Re: What's your preferred way of returning a list of items?
On 12.05.2010 10:18, * DeMarcus:
Hi,
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; }
This one is OK, and will be efficient with modern compiler.
void getList( std::vector<A>& v )
{
std::copy( aList.begin(), aList.end(), v.begin() );
}
This one's signature is OK as an opt-in alternative to the first one (i.e.
provide /both/, or just the first one).
However, the implementation is incorrect unless you assume that the argument is
of exactly the right size for the result (and that assumption would be
unrealistic, to put it mildly).
You could write it like
void getList( std::vector< A >& v )
{
std::vector< A >( aList.begin(), aList.end() ).swap( v );
}
void getList( std::vector<A>* v )
{
std::copy( aList.begin(), aList.end(), v->begin() );
}
This one is just bad. Why would you want to support nullpointer argument? If
someone calls 'getList' it's in order to get that list, not in order to do nothing.
Anyway, under normal assumptions the implementation is incorrect.
// Returning a reference to aList.
const std::vector<A>& getList() { return aList; }
This one's OK if you're clear on what it does.
const std::vector<A>::const_iterator& getList()
{
return aList.begin();
}
This is one is just silly, the caller can't do anything reasonable with the
returned iterator.
Do you know more ways to return a list?
How about ways to return lists, instead of vectors (arrays)?
What's your preferred way to
return a list of items?
That doesn't make sense without a lot more explanation of exactly what you mean
by "list" and in what context you'd want to return -- what?
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?
const std::vector<std::string>& getList() { /* What here? */ }
const std::vector<std::string>::const_iterator& getList()
{
/* What here? */
}
How about defining an iterator that walks through the keys. Then you don't have
to create all those string objects.
How do you usually deal with these kind of list returns?
Sorry, again the question is to vague to be meaningful to me.
But see above.
Cheers & hth.,
- Alf
(blog at <url: http://alfps.wordpress.com>)