C++ – Incrementing iterators: Is ++it more efficient than it++?

c++iteratorstl

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Is there a performance difference between i++ and ++i in C++?

I am writing a program where an iterator is used to loop through a std::vector. Somebody told me that doing ++it in the for statement leads to more efficient code. In other words, they are saying that:

for ( vector<string>::iterator it=my_vector.begin(); it != my_vector.end(); ++it )

runs faster than

for ( vector<string>::iterator it=my_vector.begin(); it != my_vector.end(); it++ )

Is this true? If it is, what is the reason behind the efficiency improvement? All it++/++it does is move the iterator to the next item in the vector, isn't it?

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The reason behind the preincrement being faster is that post-increment has to make a copy of the old value to return. As GotW #2 put it, "Preincrement is more efficient than postincrement, because for postincrement the object must increment itself and then return a temporary containing its old value. Note that this is true even for builtins like int."

GotW #55 provides the canonical form of postincrement, which shows that it has to do preincrement plus some more work:

T T::operator++(int)
{
  T old( *this ); // remember our original value
  ++*this;        // always implement postincrement
                  //  in terms of preincrement
  return old;     // return our original value
}

As others have noted, it's possible for some compiler to optimize this away in some cases, but if you're not using the return value it's a good idea not to rely on this optimization. Also, the performance difference is likely to be very small for types which have trivial copy constructors, though I think using preincrement is a good habit in C++.

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C++ – Why use iterators instead of array indices

The first form is efficient only if vector.size() is a fast operation. This is true for vectors, but not for lists, for example. Also, what are you planning to do within the body of the loop? If you plan on accessing the elements as in

T elem = some_vector[i];

then you're making the assumption that the container has operator[](std::size_t) defined. Again, this is true for vector but not for other containers.

The use of iterators bring you closer to container independence. You're not making assumptions about random-access ability or fast size() operation, only that the container has iterator capabilities.

You could enhance your code further by using standard algorithms. Depending on what it is you're trying to achieve, you may elect to use std::for_each(), std::transform() and so on. By using a standard algorithm rather than an explicit loop you're avoiding re-inventing the wheel. Your code is likely to be more efficient (given the right algorithm is chosen), correct and reusable.

Python – Difference between Python’s Generators and Iterators

iterator is a more general concept: any object whose class has a __next__ method (next in Python 2) and an __iter__ method that does return self.

Every generator is an iterator, but not vice versa. A generator is built by calling a function that has one or more yield expressions (yield statements, in Python 2.5 and earlier), and is an object that meets the previous paragraph's definition of an iterator.

You may want to use a custom iterator, rather than a generator, when you need a class with somewhat complex state-maintaining behavior, or want to expose other methods besides __next__ (and __iter__ and __init__). Most often, a generator (sometimes, for sufficiently simple needs, a generator expression) is sufficient, and it's simpler to code because state maintenance (within reasonable limits) is basically "done for you" by the frame getting suspended and resumed.

For example, a generator such as:

def squares(start, stop):
    for i in range(start, stop):
        yield i * i

generator = squares(a, b)

or the equivalent generator expression (genexp)

generator = (i*i for i in range(a, b))

would take more code to build as a custom iterator:

class Squares(object):
    def __init__(self, start, stop):
       self.start = start
       self.stop = stop
    def __iter__(self): return self
    def __next__(self): # next in Python 2
       if self.start >= self.stop:
           raise StopIteration
       current = self.start * self.start
       self.start += 1
       return current

iterator = Squares(a, b)

But, of course, with class Squares you could easily offer extra methods, i.e.

    def current(self):
       return self.start

if you have any actual need for such extra functionality in your application.

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C++ – Why use iterators instead of array indices

Python – Difference between Python’s Generators and Iterators

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