vector assign list

Replaces the contents of the container.

1) replaces the contents with count copies of value value

2) replaces the contents with copies of those in the range [first, last)

3) replaces the contents with the elements from the initializer list ilist .

[ edit ] Parameters

[ edit ] Complexity

1) linear in count

2) linear in distance between first and last

3) linear in ilist. size ( )

[ edit ] Example

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

[ edit ] See also

std::vector<T,Allocator>:: 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<T,Allocator>::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 > :

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C++ <vector> - assign() Function

The C++ vector::assign function is used to assign new values to the vector, replacing its old values, and modifying its size if necessary.

Return Value

Time complexity.

Linear i.e, Θ(n)

In the example below, the vector::assign function is used to assign new values to the given vector, replacing its old values.

The output of the above code will be:

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<vector>

Std:: vector ::assign.

Return value

Iterator validity

Exception safety.

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member functions:

• vector::assign
• vector::back
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• vector::swap

non-member overloads:

• relational operators (vector)
• swap (vector)

cppreference.com

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

Constructs a new container from a variety of data sources, optionally using a user supplied allocator alloc .

[ edit ] Parameters

[ edit ] Complexity

• If first and last are both forward, bidirectional or random-access iterators,
• The copy constructor of T is only called N   times, and
• No reallocation occurs.
• Otherwise ( first and last are just input iterators),
• The copy constructor of T is called O(N) times, and
• Reallocation occurs O(log N) times.
• If R models ranges::forward_range or ranges::sized_range ,
• Initializes exactly N elements from the result of dereferencing successive iterators of rg , and
• Otherwise ( R models input range),
• The copy or move constructor of T is called O(N) times, and

[ edit ] Exceptions

Calls to Allocator :: allocate may throw.

[ edit ] Notes

After container move construction (overload (8) ), references, pointers, and iterators (other than the end iterator) to other remain valid, but refer to elements that are now in * this . The current standard makes this guarantee via the blanket statement in [container.reqmts]/67 , and a more direct guarantee is under consideration via LWG issue 2321 .

The overload (4) zeroes out elements of non-class types such as int , which is different from the behavior of new[] , which leaves them uninitialized. To match the behavior of new [ ] , a custom Allocator::construct can be provided which leaves such elements uninitialized.

Note that the presence of list-initializing constructor (10) means list initialization and direct initialization do different things:

[ edit ] Example

[ edit ] defect reports.

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

[ edit ] See also

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vector class

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The C++ Standard Library vector class is a class template for sequence containers. A vector stores elements of a given type in a linear arrangement, and allows fast random access to any element. A vector is the preferred container for a sequence when random-access performance is at a premium.

Type The element data type to be stored in the vector

Allocator The type that represents the stored allocator object that encapsulates details about the vector's allocation and deallocation of memory. This argument is optional and the default value is allocator<Type> .

Vectors allow constant time insertions and deletions at the end of the sequence. Inserting or deleting elements in the middle of a vector requires linear time. The deque class container is faster at insertions and deletions at the beginning and end of a sequence. The list class container is faster at insertions and deletions at any location within a sequence.

Vector reallocation occurs when a member function must increase the sequence contained in the vector object beyond its current storage capacity. Other insertions and erasures may alter various storage addresses within the sequence. In all such cases, iterators or references that point at altered portions of the sequence become invalid. If no reallocation happens, only iterators and references before the insertion/deletion point remain valid.

The vector<bool> class is a full specialization of the class template vector for elements of type bool . It has an allocator for the underlying type used by the specialization.

The vector<bool> reference class is a nested class whose objects can provide references to elements (single bits) within a vector<bool> object.

Constructors

allocator_type

A type that represents the allocator class for the vector object.

allocator_type is a synonym for the template parameter Allocator .

See the example for get_allocator for an example that uses allocator_type .

Erases a vector and copies the specified elements to the empty vector.

first Position of the first element in the range of elements to be copied.

last Position of the first element beyond the range of elements to be copied.

count The number of copies of an element being inserted into the vector.

value The value of the element being inserted into the vector.

init_list The initializer_list containing the elements to insert.

First, assign erases any existing elements in a vector. Then, assign either inserts a specified range of elements from the original vector into a vector, or it inserts copies of a new specified value element into a vector.

Returns a reference to the element at a specified location in the vector.

position The subscript or position number of the element to reference in the vector.

Return value

A reference to the element subscripted in the argument. If position is greater than the size of the vector, at throws an exception.

If the return value of at is assigned to a const_reference , the vector object can't be modified. If the return value of at is assigned to a reference , the vector object can be modified.

Returns a reference to the last element of the vector.

