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Again, if NewCopy throws, then our own Stack's state is unchanged and the exception propagates through cleanly. Deleting the original buffer and taking ownership of the new one involves only operations that are known not to throw, so the entire if block is exception-safe.
This way, if the assignment throws, the increment is not performed and our Stack's state is unchanged. If the assignment succeeds, the Stack's state is changed to recognize the presence of the new value, and all is well.
Guideline Observe the canonical exception-safety rules: In each function, take all the code that might emit an exception and do all that work safely off to the side; only then, when you know that the real work has succeeded, should you modify the program state and clean up using only nonthrowing operations.
Pop Goes the Weasel Only one function left. That wasn't so hard, was it? Well, don't get too happy yet, because it turns out that Pop is the most problematic of these functions to write with complete exception safety. Otherwise, we create a copy of the T object to be returned, update our state, and return the T object.
If the initial copy succeeds, our state is updated and the Stack is in its new consistent state, which is also what we want. So this works, right? Well, kind of. There is a subtle flaw here that's completely outside the purview of Stack::Pop. Consider the following client code: string s1 s. That's because there is another copy[5] to worry about in either of the above cases—namely, the copy of the returned temporary into the destination.
If that copy construction or copy assignment fails, then the Stack has completed its side effect the top element has been popped off , but the popped value is now lost forever, because it never reached its destination oops. This is bad news. In effect, it means that any version of Pop that is written to return a temporary like this—and that therefore is responsible for two side effects—cannot be made completely exception-safe, because even though the function's implementation itself may look technically exception-safe, it forces clients of Stack to write exception-unsafe code.
More generally, mutator functions should not return T objects by value. See Item 19 for more about exception-safety issues for functions with multiple side effects. The point is that there can be a copy, so you have to be ready for it.
The bottom line—and it's significant—is this: Exception safety affects your class's design. In other words, you must design for exception safety from the outset, and exception safety is never "just an implementation detail. Exception safety affects a class's design.
It is never "just an implementation detail. But this business of returning references to "I no longer consider it there" resources is purely evil. If you change your implementation in the future, this may no longer be possible! Don't go there. But the real problem is that, as specified, Pop has two responsibilities—namely, to pop the top-most element and to return the just-popped value. Guideline Prefer cohesion.
Always endeavor to give each piece of code—each module, each class, each function—a single, well-defined responsibility. So another option and preferable, in my opinion is to separate the functions of "querying the top-most value" and "popping the top-most value off the stack. Well, here's one reason to do this: It avoids weakening exception safety. In fact, you've probably noticed that the above separated Top and Pop now match the signatures of the top and pop members of the standard library's stack adapter.
That's no coincidence. There's just one more fundamental point I want to drive home. I'll leave the following with you to ponder. Common Mistake "Exception-unsafe" and "poor design" go hand in hand. If a piece of code isn't exception-safe, that's generally okay and can simply be fixed. But if a piece of code cannot be made exception-safe because of its underlying design, that almost always is a signal of its poor design. Example 1: A function with two different responsibilities is difficult to make exception-safe.
Example 2: A copy assignment operator that is written in such a way that it must check for selfassignment is probably not strongly exception-safe either You will see Example 2 demonstrated very soon in this miniseries.
Note that copy assignment operators may well elect to check for self-assignment even if they don't have to—for example, they might do so for efficiency. But a copy assignment operator that has to check for self-assignment and else would not work correctly for self-assignment is probably not strongly exception-safe. Now that we have implemented an exception-safe and exception-neutral Stack, answer these questions as precisely as possible: 1.
What are the important exception-safety guarantees? For the Stack that was just implemented, what are the requirements on T, the contained type? I [email protected] RuBoard I [email protected] RuBoard Solution Just as there's more than one way to skin a cat I have a feeling I'm going to get enraged e-mail from animal lovers , there's more than one way to write exception-safe code.
In fact, there are two main alternatives we can choose from when it comes to guaranteeing exception safety. These guarantees were first set out in this form by Dave Abrahams. Basic guarantee: Even in the presence of exceptions thrown by T or other exceptions, Stack objects don't leak resources.
