Re: STL Vector - pass by reference?

Gerry Hickman wrote:
Hi,

In an earlier thread entitled "STL vector - which style of for() loop?" I had a function that could populate a vector (with a variable number of strings) and pass it back to the caller, but people pointed out this would create a "copy" of the vector and it may be better to pass by reference. Doug Harrison offered this example:

----- example start -----

I would use pass-by-reference to avoid this needless cost, e.g.

vector<string>::size_type
void GetDeviceClasses(vector<string>& guids)
{
guids.clear();
// If you can estimate n, reserve can eliminate reallocations.
// guids.reserve(n);
...
returns guids.size();
}

----- example end -----

but when I came to actually code this, I ran into some problems. I managed to code something that appears to achieve the objective, but my code is almost "back-to-front" (in terms of * and &) of what Doug posted. Can someone clarify?

----- my attempt -----

using namespace std; // just for this demo

vector<string> guids;
PopulateStrings(&guids);
cout << "Count of guids is now " << guids.size(); // prints 2

void PopulateStrings(vector<string> * guids)
{
guids->clear();
guids->push_back("test1");
guids->push_back("test2");
}

----- end my attempt -----


There are two ways to "pass by reference." They are to pass a reference, or to pass a pointer. Doug showed using a reference, your version is using a pointer. Performance would be equal either way.

--
Scott McPhillips [MVP VC++]

.

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