正确的方法来销毁具有指针值的地图

Question

I am using std::map to map string values to MyType * . My map declaration looks like this:

map<string, MyType *> *my_map = new map<string, MyType>;

my_map is a private member variable of one of my classes. My problem is that I am unsure of how to destroy the map. When deleteing the map, I would also like to call delete on all of the MyType * contained in the map. Here is my current destructor:

my_map->erase(my_map->begin(), my_map->end());
delete my_map;

Will this delete the pointers contained in the map, or do I need to iterate through the map to delete each pointer before calling erase?

Answer 1

Pointers merely point. When using raw pointers, you need to know which part of your app owns the resources that the pointers point to. If they are owned by the map, you will need to iterate over the map and call delete on each pointer before the map is destroyed. But if the map just holds pointers to objects that are owned by other parts of your code, you don't need to do anything.

A safer solution is to use shared_ptr to manage object lifetime, which will ensure that the object gets deleted properly when the last shared_ptr is destroyed. You can store shared_ptrs inside the map and if no other shared_ptr instances reference the objects within the map, the objects will be destroyed when the map is destroyed, as desired.

Answer 2

If you use smart pointers instead of raw pointers, everything will be cleaned up for you automatically.

// header:
using MapType = std::map<std::string, std::shared_ptr<MyType>>;
shared_ptr<MapType> my_map;

// usage:
my_map.emplace("foo", std::make_shared<MyType>());

// destructor:
MyClass::~MyClass()
{
    // nothing!
}

Answer 3

Will this delete the pointers contained in the map [...]?

No, given the code you have provided, you will leak every member of the map.

As a rule, for every new there must be a matching delete . You have a delete for the map, but none for the elements within.

The most correct solution to this problem is to not use dynamic allocation at all. Just store MyType s directory, if possible:

map<string, MyType>

... and instead of dynamically allocating the map itself, store that automatically:

map<string,MyType> my_map;

If automatic storage duration is not possible for some reason, then use a smart pointer for the dynamic allocations. Given a C++11 compiler, use unique_ptr (or, rarely, shared_ptr or even weak_ptr ) for the elements in the map :

map<string, unique_ptr<MyType>> my_map;

(Given a C++03 compiler, use the Boost equivalents thereof.) Then when my_map is destroyed, all the elements will be delete d.

Baring all of this, if you are in a situation where none of the above will work for you (I would by highly suspect), then you will need to iterate the map youself:

struct deleter
{
  template <typename T> operator() (const T& rhs) const
  { 
    delete rhs.second;
  }
};

for_each (my_map->begin(), my_map->end(), deleter());

In C++11, this could be made a lambda, something along the line of:

for_each (my_map->begin(), my_map->end(), [](auto item) -> void
{
  delete item.second;
});

Answer 4

In modern C++, just make your life easier and use pointers only if strictly required.

You started with this code:

 map<string, MyType *> *my_map = new map<string, MyType>; 

The first thing you can do is to consider using a std::map instance as data member, instead of a pointer to it.

Then, if MyType is not super-expensive to copy and its instances are only owned by the map, just consider a simple map from string to MyType (instead of MyType* ):

// my_map data member - no pointers --> automatically deleted in class destructor
map<string, MyType> my_map;

If you really need a map containing pointers, consider using smart pointers , like std::shared_ptr (available in C++11/14) for shared ownership, or std::unique_ptr for unique non-shared ownership.
(If you target C++98/03, an option is to use boost::shared_ptr . Since there is no move semantics, you can't have unique_ptr , which is heavily based on the move semantics feature.)
eg:

// Map containing _smart_ pointers 
//     --> default destructor is fine (no need for custom delete code)
map<string, shared_ptr<MyType>> my_map;

As you can see, using value semantics (instead of raw pointers), or smart pointers , you can simplify your code and use the automatic destruction provided by C++.