Note that this also implies that the container will be destructible and usable even if an exception is thrown while performing some container operation. However, if an exception is thrown, the container will be in a consistent, but not necessarily predictable, state. Containers that support the basic guarantee can work safely in some settings. Strong guarantee: If an operation terminates because of an exception, program state will remain unchanged. This always implies commit-or-rollback semantics, including that no references or iterators into the container be invalidated if an operation fails.
For example, if a Stack client calls Top and then attempts a Push that fails because of an exception, then the state of the Stack object must be unchanged and the reference returned from the prior call to Top must still be valid. Probably the most interesting point here is that when you implement the basic guarantee, the strong guarantee often comes along for free.
This last flaw in Stack::Push can be fixed fairly easily by moving some code and adding a try block. For a better solution, however, see the Stack presented in the second half of this miniseries. That Stack does not have the problem, and it does satisfy the strong commit-or-rollback guarantee. In addition to these two guarantees, there is one more guarantee that certain functions must provide in order to make overall exception safety possible: 3.
Nothrow guarantee: The function will not emit an exception under any circumstances. Overall exception safety isn't possible unless certain functions are guaranteed not to throw. In particular, 3. Guideline Understand the basic, strong, and nothrow exception-safety guarantees. Now we have some points to ponder.
As we'll see next time, using better encapsulation techniques can get rid of even this try block. That means we can write a strongly exception-safe and exception-neutral generic container, without using try or catch—very natty, very elegant. Note that this is the only T member function that must be exception-safe in order for our Stack to be exception-safe. In the second half of this miniseries, we'll also see how to reduce even these requirements, without compromising exception safety.
Along the way, we'll get an even more-detailed look at the standard operation of the statement delete[] x;.
Writing Exception-Safe Code—Part 5 Difficulty: 7 All right, you've had enough rest—roll up your sleeves, and get ready for a wild ride. Now we're ready to delve a little deeper into the same example, and write not just one but two new-andimproved versions of Stack. Not only is it, indeed, possible to write exception-safe generic containers, but by the time this miniseries is over, we'll have created no fewer than three complete solutions to the exception-safe Stack problem.
How can we improve Stack by reducing the requirements on T, the contained type? What do new[] and delete[] really do? The answer to the last question may be quite different from what one might expect. In particular, it helps to develop a habit of eyeing with mild suspicion anything that might turn out to be a function call—including user-defined operators, user-defined conversions, and silent temporary objects among the more subtle culprits—because any function call might throw.
One way to greatly simplify an exception-safe container like Stack is to use better encapsulation. Specifically, we'd like to encapsulate the basic memory management work. Most of the care we had to take while writing our original exception-safe Stack was needed just to get the basic memory allocation right, so let's introduce a simple helper class to put all that work in one place.
StackImpl also has a helper function named Swap, which exchanges the guts of our StackImpl object with those of another StackImpl. Your tasks: 1. Implement all three member functions of StackImpl, but not just any old way. Describe StackImpl's responsibilities. Why does it exist? How does the choice affect how StackImpl will be used?
Be as specific as possible. I [email protected] RuBoard I [email protected] RuBoard Solution We won't spend much time analyzing why the following functions are fully exception-safe work properly in the presence of exceptions and exception-neutral propagate all exceptions to the caller , because the reasons are pretty much the same as those we discussed in detail in the first half of this miniseries.
But do take a few minutes now to analyze these solutions, and note the commentary. Constructor The constructor is fairly straightforward. We'll use operator new to allocate the buffer as raw memory. Note that if we used a new-expression like new T[size], then the buffer would be initialized to defaultconstructed T objects, which was explicitly disallowed in the problem statement.
Again, remember what we learned about operator delete earlier in this miniseries. See "Some Standard Helper Functions" for full details about functions such as destroy and swap that appear in the next few pieces of code. Some Standard Helper Functions The Stack and StackImpl presented in this solution use three helper functions, one of which swap also appears in the standard library: construct , destroy , and swap.
Any object new'd in this way should generally be destroyed by calling its destructor explicitly as in the following two functions , rather than by using delete. We'll return to it a little later in the main miniseries; it illustrates more than one might think!
Swap Finally, a simple but very important function. Figure 1. Two StackImpl objects a and b Then executing a. Swap b changes the state to that shown in Figure 2. Figure 2. The same two StackImpl objects, after a.
