Iterate through a C++ Vector using a 'for'

问题:

I am new to the C++ language. I have been starting to use vectors, and have noticed that in all of the code I see to iterate though a vector via indices, the first parameter of the for loop is always something based on the vector. In Java I might do something like this with an ArrayList:

for(int i=0; i < vector.size(); i++){
   vector[i].doSomething();
}

Is there a reason I don't see this in C++? Is it bad practice?

回答1:

Is there any reason I don't see this in C++? Is it bad practice?

No. It is not a bad practice, but it renders your code certain flexibility.

Usually, pre-C++11 the code for iterating over container elements uses iterators, something like:

std::vector<int>::iterator it = vector.begin();

This is because it makes the code more flexible.

All standard library containers support and provide iterators and given that if at a later point of development you need to switch another container then this code does not need to be changed.

Note: Writing code which works with every possible standard library container is not as easily possible as it might seemingly seem to be.

回答2:

The reason why you don't see such practice is quite subjective and cannot have a definite answer, because I have seen many of the code which uses your mentioned way rather than iterator style code.

Following can be reasons of people not considering vector.size() way of looping:

1. Being paranoid about calling size() every time in the loop condition. However either it's a non-issue or it can be trivially fixed
2. Preferring std::for_each() over the for loop itself
3. Later changing the container from std::vector to other one (e.g. map, list) will also demand the change of the looping mechanism, because not every container support size() style of looping

C++11 provides a good facility to move through the containers. That is called "range based for loop" (or "enhanced for loop" in Java).

With little code you can traverse through the full (mandatory!) std::vector:

vector<int> vi;
...
for(int i : vi) 
  cout << "i = " << i << endl;

回答3:

The cleanest way of iterating through a vector is via iterators:

for (auto it = begin (vector); it != end (vector); ++it) {
    it->doSomething ();
}

or (equivalent to the above)

for (auto & element : vector) {
    element.doSomething ();
}

Prior to C++0x, you have to replace auto by the iterator type and use member functions instead of global functions begin and end.

This probably is what you have seen. Compared to the approach you mention, the advantage is that you do not heavily depend on the type of vector. If you change vector to a different "collection-type" class, your code will probably still work. You can, however, do something similar in Java as well. There is not much difference conceptually; C++, however, uses templates to implement this (as compared to generics in Java); hence the approach will work for all types for which begin and end functions are defined, even for non-class types such as static arrays. See here: How does the range-based for work for plain arrays?

回答4:

The right way to do that is:

for(std::vector<T>::iterator it = v.begin(); it != v.end(); ++it) {
    it->doSomething();
 }

Where T is the type of the class inside the vector. For example if the class was CActivity, just write CActivity instead of T.

This type of method will work on every STL (Not only vectors, which is a bit better).

If you still want to use indexes, the way is:

for(std::vector<T>::size_type i = 0; i != v.size(); i++) {
    v[i].doSomething();
}

回答5:

There's a couple of strong reasons to use iterators, some of which are mentioned here:

Switching containers later doesn't invalidate your code.

i.e., if you go from a std::vector to a std::list, or std::set, you can't use numerical indices to get at your contained value. Using an iterator is still valid.

Runtime catching of invalid iteration

If you modify your container in the middle of your loop, the next time you use your iterator it will throw an invalid iterator exception.

回答6:

With STL, programmers use iterators for traversing through containers, since iterator is an abstract concept, implemented in all standard containers. For example, std::list has no operator [] at all.

回答7:

I was surprised nobody mentioned that iterating through an array with an integer index makes it easy for you to write faulty code by subscripting an array with the wrong index. For example, if you have nested loops using i and j as indices, you might incorrectly subscript an array with j rather than i and thus introduce a fault into the program.

In contrast, the other forms listed here, namely the range based for loop, and iterators, are a lot less error prone. The language's semantics and the compiler's type checking mechanism will prevent you from accidentally accessing an array using the wrong index.