std vector assign from pointer

std::vector:: assign

Replaces the contents of the container.

All iterators, pointers and references to the elements of the container are invalidated. The past-the-end iterator is also invalidated.

The following code uses assign to add several characters to a std:: vector < char > :

std:: vector

std::vector is a sequence container that encapsulates dynamic size arrays.

The elements are stored contiguously, which means that elements can be accessed not only through iterators, but also using offsets on regular pointers to elements. This means that a pointer to an element of a vector may be passed to any function that expects a pointer to an element of an array.

The storage of the vector is handled automatically, being expanded and contracted as needed. Vectors usually occupy more space than static arrays, because more memory is allocated to handle future growth. This way a vector does not need to reallocate each time an element is inserted, but only when the additional memory is exhausted. The total amount of allocated memory can be queried using capacity() function. Extra memory can be returned to the system via a call to shrink_to_fit() .

Reallocations are usually costly operations in terms of performance. reserve() function can be used to eliminate reallocations if the number of elements is known beforehand.

The complexity (efficiency) of common operations on vectors is as follows:

• Random access - constant O(1)
• Insertion or removal of elements at the end - amortized constant O(1)
• Insertion or removal of elements - linear in distance to the end of the vector O(n)

std::vector meets the requirements of Container , AllocatorAwareContainer , SequenceContainer and ReversibleContainer .

[ edit ] Specializations

The standard library provides a specialization of std::vector for the type bool , which is optimized for space efficiency.

[ edit ] Member types

[ edit ] member functions, [ edit ] non-member functions.

Defined in: <vector>

• Polymorphic (since C++17)

std::vector is a container that encapsulates dynamic size arrays.

The elements are stored contiguously, one after another. This means that a pointer to an element of a vector may be passed to any function that expects a pointer to an element of an array.

Storage size ​

The storage of the vector is handled automatically, being expanded and contracted as needed. Vectors usually occupy more space than static arrays, because more memory is allocated to handle future growth. This way a vector does not need to reallocate each time an element is inserted, but only when the additional memory is exhausted. The total amount of allocated memory can be queried using capacity() function. Extra memory can be returned to the system via a call to shrink_to_fit()   ( since C++11 )

Reallocations are usually costly operations in terms of performance. The reserve() function can be used to eliminate reallocations if the number of elements is known beforehand.

Technical details ​

The complexity (efficiency) of common operations on vectors is as follows:

• random access - constant 𝓞(1)
• insertion or removal of elements at the end - amortized constant 𝓞(1)
• insertion or removal of elements - linear in the distance to the end of the vector 𝓞(n)

std::vector (for T other than bool ) meets the requirements of:

• AllocatorAwareContainer
• SequenceContainer
• ContiguousContainer   ( since C++17 )
• ReversibleContainer
• since C++17
• until C++17
• until C++11
• since C++20
• until C++20

std :: vector

Template parameters ​.

See technical details for more detailed information.

Type names ​

Member functions ​, element access ​, iterators ​, operations ​, non-member functions ​, deduction guides (since c++17) ​, deduction guides ​.

(1) allows deduction from an iterator range .

Overload resolution ​

In order for any of the deduction guides to participate in overload resolution, the folllowing requirements must be met:

• InputIt satisfies the requirements of LegacyInputIterator
• Alloc satisfies the requirements of Allocator

The extent to which the library determines that a type does not satisfy LegacyInputIterator is unspecified , except that as a minimum:

• Integral types do not qualify as input iterators.

Likewise, the extent to which it determines that a type does not satisfy Allocator is unspecified , except that as a minimum:

• The member type Alloc::value_type must exist.
• The expression std::declval<Alloc&>().allocate(std::size_t{}) must be well-formed when treated as an unevaluated operand.

Basic usage ​

Calculate arithmetic mean ​, find the maximum element ​, erase nth element ​.

• Storage size
• Technical details
• Template parameters
• Element access
• Non-member functions
• Deduction guides (since C++17)
• Basic usage

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6.4. Storing Pointers in a vector

For efficiency or other reasons, you can’t store copies of your objects in a vector , but you need to keep track of them somehow.

Store pointers to your objects in a vector instead of copies of the objects themselves. But if you do, don’t forget to delete the objects that are pointed to, because the vector won’t do it for you. Example 6-4 shows how to declare and work with vector s of pointers.

