop? Probably the program will just terminate, but if the out of memory error is handled further up in the call stack you'll have a memory leak. -
Seth Carnegie over 12 years@EmilStyrke please review my latest edit to see if that handles the situation you are talking about.
-
someguy over 12 years@Seth Carnegie: You just need to add a little check in case
*i
is a null pointer (in which case you would just break out of the loop) :) -
Seth Carnegie over 12 years@someguy why (besides efficiency)?
-
someguy over 12 years@Seth Carnegie: because copying the vector may have failed half way, so the other half would be null pointers
-
Seth Carnegie over 12 years@someguy yes but it would still work fine (although doing a little extra work) so it's not absolutely necessary. I will add it though since it would be more efficient.
-
someguy over 12 years@Seth Carnegie: Deleting a null pointer results in undefined behaviour. Also, you don't need to set
*i
to NULL. -
Seth Carnegie over 12 years@someguy actually calling
delete
on a null pointer is defined to do nothing. Not sure why I set*i
to null, seemed like a good idea at the time :) At least now I can get rid of the curly braces. -
someguy over 12 years@Seth Carnegie: Oh yeah you're right. I think I got it mixed up with dereferencing a null pointer :p.
-
Emil Styrke over 12 years@SethCarnegie: Looks like it handles the case I had in mind, yes.