Suppose I have a STL map where the values are pointers, and I want to delete them all. How would I represent the following code, but making use of std::for_each? I'm happy for solutions to use Boost.
for( stdext::hash_map<int, Foo *>::iterator ir = myMap.begin();
ir != myMap.end();
++ir )
{
delete ir->second; // delete all the (Foo *) values.
}
(I've found Boost's checked_delete
, but I'm not sure how to apply that to the pair<int, Foo *>
that the iterator represents).
(Also, for the purposes of this question, ignore the fact that storing raw pointers that need deleting in an STL container isn't very sensible).
Note: I have subsequently found and listed a one-line answer below... but the code is pretty awful so I've accepted GMan's saner answer.
You have to make a function object:
struct second_deleter
{
template <typename T>
void operator()(const T& pX) const
{
delete pX.second;
}
};
std::for_each(myMap.begin(), myMap.end(), second_deleter());
If you're using boost, you could also use the lambda library:
namespace bl = boost::lambda;
std::for_each(myMap.begin(), myMap.end(), second_deleter(),
bl::bind(bl::delete_ptr(),
bl::bind(std::select2nd<myMap::value_type>(), _1));
But you might try the pointer containers library which does this automatically.
Note you are not using a map, but a hash_map
. I recommend you switch to boost's unordered_map
, which is more current. However, there doesn't seem to be a ptr_unordered_map
.
For safety, you should wrap this thing up. For example:
template <typename T, typename Deleter>
struct wrapped_container
{
typedef T container_type;
typedef Deleter deleter_type;
wrapped_container(const T& pContainer) :
container(pContainer)
{}
~wrapped_container(void)
{
std::for_each(container.begin(), container.end(), deleter_type());
}
T container;
};
And use it like:
typedef wrapped_container<
boost::unordered_map<int, Foo*>, second_deleter> my_container;
my_container.container./* ... */
This ensures no matter what, your container will be iterated through with a deleter. (For exceptions, for example.)
Compare:
std::vector<int*> v;
v.push_back(new int);
throw "leaks!"; // nothing in vector is deleted
wrapped_container<std::vector<int*> > v;
v.container.push_back(new int);
throw "no leaks!"; // wrapped_container destructs, deletes elements
Have you tried using BOOST_FOREACH ? That should allow you to do that in a line without creating your own functor.
I have not tested the following code but it should look something like this(if not exactly):
typedef stdext::hash_map<int, Foo *> MyMapType; //see comment.
BOOST_FOREACH( MyMapType::value_type& p, myMap )
{
delete p.second;
}
Well thats more than 1 line, due to the typedef :)
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