The last element of the vector. If the vector is empty, the return value is undefined.

If the return value of back is assigned to a const_reference , the vector object can't be modified. If the return value of back is assigned to a reference , the vector object can be modified.

When compiled by using _ITERATOR_DEBUG_LEVEL defined as 1 or 2, a runtime error occurs if you attempt to access an element in an empty vector. For more information, see Checked iterators .

Returns a random-access iterator to the first element in the vector.

A random-access iterator addressing the first element in the vector or to the location succeeding an empty vector . Always compare the value returned with vector::end to ensure it's valid.

If the return value of begin is assigned to a vector::const_iterator , the vector object can't be modified. If the return value of begin is assigned to an vector::iterator , the vector object can be modified.

Returns the number of elements that the vector could contain without allocating more storage.

The current length of storage allocated for the vector.

The member function resize will be more efficient if sufficient memory is allocated to accommodate it. Use the member function reserve to specify the amount of memory allocated.

Returns a const iterator that addresses the first element in the range.

A const random-access iterator that points at the first element of the range, or the location just beyond the end of an empty range (for an empty range, cbegin() == cend() ).

With the return value of cbegin , the elements in the range can't be modified.

You can use this member function in place of the begin() member function to guarantee that the return value is const_iterator . Typically, it's used in with the auto type deduction keyword, as shown in the following example. In the example, consider Container to be a modifiable (non- const ) container of any kind that supports begin() and cbegin() .

Returns a const past-the-end iterator that points to the element following the last element of the vector.

A const past-the-end iterator for the vector. It points to the element following the last element of the vector. That element is a placeholder and shouldn't be dereferenced. Only use it for comparisons. If the vector is empty, then vector::cend() == vector::cbegin() .

cend is used to test whether an iterator has passed the end of its range.

You can use this member function in place of the end() member function to guarantee that the return value is const_iterator . Typically, it's used with the auto type deduction keyword, as shown in the following example. In the example, consider Container to be a modifiable (non- const ) container of any kind that supports end() and cend() .

The value returned by cend shouldn't be dereferenced. Only use it for comparisons.

Erases the elements of the vector.

const_iterator

A type that provides a random-access iterator that can read a const element in a vector.

A type const_iterator can't be used to modify the value of an element.

See the example for back for an example that uses const_iterator .

const_pointer

A type that provides a pointer to a const element in a vector.

A type const_pointer can't be used to modify the value of an element.

An iterator is more commonly used to access a vector element.

const_reference

A type that provides a reference to a const element stored in a vector. It's used for reading and doing const operations.

A type const_reference can't be used to modify the value of an element.

const_reverse_iterator

A type that provides a random-access iterator that can read any const element in the vector.

A type const_reverse_iterator can't modify the value of an element and is used to iterate through the vector in reverse.

See rbegin for an example of how to declare and use an iterator.

Returns a const iterator to the first element in a reversed vector.

A const reverse random-access iterator addressing the first element in a reversed vector or addressing what had been the last element in the unreversed vector .

With the return value of crbegin , the vector object can't be modified.

Returns a const past-the-end reverse iterator that points to the element following the last element of the reversed vector.

A const reverse past-the-end iterator for the reversed vector. It points to the element following the last element of the reversed vector, which is the same as the element before the first element of the non-reversed vector. That element is a placeholder and shouldn't be dereferenced. Only use it for comparisons.

crend is used with a reversed vector just as vector::cend is used with a vector .

With the return value of crend (suitably decremented), the vector object can't be modified.

crend can be used to test to whether a reverse iterator has reached the end of its vector .

The value returned by crend shouldn't be dereferenced. Only use it for comparisons.

Returns a pointer to the first element in the vector.

A pointer to the first element in the vector or to the location succeeding an empty vector .

difference_type

A type that provides the difference between two iterators that refer to elements within the same vector.

A difference_type can also be described as the number of elements between two pointers, because a pointer to an element contains its address.

Inserts an element constructed in place into the vector at a specified position.

position The position in the vector where the first element is inserted.

args Constructor arguments. The function infers which constructor overload to invoke based on the arguments provided.

The function returns an iterator that points to the position where the new element was inserted into the vector .

Any insertion operation can be expensive, see vector class for a discussion of vector performance.

emplace_back

Adds an element constructed in place to the end of the vector.

Tests if the vector is empty.

true if the vector is empty; false if the vector isn't empty.

Returns a past-the-end iterator that points to the element following the last element of the vector.

A past-the-end iterator for the vector. It points to the element following the last element of the vector. That element is a placeholder and shouldn't be dereferenced. Only use it for comparisons. If the vector is empty, then vector::end() == vector::begin() .

