In p = new fred(), does the fred memory "leak" if fred , C/C++ Programming

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Q: In p = new Fred(), does the Fred memory "leak" if  Fred constructor throws an exception?

A: No.        

If an exception take place during the Fred constructor of p = new Fred(), the C++ language guarantees that the memory sizeof(Fred) bytes which were allocated will auto magically be released back to the heap.

Here the details: new Fred() is a two-step procedure:

sizeof(Fred) bytes of memory are allocated via the primitive void* operator new(size_t nbytes). This primitive is alike in spirit to malloc(size_t nbytes). (Note, though, that these two are not interchangeable; for example there is no guarantee that the two memory allocation primitives even employ the same heap!).

It develops an object in that memory by calling the Fred constructor. The pointer returned from the primary step is passed as the this parameter to the constructor. This step is wrapped out in a try catch block to manage the case while an exception is thrown during this step.

Therefore the real generated code is functionally similar to:

// Original code: Fred* p = new Fred();

Fred* p = (Fred*) operator new(sizeof(Fred));

try {

new(p) Fred(); // Placement new

}

catch (...) {

operator delete(p); // Deallocate memory throw; // Re-throw exception

}

The statement marked "Placement new" calls the Fred constructor. Inside the constructor the pointer p becomes the this pointer, Fred::Fred().

 


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