Avoiding memory leaks

问题:

How can we use an overloaded operator to prevent memory leaks in C++?

Any complete example..

Regards,

PKV

回答1:

If you want to avoid memory leaks, don't use delete.

It may seem paradoxical, but the truth is that manual memory management is error prone, it is best to use automatic (or library) technics.

In C++, for each object that you create, there should be a clear ownership. That is, you should be able to identify the object lifetime, possibly depending on some others.

The first step is to avoid dynamic memory allocation: if you do not use new, you don't have anything to manage -- caveat: some library will hand you memory over and expect you to free it. Therefore, whenever possible, use the stack.

Many use of new can be avoided by using the STL containers (std::vector<T> for example) instead of rolling your own situations.

The second step is to use new sparingly, and to always hand over the memory to a single owner immediately after it's been allocated. These owners include:

• std::unique_ptr (C++0x) or boost::scoped_ptr, in a last resort std::auto_ptr.
• boost::ptr_vector and the whole collection of Boost.Pointer Container library

A single owner is easy to track down, and since the object's lifetime is tied to its owner, therefore the object's lifetime is easy to track down too.

The third step is the delicate one, the introduction of shared ownership. It really complicates all reasoning around the object's lifetime, and introduces the risk of cycles of references, which effectively mean memory leaks. They are required in some situations, but best avoided whenever possible.

• std::shared_ptr (C++0x) or equivalent (std::tr1::shared_ptr, boost::shared_ptr)
• std::weak_ptr (C++0x) or equivalent

The latter is used to "break" cycles. However it can quickly become difficult to understand where to introduce the weak_ptr, even with a graph of the relationships.

EDIT:

As noted by Tobias, this idiom is known as Resources Acquisition Is Initialization (RAII), which is awkwardly named. A newer term is emerging: Scoped Bound Resources Management (SBRM) to describe a subset of it --> binding the resources to a scope.

回答2:

Just to add some more generality to Matthieus answer:

Whenever you use a resource that needs to be freed (memory, network connections, file handles, windows handles, ...) use Resource Acquisition Is Initialization (RAII).

One manifestation of this idiom are the std::unique_ptr and boost::scoped_ptr mentioned above. If you do not have a RAII container for the needed resource available - build one. It's always worth it.

回答3:

Most people recommend using Boost or STL but there are cases where this is not possible (on operating system development, embedded systems with limited resources, etc.). In that case make sure that you use the stack whenever possible and that you only use new inside the constructor of a class and delete inside its desctructor. For double checking, there are some tools that help you find memory leaks, like valgrind.

回答4:

If you want to avoid memory leaks don't roll your own solution use boost.shared_ptr. If you really want to do it manually then put your clean up code in the destructor.