In p = new Fred(), does the Fred memory “leak” if the Fred constructor throws an exception?

No.

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

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

1. sizeof(Fred) bytes of memory are allocated using the primitive void* operator new(size_t nbytes). This primitive is similar in spirit to malloc(size_t nbytes). (Note, however, that these two are not interchangeable; e.g., there is no guarantee that the two memory allocation primitives even use the same heap!).
2. It constructs an object in that memory by calling the Fred constructor. The pointer returned from the first step is passed as the this parameter to the constructor. This step is wrapped in a try … catch block to handle the case when an exception is thrown during this step.

Thus the actual generated code is functionally similar to:

1. // Original code: Fred* p = new Fred();
2. Fred* p;
3. void* tmp = operator new(sizeof(Fred));
4. try {
5. new(tmp) Fred(); // Placement new
6. p = (Fred*)tmp; // The pointer is assigned only if the ctor succeeds
7. }
8. catch (...) {
9. operator delete(tmp); // Deallocate the memory
10. throw; // Re-throw the exception
11. }

The statement marked “Placement new” calls the Fred constructor. The pointer p becomes the this pointer inside the constructor, Fred::Fred().

转载于:https://www.cnblogs.com/hustxujinkang/p/5069971.html