Re: How to pass STL containers (say a vector) ?
Sanjay Kumar wrote:
Folks,
I am getting back into C++ after a long time and I have
this simple question: How do pyou ass a STL container
like say a vector or a map (to and from a function) ?
Prefer to return by value and pass by const reference.
function:
vector<string> tokenize(string s){
vector<string> tokenize(string const& s){
vector<string> myvector;
//split s and push_back into myvector;
//is this ok ? vector destroyed on exit from funcion ?
return myvector;
}
main:
vector<string> result = tokenize(s);
For it to work, there has to be deep copy of the result of vector inside
function (myvector) into the "result" vector before myvector is destroyed.
Is that how it works?
Most likely not (at least in a non-debug build). RVO (google for that
one) will kick in and remove the redundant copy. This is the case for
all modern (2000 or later) compilers I know.
Could this be inefficient if there is large amount
of data to be copied from the container ?
It could if your compiler cant optimise (which I doubt). If it can't
and you spend to much time returning your container, pass the
returnvalue by reference and finish with a swap instead of the return:
void tokenize(string const& s,vector<string>& result){
vector<string> myvector;
//split s and push_back into myvector;
//is this ok ? vector destroyed on exit from funcion ?
std::swap(result,myvector);
return;
}
Notice that the function now is not so easy to use. Also, it will most
likely be slightly slower than the original function.
In that case should I user pointers ? Or most likely say auto_ptr to the
Container ? Like below:
Never!
function:
auto_ptr<vector<string>> tokenize(string s){
auto_ptr <vector<string> > myvector(new vector<string>);
//split s and push_back into (*myvector)push_back(xx);
//now return auto_ptr
return myvector;
}
main:
vector<string> result = tokenize(s);
Or is this an overkill (and may be even incorrect).
This isnt even legal C++ - you are assigning a std::auto_ptr to a
std::vector-
I have read about passing iterators instead. How would you do about two
with iterators ?
Iterators can be useful for passing ranges to a function. It is not
faster than passing the container by constant reference, but it is more
flesible in case you do not want to pass an entire container.
Iterators can also be useful when you do not want to return a
container, but rather would e.g. append some values. For now, I
recommend that you stick to containers.
/Peter
Any help would be appreciated.
thanks you,
-Sanjay Kumar