std::next vs std::advance in C++

std::advance and std::next are used to advance the iterator by a certain position, such that we can make the iterator point to a desired position. Although both have same purpose, but their implementation is different from each other. This makes it important for us to understand the difference between the two. In C++11, std::next() will advance by one by default, whereas std::advance() requires a distance.

1. Syntactical Difference: Syntax of std::advance and std::next is:

// Definition of std::advance()
template
void advance( InputIt& it, Distance n );

it: Iterator to be advanced
n: Distance to be advanced

// Definition of std::next()
ForwardIterator next (ForwardIterator it,
 typename iterator_traits::difference_type n = 1);

it: Iterator pointing to base position
n: Distance to be advanced from base position.

• Return type: std::advance does not return anything, whereas std::next returns an iterator after advancing n positions from the given base position.
• As in the syntax of std::next(), it will at least advance the iterator by one position, even if we do not specify the position which it has to advance as it has a default value one, whereas if we use std::advance, it has no such default argument.

1. Working
   • Argument Modification: std::advance modifies it arguments such that it points to the desired position, whereas, std::next does not modify its argument, infact it returns a new iterator pointing to the desired position. 

• Output:

v1 = 1 2 3
v2 = 4 5 6 1 2 

• Explanation: As can be seen, we want to make ii point to 2 spaces ahead of where it is pointing, so if we use std::advance ii will be pointing to two spaces ahead, whereas if we use std::next, then ii will not be advanced, but an iterator pointing to the new position will be returned, and will be stored in iii.

1. Pre-requisite: std::next requires the iterator passed as argument to be of type at least forward iterator, whereas std::advance does not have such restrictions, as it can work with any iterator, even with input iterator or better than it.

Let us see the differences in a tabular form -: