与Java相比，C ++ STL数据结构

Question

I'm currently learning C++ and trying to get used to the standard data structures that come with it, but they all seem very bare. For example, list doesn't have simple accessors like get(index) that I'm used to in Java. Methods like pop_back and pop_front don't return the object in the list either. So you have to do something like:

Object blah = myList.back();
myList.pop_back();

Instead of something simple like: Object blah = myList.pop_back();

In Java, just about every data structure returns the object back so you don't have to make these extra calls. Why is the STL containers for C++ designed like this? Are common operations like this that I do in Java not so common for C++?

edit: Sorry, I guess my question was worded very poorly to get all these downvotes, but surely somebody could have edited it. To clarify, I'm wondering why the STL data structures are created like this in comparison to Java. Or am I using the wrong set of data structures to begin with? My point is that these seem like common operations you might use on (in my example) a list and surely everybody does not want to write their own implementation each time.

edit: reworded the question to be more clear.

Answer 1

Quite a few have already answered the specific points you raised, so I'll try to take a look for a second at the larger picture.

One of the must fundamental differences between Java and C++ is that C++ works primarily with values, while Java works primarily with references.

For example, if I have something like:

class X {
    // ...
};

// ...
X x;

In Java, x is only a reference to an object of type X. To have an actual object of type X for it to refer to, I normally have something like: X x = new X; . In C++, however, X x; , by itself, defines an object of type X , not just a reference to an object. We can use that object directly, not via a reference (ie, a pointer in disguise).

Although this may initially seem like a fairly trivial difference, the effects are substantial and pervasive. One effect (probably the most important in this case) is that in Java, returning a value does not involve copying the object itself at all. It just involves copying a reference to the value. This is normally presumed to be extremely inexpensive and (probably more importantly) completely safe -- it can never throw an exception.

In C++, you're dealing directly with values instead. When you return an object, you're not just returning a reference to the existing object, you're returning that object's value, usually in the form of a copy of that object's state. Of course, it's also possible to return a reference (or pointer) if you want, but to make that happen, you have to make it explicit.

The standard containers are (if anything) even more heavily oriented toward working with values rather than references. When you add a value to a collection, what gets added is a copy of the value you passed, and when you get something back out, you get a copy of the value that was in the container itself.

Among other things, this means that while returning a value might be cheap and safe just like in Java, it can also be expensive and/or throw an exception. If the programmer wants to store pointers, s/he can certainly do so -- but the language doesn't require it like Java does. Since returning an object can be expensive and/or throw, the containers in the standard library are generally built around ensuring they can work reasonably well if copying is expensive, and (more importantly) work correctly, even when/if copying throws an exception.

This basic difference in design accounts not only for the differences you've pointed out, but quite a few more as well.

Answer 2

back() returns a reference to the final element of the vector, which makes it nearly free to call. pop_back() calls the destructor of the final element of the vector.

So clearly pop_back() cannot return a reference to an element that is destroyed. So for your syntax to work, pop_back() would have to return a copy of the element before it is destroyed.

Now, in the case where you do not want that copy, we just needlessly made a copy.

The goal of C++ standard containers is to give you nearly bare-metal performance wrapped up in nice, easy to use dressing. For the most part, they do NOT sacrifice performance for ease of use -- and a pop_back() that returned a copy of the last element would be sacrificing performance for ease of use.

There could be a pop-and-get-back method, but it would duplicate other functionality. And it would be less efficient in many cases than back-and-pop.

As a concrete example,

vector<foo> vec; // with some data in it
foo f = std::move( vec.back() ); // tells the compiler that the copy in vec is going away
vec.pop_back(); // removes the last element

note that the move had to be done before the element was destroyed to avoid creating an extra temporary copy... pop_back_and_get_value() would have to destroy the element before it returned, and the assignment would happen after it returned, which is wasteful.

Answer 3

A list doesn't have a get(index) method because accessing a linked list by index is very inefficient. The STL has a philosophy of only providing methods that can be implemented somewhat efficiently. If you want to access a list by index in spite of the inefficiency, it's easy to implement yourself.

The reason that pop_back doesn't return a copy is that the copy constructor of the return value will be called after the function returns (excluding RVO/NRVO). If this copy constructor throws an exception, you have removed the item from the list without properly returning a copy. This means that the method would not be exception-safe. By separating the two operations, the STL encourages programming in an exception-safe manner.

Answer 4

Why is the STL containers for C++ designed like this?

I think Bjarne Stroustrup put it best :

C++ is lean and mean. The underlying principle is that you don't pay for what you don't use.

In the case of a pop() method that would return the item, consider that in order to both remove the item and return it, that item could not be returned by reference. The referent no longer exists because it was just pop() ed. It could be returned by pointer, but only if you make a new copy of the original, and that's wasteful. So it would most likely be returned by value which has the potential to make a deep copy. In many cases it won't make a deep copy (through copy elision), and in other cases that deep copy would be trivial. But in some cases, such as large buffers, that copy could be extremely expensive and in a few, such as resource locks, it might even be impossible.

C++ is intended to be general-purpose, and it is intended to be fast as possible. General-purpose doesn't necessarily mean "easy to use for simple use cases" but "an appropriate platform for the widest range of applications."

Answer 5

Concerning pop() -like functions, there are two things (at least) to consider:

1) There is no clear and safe action for a returning pop_back() or pop_front() for cases when there is nothing there to return.

2) These functions would return by value. If there were an exception thrown in the copy constructor of the type stored in the container, the item would be removed from the container and lost. I guess this was deemed to be undesirable and unsafe.

Concerning access to list, it is a general design principle of the standard library not to avoid providing inefficient operations. std::list is a double-linked list, and accessing a list element by index means traversing the list from the beginning or end until you get to the desired position. If you want to do this, you can provide your own helper function. But if you need random access to elements, then you should probably use a structure other than a list.

Answer 6

list doesn't even have simple accessors like get(index)

Why should it? A method that lets you access the n-th element from the list would hide the complexity of O(n) of the operation, and that's the reason C++ doesn't offer it. For the same reason, C++'s std::vector doesn't offer a pop_front() function, since that one would also be O(N) in the size of the vector.

Methods like pop_back and pop_front don't return the object in the list either.

The reason is exception safety. Also, since C++ has free functions, it's not hard to write such an extension to the operations of std::list or any standard container.

template<class Cont>
typename Cont::value_type return_pop_back(Cont& c){
  typename Cont::value_type v = c.back();
  c.pop_back();
  return v;
}

It should be noted, though, that the above function is not exception-safe, meaning if the return v; throws, you'll have a changed container and a lost object.

Answer 7

In Java a pop of a general interface can return a reference to the object popped.

In C++ returning the corresponding thing is to return by value.

But in the case of non-movable non-POD objects the copy construction might throw an exception. Then, an element would have been removed and yet not have been made accessible to the client code. A convenience return-by-value popper can always be defined in terms of more basic inspector and pure popper, but not vice versa.

This is also a difference in philosophy.

With C++ the standard library only provides basic building blocks, not directly usable functionality (in general). The idea is that you're free to choose from thousands of third party libraries, but that freedom of choice comes at a great cost , in usability, portability, training, etc. In contrast, with Java you have mostly all you need (for typical Java programming) in the standard library, but you're not effectively free to choose (which is another kind of cost).