Wednesday, January 26, 2011

prevent heap allocation for my class

(Much of this write-up is based on http://www.cprogramming.com/tutorial/operator_new.html) It's useful if we can restrict a special class's instantiation to the stack only, and disallow heap allocation. Overload op-new for your class, and make it throw exception unconditionally? Alternatively, can we declare it but leave it unimplemented like --

class Myclass {
public:
       void* operator new(size_t);
       void operator delete(void*);
};

Both of them are by default static members. Overloading can be used for many purposes. For example, we may need to alter the exception thrown in case of failure--std::bad_alloc--and throw something else:

The relationship between Operator New and the New Keyword


Don't be confused by the fact that there is both a new keyword and an operator new. When you write:

MyClass *x = new MyClass;

there are actually two things that happen--memory allocation and object construction; the new keyword is responsible for both. One step in the process is to call operator new in order to allocate memory; the other step is to actually invoke the constructor. Operator new lets you change the memory allocation method, but does not have any responsibility for calling the constructor. That's the job of the new keyword.

It is possible to invoke the constructor without calling operator new. That's the job of placement new

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