How would I use for_each to delete every value in an STL map?

Question

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

Answer 1

You have to make a function object:

struct second_deleter
{
    template 
    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(), _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 
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, 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 v;
v.push_back(new int);

throw "leaks!"; // nothing in vector is deleted

wrapped_container > v;
v.container.push_back(new int);

throw "no leaks!"; // wrapped_container destructs, deletes elements