When is destructor invoked

In the preceding example, the destructor for Base2 is called before the destructor for Base1. Calling a destructor explicitly is seldom necessary. However, it can be useful to perform cleanup of objects placed at absolute addresses. These objects are commonly allocated using a user-defined new operator that takes a placement argument. The delete operator cannot deallocate this memory because it is not allocated from the free store for more information, see The new and delete Operators.

A call to the destructor, however, can perform appropriate cleanup. To explicitly call the destructor for an object, s , of class String , use one of the following statements:. The notation for explicit calls to destructors, shown in the preceding, can be used regardless of whether the type defines a destructor. This allows you to make such explicit calls without knowing if a destructor is defined for the type.

An explicit call to a destructor where none is defined has no effect. A class needs a destructor if it acquires a resource, and to manage the resource safely it probably has to implement a copy constructor and a copy assignment. If these special functions are not defined by the user, they are implicitly defined by the compiler.

The implicitly generated constructors and assignment operators perform shallow, memberwise copy, which is almost certainly wrong if an object is managing a resource. In the next example, the implicitly generated copy constructor will make the pointers str1. Explicitly defining a destructor, copy constructor, or copy assignment operator prevents implicit definition of the move constructor and the move assignment operator.

In this case, failing to provide move operations is usually, if copying is expensive, a missed optimization opportunity. Feedback will be sent to Microsoft: By pressing the submit button, your feedback will be used to improve Microsoft products and services.

Privacy policy. Skip to main content. Exception Handling. File Handling. Table of Contents. Save Article. Improve Article. Like Article. Recommended Articles. Article Contributed By :. Neither the compiler nor the run-time system make any attempt to check whether you did this right. You have been warned. You are also solely responsible for destructing the placed object. This is done by explicitly calling the destructor:. But how can we later delete those objects correctly?

A pointer to any X allocated anywhere can be assigned to p1. If an Arena keeps track of what objects it holds, you can even write destroy to defend itself against mistakes. You never need to explicitly call a destructor except with placement new. They are destroyed in the reverse order they appear within the declaration for the class.

Base classes are destructed after member objects. In the event of multiple inheritance, direct base classes are destructed in the reverse order of their appearance in the inheritance list. Note: Order dependencies with virtual inheritance are trickier. See this FAQ for details. Starting with a simple memory allocator function, allocate , you would use placement new to construct an object in that memory. In other words, the following is morally equivalent to new Foo :. Each of these memory pool objects will allocate a big chunk of memory using some specific system call e.

Your memory-pool class might look something like this:. Now one of your users might have a Pool called pool , from which they could allocate objects like this:. That allows users to do lots of funky things.

For example, if they have a chunk of the system that allocates memory like crazy then goes away, they could allocate all their memory from a Pool , then not even bother doing any delete s on the little pieces: just deallocate the entire pool at once.

Another angle: many systems support a non-standard function often called alloca which allocates a block of memory from the stack rather than the heap. Naturally this block of memory automatically goes away when the function returns, eliminating the need for explicit delete s. Assuming you survived the 6 or 8 lines of code needed to wrap your allocate function as a method of a Pool class, the next step is to change the syntax for allocating objects. The goal is to change from the rather clunky syntax new pool.

To make this happen, you need to add the following two lines of code just below the definition of your Pool class:.