Swap b Note that Swap supports the strongest exception guarantee of all—namely, the nothrow guarantee; Swap is guaranteed not to throw an exception under any circumstances. It turns out that this feature of Swap is essential, a linchpin in the chain of reasoning about Stack's own exception safety. Why does StackImpl exist? Well, there's nothing magical going on here: StackImpl is responsible for simple raw memory management and final cleanup, so any class that uses it won't have to worry about those details.
Technique 1: Private Base Class. If it were private , no one could use the class. First, consider what happens if we make it protected. Using protected means that StackImpl is intended to be used as a private base class. So Stack will be "implemented in terms of " StackImpl, which is what private inheritance means, and we have a clear division of responsibilities. The StackImpl base class will take care of managing the memory buffer and destroying all remaining T objects during Stack destruction, while the Stack derived class will take care of constructing all T objects within the raw memory.
The raw memory management takes place pretty much entirely outside Stack itself, because, for example, the initial allocation must fully succeed before any Stack constructor body can be entered. Item 13 begins the final phase of this miniseries, in which we'll concentrate on implementing this version.
Technique 2: Private Member. Using public hints that StackImpl is intended to be used as a struct by some external client, because its data members are public. So again, Stack will be "implemented in terms of " StackImpl, only this time using a HAS-A containment relationship instead of private inheritance.
We still have the same clear division of responsibilities. The StackImpl object will take care of managing the memory buffer and destroying all T objects remaining during Stack destruction, and the containing Stack will take care of constructing T objects within the raw memory. Because data members are initialized before a class's constructor body is entered, the raw memory management still takes place pretty much entirely outside Stack, because, for example, the initial allocation must fully succeed before any Stack constructor body can be entered.
As we'll see when we look at the code, this second technique is only slightly different from the first. Implement all the member functions of the following version of Stack, which is to be implemented in terms of StackImpl by using StackImpl as a private base class. Can you spot it? The only operation here that might throw is the new done in StackImpl's constructor, and that's unimportant when considering Stack's own exception safety.
If it does happen, we won't enter the Stack constructor body and there will never have been a Stack object at all, so any initial allocation failures in the base class don't affect Stack. Note that we slightly changed Stack's original constructor interface to allow a starting "hint" at the amount of memory to allocate. We'll make use of this in a minute when we write the Push function.
Guideline Observe the canonical exception-safety rules: Always use the "resource acquisition is initialization" idiom to isolate resource ownership and management. The Destructor Here's the first elegance: We don't need to provide a Stack destructor. The default compiler-generated Stack destructor is fine, because it just calls the StackImpl destructor to destroy any objects that were constructed and actually free the memory.
See the previous solution for a discussion of what construct does. The worst that can happen here is that a T constructor could fail, in which case the StackImpl destructor will correctly destroy exactly as many objects as were successfully created and then deallocate the raw memory. One big benefit derived from StackImpl is that we could add as many more constructors as we want without putting clean-up code inside each one.
Elegant Copy Assignment The following is an incredibly elegant and nifty way to write a completely safe copy assignment operator. It's even cooler if you've never seen the technique before. Take a minute to think about it before reading on. This function is the epitome of a very important guideline that we've seen already. It's beautifully elegant, if a little subtle.
We just construct a temporary object from other, then call Swap to swap our own guts with temp's, and, finally, when temp goes out of scope and destroys itself, it automatically cleans up our old guts in the process, leaving us with the new state.
Because selfassignment is exceedingly rare, I omitted the traditional if this! See Item 38 for all the gory details. Note that because all the real work is done while constructing temp, any exceptions that might be thrown either by memory allocation or T copy construction can't affect the state of our object.
Also, there won't be any memory leaks or other problems from the temp object, because the Stack copy constructor is already strongly exception-safe. Once all the work is done, we simply swap our object's internal representation with temp's, which cannot throw because Swap has a throw exception specification, and because it does nothing but copy builtins , and we're done. Note especially how much more elegant this is than the exception-safe copy assignment we implemented in Item 9.
This version also requires much less care to ensure that it's been made properly exception-safe. Study it for a moment before reading on. Count First, consider the simple else case: If we already have room for the new object, we attempt to construct it. This is safe and straightforward.