Example 6-4. Using vectors of pointers

You can store pointers in a vector just like you would anything else. Declare a vector of pointers like this:

The important thing to remember is that a vector stores values without regard for what those values represent. It, therefore, doesn’t know that it’s supposed to delete pointer values when it’s destroyed. If you allocate memory, then put pointers to that ...

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• C++ Data Types
• C++ Input/Output
• C++ Pointers
• C++ Interview Questions
• C++ Programs
• C++ Cheatsheet
• C++ Projects
• C++ Exception Handling
• C++ Memory Management

C++ Vector of Pointers

• C++ Vector of Structs
• void Pointer in C++
• Vector in C++ STL
• Smart Pointers in C++
• 'this' pointer in C++
• Modifiers for Vector in C++ STL
• vector insert() Function in C++ STL
• Pointers vs Array in C++
• C++ - Pointer to Structure
• vector : : resize() in C++ STL
• Copy File To Vector in C++ STL
• How to Reverse Iterate a Vector in C++?
• Vector of sets in C++
• void Pointer in C
• Structure Pointer in C
• Dope Vector
• Array of Pointers in C

Prerequisites

• Pointers in C++
• Vectors in C++

Vector of pointers are vectors that can hold multiple pointers. Each pointer within a vector of pointers points to an address storing a value. We can use the vector of pointers to manage values that are not stored in continuous memory.

How to Create Vector of Pointers in C++?

Similar to any other vector declaration we can declare a vector of pointers. In C++ we can declare vector pointers using 3 methods:

•  Using std::vector container 
•  Using [ ] notations  
•  Using the new keyword (Dynamic Memory)

1. Using std::vector container 

Using vectors to create vector pointers is the easiest and most effective method as it provides extra functionality of STL.

A. Insert elements in a vector:

  We can insert elements by 2 methods:

• while initialization
• using push_back( )

Insertion while initialization: Although it’s an option that can be used we should avoid such type of insertion as vectors store addresses within them.

Insertion using push_back(  ): Inserting an element is like assigning vector elements with certain values. We can perform this task in certain steps.

• Create a variable and insert a value in it. 
• Insert the address of the variable inside the vector.

B. Deletion of Elements

Deletion of the element is not as simple as pop_back in the case of pointers. It can be done using 2 steps:

• Free the pointer (Remove address from variable)
• Erase the variable.

2. Using [ ] notations 

Square brackets are used to declare fixed size. This can be used to operate over to create an array containing multiple pointers. This is a type of array that can store the address rather than the value. So, can be called a pointer array, and the memory address is located on the stack memory rather than the heap memory.

3. Using the new keyword 

The new Keyword in C++ represents dynamic memory allocation i.e, heap memory. The code will suffer from a memory leak if the programmer does not free up the memory before exiting. This can lead to a huge problem in long-running applications or resource-constrained hardware environments.

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<vector>

Std:: vector ::assign, return value, iterator validity, exception safety.

• Information
• <cassert> (assert.h)
• <cctype> (ctype.h)
• <cerrno> (errno.h)
• <cfenv> (fenv.h)
• <cfloat> (float.h)
• <cinttypes> (inttypes.h)
• <ciso646> (iso646.h)
• <climits> (limits.h)
• <clocale> (locale.h)
• <cmath> (math.h)
• <csetjmp> (setjmp.h)
• <csignal> (signal.h)
• <cstdarg> (stdarg.h)
• <cstdbool> (stdbool.h)
• <cstddef> (stddef.h)
• <cstdint> (stdint.h)
• <cstdio> (stdio.h)
• <cstdlib> (stdlib.h)
• <cstring> (string.h)
• <ctgmath> (tgmath.h)
• <ctime> (time.h)
• <cuchar> (uchar.h)
• <cwchar> (wchar.h)
• <cwctype> (wctype.h)

Containers:

• <array>
• <deque>
• <forward_list>
• <list>
• <map>
• <queue>
• <set>
• <stack>
• <unordered_map>
• <unordered_set>

Input/Output:

• <fstream>
• <iomanip>
• <ios>
• <iosfwd>
• <iostream>
• <istream>
• <ostream>
• <sstream>
• <streambuf>

Multi-threading:

• <atomic>
• <condition_variable>
• <future>
• <mutex>
• <thread>
• <algorithm>
• <bitset>
• <chrono>
• <codecvt>
• <complex>
• <exception>
• <functional>
• <initializer_list>
• <iterator>
• <limits>
• <locale>
• <memory>
• <new>
• <numeric>
• <random>
• <ratio>
• <regex>
• <stdexcept>
• <string>
• <system_error>
• <tuple>
• <typeindex>
• <typeinfo>
• <type_traits>
• <utility>
• <valarray>
• vector<bool>
• vector::vector
• vector::~vector

member functions:

