Sunday, August 11, 2013

RAII - heap alloc/deleted in separate functions

I think the standard pattern of RAII smart ptr is via a local variable. When it goes out-of-scope RAII kicks in. (P199 [[Essential C++]] is authoritative...)

Q: Now suppose a function is designed to new up an object on the heap, and pass (by several means) it out of the function. The pointee object on heap needs to be cleaned up in another method such as mymain(). So the smart ptr is not local variable. How can we use RAII smart ptr for this?

1) A tested technique - in some outer function such as mymain(), declare/create the smart ptr local variable and pass it (by ref NOT by value -- tested in MSVS) into the factory function, which puts the heap ptr into the smart ptr. After using it in some other function, eventually we reach the end of mymain() and the local variable goes out of scope. Exception-safe.

2) A tested technique - a global vector holding a bunch of smart pointers. From anywhere we can save a "resource pointer" into the vector. From anywhere we can also clear the vector, and those pointee objects will receive the delete call. However, there's no automatic, fool proof clean-up because a careless developer could forget to call clear(). Also, no exception-safety.

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