Why the Destructor in C++ de-allocated memory in reverse order of how they were initialised?

Question

What is the advantage in de-allocating memory in reverse order to variables?

Answer 1

It's not just about deallocating memory, it's about symmetry in a broader sense.

Each time you create an object you are creating a new context to work in. You "push" into these contexts as you need them and "pop" back again later -- symmetry is necessary.

It's a very powerful way of thinking when it comes to RAII and exception-safety, or proving correctness w.r.t. preconditions and postconditions (constructors establish invariants, destructors ought to assert() them, and in well-designed classes each method clearly preserves them).

IMHO, lack of this feature is Java's single biggest flaw. Consider objects whose constructors open file handles or mutexes or whatever -- Armen's answer brilliantly illustrates how this symmetry enforces some common-sense constraints (languages such as Java may let Object1 go out of scope before Object2 but Object2 keeps Object1 alive by reference counting) but there's a whole wealth of design issues that fall neatly into place when considered in terms of object lifetimes.

Lots of C++ gotchas explain themselves when you bear this in mind

• why gotos can't cross initialisations
• why you may be advised to have only one return in any function (this only applies to non-RAII languages such as C and Java)
• why an exception is the only reasonable way for a constructor to fail, and likewise why destructors can never reasonably throw
• why you shouldn't call virtual functions in a constructor

etc etc...