• vector::assign
• vector::back
• vector::begin
• vector::capacity
• vector::cbegin
• vector::cend
• vector::clear
• vector::crbegin
• vector::crend
• vector::data
• vector::emplace
• vector::emplace_back
• vector::empty
• vector::end
• vector::erase
• vector::front
• vector::get_allocator
• vector::insert
• vector::max_size
• vector::operator=
• vector::operator[]
• vector::pop_back
• vector::push_back
• vector::rbegin
• vector::rend
• vector::reserve
• vector::resize
• vector::shrink_to_fit
• vector::size
• vector::swap

non-member overloads:

• relational operators (vector)
• swap (vector)

• <cassert> (assert.h)
• <cctype> (ctype.h)
• <cerrno> (errno.h)
• C++11 <cfenv> (fenv.h)
• <cfloat> (float.h)
• C++11 <cinttypes> (inttypes.h)
• <ciso646> (iso646.h)
• <climits> (limits.h)
• <clocale> (locale.h)
• <cmath> (math.h)
• <csetjmp> (setjmp.h)
• <csignal> (signal.h)
• <cstdarg> (stdarg.h)
• C++11 <cstdbool> (stdbool.h)
• <cstddef> (stddef.h)
• C++11 <cstdint> (stdint.h)
• <cstdio> (stdio.h)
• <cstdlib> (stdlib.h)
• <cstring> (string.h)
• C++11 <ctgmath> (tgmath.h)
• <ctime> (time.h)
• C++11 <cuchar> (uchar.h)
• <cwchar> (wchar.h)
• <cwctype> (wctype.h)

Containers:

• C++11 <array>
• <deque>
• C++11 <forward_list>
• <list>
• <map>
• <queue>
• <set>
• <stack>
• C++11 <unordered_map>
• C++11 <unordered_set>
• <vector>

Input/Output:

• <fstream>
• <iomanip>
• <ios>
• <iosfwd>
• <iostream>
• <istream>
• <ostream>
• <sstream>
• <streambuf>

Multi-threading:

• C++11 <atomic>
• C++11 <condition_variable>
• C++11 <future>
• C++11 <mutex>
• C++11 <thread>
• <algorithm>
• <bitset>
• C++11 <chrono>
• C++11 <codecvt>
• <complex>
• <exception>
• <functional>
• C++11 <initializer_list>
• <iterator>
• <limits>
• <locale>
• <memory>
• <new>
• <numeric>
• C++11 <random>
• C++11 <ratio>
• C++11 <regex>
• <stdexcept>
• <string>
• C++11 <system_error>
• C++11 <tuple>
• C++11 <type_traits>
• C++11 <typeindex>
• <typeinfo>
• <utility>
• <valarray>
• vector<bool>
• vector::~vector
• vector::vector

member functions

• vector::assign
• vector::back
• vector::begin
• vector::capacity
• C++11 vector::cbegin
• C++11 vector::cend
• vector::clear
• C++11 vector::crbegin
• C++11 vector::crend
• C++11 vector::data
• C++11 vector::emplace
• C++11 vector::emplace_back
• vector::empty
• vector::end
• vector::erase
• vector::front
• vector::get_allocator
• vector::insert
• vector::max_size
• vector::operator[]
• vector::operator=
• vector::pop_back
• vector::push_back
• vector::rbegin
• vector::rend
• vector::reserve
• vector::resize
• C++11 vector::shrink_to_fit
• vector::size
• vector::swap

non-member overloads

• relational operators (vector)
• swap (vector)

std:: vector ::data

Return value, iterator validity, exception safety.

cppreference.com

Std::vector<t,allocator>:: data.

Returns pointer to the underlying array serving as element storage. The pointer is such that range [ data ( ) ,  data ( ) +   size() ) is always a valid range , even if the container is empty ( data() is not dereferenceable in that case).

[ edit ] Parameters

[ edit ] return value.

Pointer to the underlying element storage. For non-empty containers, the returned pointer compares equal to the address of the first element.

[ edit ] Complexity

[ edit ] notes.

If size() is ​ 0 ​ , data() may or may not return a null pointer.

[ edit ] Example

Defect reports.

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

[ edit ] See also

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