Placement syntax: Difference between revisions

Revision as of 12:44, 16 November 2012

Revision as of 11:57, 21 November 2012

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placement new is used when you do not want operator new to allocate memory (you have pre-allocated it and you want to place the object there), but you do want the object to be constructed. examplesExamples of typical situations where this may be required are:

a.* youYou want to create objects in memory shared between two different processes.

b.* youYou want objects to be created in non-pageable memory.

c.* youYou want to separate memory allocation from construction e.g. in implementing a std::vector<> (see std::vector<>::reserve).

The basic problem is that the constructor is a peculiar function; when it starts off, there is no object, only raw memory. And by the time it finishes, you have a fully initialized object. Therefore i) The constructor cannot be called on an object ii) However, it needs to access (and initialize) non-static members. This makes calling the constructor directly an error. The solution is the placement form of operator new.