Re: What's your preferred way of returning a list of items?
On May 12, 9:18 am, DeMarcus <use_my_alias_h...@hotmail.com> wrote:
Here are a couple of ways to return some list of items.
struct A
{
};
std::vector<A> aList; // Some list of items.
// Returning a copy.
std::vector<A> getList() { return aList; }
That's generally the preferred way. Until the profiler says
otherwise.
void getList( std::vector<A>& v )
{
std::copy( aList.begin(), aList.end(), v.begin() );
}
That should be:
std::copy( aList.begin(), aList.end(), std::back_inserter(v) );
void getList( std::vector<A>* v )
{
std::copy( aList.begin(), aList.end(), v->begin() );
}
Same thing here. You should be using sd::back_inserter(*v),
rather than v->begin().
Whether you use a pointer or a reference in this case depends on
the local coding conventions.
// Returning a reference to aList.
const std::vector<A>& getList() { return aList; }
Where does aList live? The semantics here are distinctly
different from those of the previous versions, and the choice
between this version and one of the previous should be made
according to the desired semantics.
const std::vector<A>::const_iterator& getList()
{
return aList.begin();
}
This simply doesn't work. You're returning a reference to a
temporary.
Do you know more ways to return a list? What's your preferred
way to return a list of items?
The only way to "return" a list is to return it.
Also, here comes another trickier one. Let's say I have a map
instead and want to return the keys.
std::map<std::string, A> aMap;
// Returning a copy of the keys.
std::vector<std::string> getList()
{
std::vector<std::string> aKeys;
auto keysEnd = aMap.end();
for( auto i = aMap.begin(); i != keysEnd; ++i )
aKeys.push_back( (*i).first );
return aKeys;
}
void getList( std::vector<std::string>& v )
{
auto keysEnd = aMap.end();
for( auto i = aMap.begin(); i != keysEnd; ++i )
v.push_back( (*i).first );
}
void getList( std::vector<std::string>* v )
{
auto keysEnd = aMap.end();
for( auto i = aMap.begin(); i != keysEnd; ++i )
v->push_back( (*i).first );
}
// But is it even possible to return a reference to // the
keys in a map?
It's possible to return a reference to a single key, but it's
not possible to return a reference to some collection which
doesn't exist otherwise.
const std::vector<std::string>& getList() { /* What here? */ }
const std::vector<std::string>::const_iterator& getList()
{
/* What here? */
}
How do you usually deal with these kind of list returns?
I return what I should return, until the profiler says
otherwise. In which case, I use an out argument, whose format
(reference or pointer) depends on the local coding conventions.
--
James Kanze