Otherwise, like last time, if we don't have enough room for the new element, we trigger a reallocation. In this case, we simply construct a temporary Stack object, push the new element onto that, and finally swap out our original guts to it to ensure they're disposed of in a tidy fashion.
But is this exception-safe? Consider: If the construction of temp fails, our state is unchanged and no resources have been leaked, so that's fine. If any part of the loading of temp's contents including the new object's copy construction fails by throwing an exception, temp is properly cleaned up when its destructor is called as temp goes out of scope.
In no case do we alter our state until all the work has already been completed successfully. Note that this provides the strong commit-or-rollback guarantee, because the Swap is performed only if the entire reallocate-and-push operation succeeds. Any references returned from Top , or iterators if we later chose to provide them, would never be invalidated by a possible internal grow operation if the insertion is not completely successful.
Stack::Top Top hasn't changed at all. Thanks to StackImpl, we can go on to write as many more constructors as we like, without having to worry about clean-up code, whereas last time each constructor would have had to know about the clean-up itself. Who says writing exception-safe code is trying? Implement all the member functions of the following version of Stack, which is to be implemented in terms of StackImpl by using a StackImpl member object. Count Whew. Which brings us to the final question in this miniseries.
The end of the line is a good place to stop and reflect, and that's just what we'll do for these last three problems. Which technique is better—using StackImpl as a private base class, or as a member object?
How reusable are the last two versions of Stack? What requirements do they put on T, the contained type? In other words, what kinds of T can our latest Stack accept? The fewer the requirements are, the more reusable Stack will be. Should Stack provide exception specifications on its functions? When deciding between private inheritance and containment, my rule of thumb is always to prefer the latter and use inheritance only when absolutely necessary.
Both techniques mean "is implemented in terms of," and containment forces a better separation of concerns because the using class is a normal client with access to only the used class's public interface.
When writing a templated class, particularly something as potentially widely useful as a generic container, always ask yourself one crucial question: How reusable is my class? Or, to put it a different way: What constraints have I put upon users of the class, and do those constraints unduly limit what those users might want to reasonably do with my class?
These Stack templates have two major differences from the first one we built. We've discussed one of the differences already. These latest Stacks decouple memory management from contained object construction and destruction, which is nice, but doesn't really affect users.
However, there is another important difference. The new Stacks construct and destroy individual objects in place as needed, instead of creating default T objects in the entire buffer and then assigning them as needed. This second difference turns out to have significant benefits: better efficiency and reduced requirements on T, the contained type.
Now, however, no default construction is needed, because the only T construction that's ever performed is copy construction. Further, no copy assignment is needed, because T objects are never assigned within Stack or StackImpl. On the other hand, we now always need a copy constructor. This means that the new Stacks require only two things of T: Copy constructor Nonthrowing destructor to be able to guarantee exception safety How does this measure up to our original question about usability?
Well, while it's true that many classes have both default constructors and copy assignment operators, many useful classes do not. In fact, some objects simply cannot be assigned to, such as objects that contain reference members, because they cannot be reseated. Now even these can be put into Stacks, whereas in the original version they could not.
That's definitely a big advantage over the original version, and one that quite a few users are likely to appreciate as Stack gets reused over time. Guideline Design with reuse in mind. In short: No, because we, the authors of Stack, don't know enough, and we still probably wouldn't want to even if we did know enough. The same is true in principle for any generic container. First, consider what we, as the authors of Stack, do know about T, the contained type: precious little.
In particular, we don't know in advance which T operations might throw or what they might throw. We could always get a little totalitarian about it and start dictating additional requirements on T, which would certainly let us know more about T and maybe add some useful exception specifications to Stack's member functions. However, doing that would run completely counter to the goal of making Stack widely reusable, and so it's really out of the question. Next, you might notice that some container operations for example, Count simply return a scalar value and are known not to throw.
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This interactive multimedia simulation is comprised of a series of ten ethnographic encounters with the culture of a fictional Mexican village set in a computer-based learning environment. These include the Quia online versions of the workbook and laboratory manual, the video program, the music playlist, and new interactive games. Also other file formats may be included in this archive: pdf, epub, fb2, mobi. Sections are listed along the left side of the window show me.
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