Iterators are one of the most useful features of containers in modern C++. Mostly we use them with vectors, maps, strings, and other containers of C++. In C++11, the begin() and end() iterators are used to define the start of iteration and the end of the iteration, mostly used in the for loops. In C++14, there are new additions to the global std::begin – std::end functions, and in this post, we explain these new begin-end iterators.
Table of Contents
What are the begin end iterators in C++11 and beyond?
In modern C++, containers are data storage arrays. They are very useful for iterating and searching data with their amazing methods and properties. An iterator (<iterator>) is an object that points to an element in a range of elements (i.e. characters of a string or members of a vector). We can use Iterators to iterate through the elements of this range using a set of operators, for example using the ++, –, and * operators.
Iteration can be done with begin/end iterators,
- The
begin()method returns an iterator pointing to the first element in the vector. - The
end()method returns an iterator pointing to the theoretical element that follows the last element in the vector.
Here is a simple example how we can use begin end iterators in the for iteration as below.
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for (auto vi= vec.begin(); vi!= vec.end(); vi++) std::cout << *vi << ','; |
What are the reverse begin end iterators in C++14?
In C++14 , there are new additions to the global std::begin – std::end functions, they were augmented with reverse iterators, std::rbegin and std::rend. These are available in C++17 too.
Reverse iteration can be done with rbegin and rend iterators.
- The
rbegin()method returns a reverse iterator pointing to the last element in the vector. It moves from last to first element. - The
rend()method returns a reverse iterator pointing to the theoretical element preceding the first element in the vector.
Here is an example with a vector vec.
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for (auto vi= vec.rbegin(); vi!= vec.rend(); vi++) std::cout << *vi << ','; |
this list vector members in reverse order.
What are the constant begin end iterators in C++14?
In C++14 and C++17, there are also constant begin end iterators. std::cbegin and std::cend , and reverse iterators std::rbegin/std::rend and std::crbegin and std::crend
Constant iteration can be done with with cbegin/cend iterators, this can be faster than normal iteration.
- The
cbegin()method returns a constant iterator pointing to the first element in the vector. - The
cend()method returns a constant iterator pointing to the theoretical element that follows the last element in the vector.
Here is an example with a vector vec.
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for (auto vi= vec.cbegin(); vi!= vec.cend(); vi++) std::cout << *vi << ','; |
this list vector members in constant iterators.
What are the constant reverse begin end iterators in C++14?
In C++14 and C++17, constant reverse iterations can be done with crbegin and crend iterators,
- The crbegin() method returns a constant reverse iterator pointing to the last element in the vector. It moves from last to first element.
- The crend() method returns a constant reverse iterator pointing to the theoretical element preceding the first element in the vector.
Here is an example with a vector vec.
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for (auto vi= vec.crbegin(); vi!= vec.crend(); vi++) std::cout << *vi << ','; |
this list vector members as a constant iterator in reverse order.
Is there a full example about new begin end iterators in C++14?
Here is a full example about how we can use new begin end iterators in C++ and beyond.
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#include <iostream> #include <string> #include <vector> int main() { // A vector in strings std::vector<std::string> vec{ "Bruneau", "David", "Ian", "Marco", "Nuno", "Yilmaz" }; // standard iteration for (auto v: vec) std::cout << v << ','; std::cout << std::endl << std::endl; // iteration with begin/end // std::vector<std::string>::iterator vi; for (auto vi= vec.begin(); vi!= vec.end(); vi++) std::cout << *vi << ','; std::cout << std::endl; // reverse iteration with rbegin/rend for (auto vi= vec.rbegin(); vi!= vec.rend(); vi++) std::cout << *vi << ','; std::cout << std::endl << std::endl; // constant iteration with cbegin/cend for (auto vi= vec.cbegin(); vi!= vec.cend(); vi++) std::cout << *vi << ','; std::cout << std::endl; // constant reverse iteration with crbegin/crend for (auto vi= vec.crbegin(); vi!= vec.crend(); vi++) std::cout << *vi << ','; std::cout << std::endl; system("pause"); return 0; } |
and the output will be as follows:
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Bruneau,David,Ian,Marco,Nuno,Yilmaz, Bruneau,David,Ian,Marco,Nuno,Yilmaz, Yilmaz,Nuno,Marco,Ian,David,Bruneau, Bruneau,David,Ian,Marco,Nuno,Yilmaz, Yilmaz,Nuno,Marco,Ian,David,Bruneau, |
Note that these can be applied to the other containers of the modern C++, such as strings, arrays, lists, maps, sets, etc.
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