If the return value of end is assigned to a variable of type const_iterator , the vector object can't be modified. If the return value of end is assigned to a variable of type iterator , the vector object can be modified.

Removes an element or a range of elements in a vector from specified positions.

position Position of the element to be removed from the vector.

first Position of the first element removed from the vector.

last Position just beyond the last element removed from the vector.

An iterator that designates the first element remaining beyond any elements removed, or a pointer to the end of the vector if no such element exists.

Returns a reference to the first element in a vector.

A reference to the first element in the vector object. If the vector is empty, the return is undefined.

If the return value of front is assigned to a const_reference , the vector object can't be modified. If the return value of front is assigned to a reference , the vector object can be modified.

get_allocator

Returns a copy of the allocator object used to construct the vector.

The allocator used by the vector.

Allocators for the vector class specify how the class manages storage. The default allocators supplied with C++ Standard Library container classes are sufficient for most programming needs. Writing and using your own allocator class is an advanced C++ feature.

Inserts an element, or many elements, or a range of elements into the vector at a specified position.

count The number of elements being inserted into the vector.

first The position of the first element in the range of elements to be copied.

last The position of the first element beyond the range of elements to be copied.

The first two insert functions return an iterator that points to the position where the new element was inserted into the vector.

As a precondition, first and last must not be iterators into the vector, or the behavior is undefined. Any insertion operation can be expensive, see vector class for a discussion of vector performance.

A type that provides a random-access iterator that can read or modify any element in a vector.

A type iterator can be used to modify the value of an element.

See the example for begin .

Returns the maximum length of the vector.

The maximum possible length of the vector.

Returns a reference to the vector element at a specified position.

position The position of the vector element.

If the position specified is greater than or equal to the size of the container, the result is undefined.

If the return value of operator[] is assigned to a const_reference , the vector object can't be modified. If the return value of operator[] is assigned to a reference, the vector object can be modified.

When compiled by using _ITERATOR_DEBUG_LEVEL defined as 1 or 2, a runtime error occurs if you attempt to access an element outside the bounds of the vector. For more information, see Checked iterators .

Replaces the elements of the vector with a copy of another vector.

right The vector being copied into the vector .

After erasing any existing elements in a vector , operator= either copies or moves the contents of right into the vector .

A type that provides a pointer to an element in a vector.

A type pointer can be used to modify the value of an element.

Deletes the element at the end of the vector.

For a code example, see vector::push_back() .

Adds an element to the end of the vector.

value The value to assign to the element added to the end of the vector.

Returns an iterator to the first element in a reversed vector.

A reverse random-access iterator addressing the first element in a reversed vector or addressing what had been the last element in the unreversed vector.

If the return value of rbegin is assigned to a const_reverse_iterator , the vector object can't be modified. If the return value of rbegin is assigned to a reverse_iterator , the vector object can be modified.

A type that provides a reference to an element stored in a vector.

See at for an example of how to use reference in the vector class.

Returns a past-the-end reverse iterator that points to the element following the last element of the reversed vector.

A reverse past-the-end iterator for the reversed vector. It points to the element following the last element of the reversed vector, which is the same as the element before the first element of the non-reversed vector. That element is a placeholder and shouldn't be dereferenced. Only use it for comparisons.

rend is used with a reversed vector just as end is used with a vector.

If the return value of rend is assigned to a const_reverse_iterator , then the vector object can't be modified. If the return value of rend is assigned to a reverse_iterator , then the vector object can be modified.

rend can be used to test to whether a reverse iterator has reached the end of its vector.

The value returned by rend shouldn't be dereferenced. Only use it for comparisons.

Reserves a minimum length of storage for a vector object, allocating space if necessary.

count The minimum length of storage to be allocated for the vector.

Specifies a new size for a vector.

new_size The new size of the vector.

value The initialization value of new elements added to the vector if the new size is larger that the original size. If the value is omitted, the new objects use their default constructor.

If the container's size is less than the requested size, new_size , resize adds elements to the vector until it reaches the requested size. When the container's size is larger than the requested size, resize deletes the elements closest to the end of the container until it reaches the size new_size . No action is taken if the present size of the container is the same as the requested size.

size reflects the current size of the vector.

reverse_iterator

A type that provides a random-access iterator that can read or modify any element in a reversed vector.

A type reverse_iterator is used to iterate through the vector in reverse.

See the example for rbegin .

shrink_to_fit

Discards excess capacity.

Returns the number of elements in the vector.

The current length of the vector.

A type that counts the number of elements in a vector.

See the example for capacity .

