namespace c++ explanation with examples

C++ Standard Library:

In the C++ programming language, the C++ Standard Library is a collection of classes and functions, which are written in the core language and part of the C++ ISO Standard itself.The C++ Standard Library provides several generic containers, functions to utilize and manipulate these containers, function objects, generic strings and streams (including interactive and file I/O), support for some language features, and functions for everyday tasks such as finding the square root of a number. The C++ Standard Library also incorporates 18 headers of the ISO C90 C standard library ending with ".h", but their use is deprecated. No other headers in the C++ Standard Library end in ".h". Features of the C++ Standard Library are declared within the std namespace.

The C++ Standard Library is based upon conventions introduced by the Standard Template Library (STL), and has been influenced by research in generic programming and developers of the STL such as Alexander Stepanov and Meng Lee^. Although the C++ Standard Library and the STL share many features, neither is a strict superset of the other.

A noteworthy feature of the C++ Standard Library is that it not only specifies the syntax and semantics of generic algorithms, but also places requirements on their performance. These performance requirements often correspond to a well-known algorithm, which is expected but not required to be used. In most cases this requires linear time O(n) or linearithmic time O(n log n), but in some cases higher bounds are 

allowed, such as quasilinear time O(n log² n) for stable sort (to allow in-place merge sort). Previously sorting was only required to take O(n log n) on average, allowing the use of quicksort, which is fast in practice but has poor worst-case performance, but introsort was introduced to allow both fast average performance and optimal worst-case complexity, and as of C++11, sorting is guaranteed to be at worst linearithmic. In other cases requirements remain laxer, such as selection, which is only required to be linear on average (as in quick select), not requiring worst-case linear as in introselect.

The C++ Standard Library underwent ISO standardization as part of the C++ ISO Standardization effort, and is undergoing further work regarding standardization of expanded functionality.

std::list:

 

std::list is a container that supports constant time insertion and removal of elements from anywhere in the container. Fast random access is not supported. It is usually implemented as a doubly-linked list. Compared to std::forward_list this container provides bidirectional iteration capability while being less space efficient.

Addition, removal and moving the elements within the list or across several lists does not invalidate the iterators or references. An iterator is invalidated only when the corresponding element is deleted. 

std::list meets the requirements of Container, Al locator Aware Container, Sequence Container and Reversible Container.

Template parameters

	-	The type of the elements. T must meet the requirements of Copy Assignable and Copy Constructible. The requirements that are imposed on the elements depend on the actual operations performed on the container. Generally, it is required that element type is a complete type and meets the requirements of Erasable, but many member functions impose stricter requirements. The requirements that are imposed on the elements depend on the actual operations performed on the container. Generally, it is required that element type meets the requirements of Erasable, but many member functions impose stricter requirements. This container (but not its members) can be instantiated with an incomplete element type if the allocator satisfies the allocator completeness requirements.
Allocator	-	An allocator that is used to acquire/release memory and to construct/destroy the elements in that memory. The type must meet the requirements of Allocator. The behavior is undefined if Allocator::value_type is not the same as T.

Member types

Member type	Definition
value_type	T
allocator_type	Allocator
size_type	Unsigned integer type (usually std::size_t)
difference_type	Signed integer type (usually std::ptrdiff_t)
reference	Allocator::reference value_type&
const_reference	Allocator::const_reference const value_type&
pointer	Allocator::pointer std::allocator_traits<Allocator>::pointer
const_pointer	Allocator::const_pointer std::allocator_traits<Allocator>::const_pointer
iterator	BidirectionalIterator
const_iterator	Constant bidirectional iterator
reverse_iterator	std::reverse_iterator<iterator>
const_reverse_iterator	std::reverse_iterator<const_iterator>

Member functions

(constructor)	constructs the list (public member function)
(destructor)	destructs the list (public member function)
operator=	assigns values to the container (public member function)
assign	assigns values to the container (public member function)
get_allocator	returns the associated allocator (public member function)
Element access
front	access the first element (public member function)
back	access the last element (public member function)
Iterators
begin cbegin	returns an iterator to the beginning (public member function)
end cend	returns an iterator to the end (public member function)
rbegin crbegin	returns a reverse iterator to the beginning (public member function)
rend crend	returns a reverse iterator to the end (public member function)
Capacity
empty	checks whether the container is empty (public member function)
size	returns the number of elements (public member function)
max_size	returns the maximum possible number of elements (public member function)
Modifiers
clear	clears the contents (public member function)
insert	inserts elements (public member function)
emplace	constructs element in-place (public member function)
erase	erases elements (public member function)
push_back	adds an element to the end (public member function)
emplace_back	constructs an element in-place at the end (public member function)
pop_back	removes the last element (public member function)
push_front	inserts an element to the beginning (public member function)
emplace_front	constructs an element in-place at the beginning (public member function)
pop_front	removes the first element (public member function)
resize	changes the number of elements stored (public member function)
swap	swaps the contents (public member function)
Operations
merge	merges two sorted lists (public member function)
splice	moves elements from another list (public member function)
removeremove_if	removes elements satisfying specific criteria (public member function)
reverse	reverses the order of the elements (public member function)
unique	removes consecutive duplicate elements (public member function)
sort	sorts the elements (public member function)

Non-member functions

operator==operator!=operator<operator<=operator>operator>=	lexicographically compares the values in the list (function template)
std::swap(std::list)	specializes the std::swap algorithm (function template)

Example

Run this code

#include <algorithm>

#include <iostream>

#include <list>

int main()

{

    // Create a list containing integers

    std::list<int> l = { 7, 5, 16, 8 };

    // Add an integer to the front of the list

    l.push_front(25);

    // Add an integer to the back of the list

    l.push_back(13);

    // Insert an integer before 16 by searching

    auto it = std::find(l.begin(), l.end(), 16);

    if (it != l.end()) {

        l.insert(it, 42);

    }

    // Iterate and print values of the list

    for (int n : l) {

        std::cout << n << '\n';

    }

}

Output:

25

7

5

42

16

8

13