Exchanges the elements of two vectors.

right A vector providing the elements to be swapped. Or, a vector whose elements are to be exchanged with the elements in the vector left .

left A vector whose elements are to be exchanged with the elements in the vector right .

A type that represents the data type stored in a vector.

value_type is a synonym for the template parameter Type .

Constructs a vector. Overloads construct a vector of a specific size, or with elements of a specific value. Or, as a copy of all or part of some other vector. Some overloads also allow you to specify the allocator to use.

allocator The allocator class to use with this object. get_allocator returns the allocator class for the object.

count The number of elements in the constructed vector.

value The value of the elements in the constructed vector.

source The vector of which the constructed vector is to be a copy.

init_list The initializer_list containing the elements to copy.

All constructors store an allocator object ( allocator ) and initialize the vector.

The first two constructors specify an empty initial vector. The second constructor explicitly specifies the allocator type ( allocator ) to use.

The third constructor specifies a repetition of a specified number ( count ) of elements of the default value for class Type .

The fourth and fifth constructors specify a repetition of ( count ) elements of value value .

The sixth constructor specifies a copy of the vector source .

The seventh constructor moves the vector source .

The eighth constructor uses an initializer_list to specify the elements.

The ninth and tenth constructors copy the range [ first , last ) of a vector.

Thread Safety in the C++ Standard Library C++ Standard Library Reference

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Additional resources

• Standard Template Library
• STL Priority Queue
• STL Interview Questions
• STL Cheatsheet
• C++ Templates
• C++ Functors
• C++ Iterators

Vector in C++ STL

• 7 ways to Initialize Vector in C++

Commonly Used Methods

• vector::begin() and vector::end() in C++ STL
• vector::empty() and vector::size() in C++ STL
• vector::operator= and vector::operator[ ] in C++ STL
• vector::front() and vector::back() in C++ STL
• vector::push_back() and vector::pop_back() in C++ STL
• vector insert() Function in C++ STL
• vector emplace() function in C++ STL
• vector :: assign() in C++ STL
• vector erase() and clear() in C++

Other Member Methods

• vector max_size() function in C++ STL
• vector capacity() function in C++ STL
• vector rbegin() and rend() function in C++ STL
• vector :: cbegin() and vector :: cend() in C++ STL
• vector::crend() & vector::crbegin() with example
• vector : : resize() in C++ STL
• vector shrink_to_fit() function in C++ STL
• Using std::vector::reserve whenever possible
• vector data() function in C++ STL
• 2D Vector In C++ With User Defined Size
• Passing Vector to a Function in C++
• How does a vector work in C++?
• How to implement our own Vector Class in C++?
• Advantages of vector over array in C++

Common Vector Programs

• Sorting a vector in C++
• How to reverse a Vector using STL in C++?
• How to find the minimum and maximum element of a Vector using STL in C++?
• How to find index of a given element in a Vector in C++

Vectors are the same as dynamic arrays with the ability to resize themselves automatically when an element is inserted or deleted, with their storage being handled automatically by the container. Vector elements are placed in contiguous storage so that they can be accessed and traversed using iterators. In vectors, data is inserted at the end. Inserting at the end takes differential time, as sometimes the array may need to be extended. Removing the last element takes only constant time because no resizing happens. Inserting and erasing at the beginning or in the middle is linear in time.

What is std::vector in C++?

std::vector in C++ is the class template that contains the vector container and its member functions. It is defined inside the <vector> header file. The member functions of the std::vector class provide various functionalities to vector containers.

Syntax to Declare Vector in C++

where the data type is the type of data of each element of the vector. You can remove the std:: if you have already used the std namespace.

Initialization of Vector in C++

We can initialize a vector in the following ways:

1. Initialization Using List

This initialization is done with a declaration. Here, we pass the list of elements to the vector constructor to create a vector with the specified elements.

2. Initialization With a Single Value

This initialization is also done with declaration. Here, we specify the size of the vector and then initialize every element of the vector with the value.

3. Initialization From Another Vector

This initialization is used to create a vector that is an exact copy of other_vec.

Some commonly used member functions of std::vector class are written below:

• begin() – Returns an iterator pointing to the first element in the vector
• end() – Returns an iterator pointing to the theoretical element that follows the last element in the vector
• rbegin() – Returns a reverse iterator pointing to the last element in the vector (reverse beginning). It moves from last to first element
• rend() – Returns a reverse iterator pointing to the theoretical element preceding the first element in the vector (considered as reverse end)
• cbegin() – Returns a constant iterator pointing to the first element in the vector.
• cend() – Returns a constant iterator pointing to the theoretical element that follows the last element in the vector.
• crbegin() – Returns a constant reverse iterator pointing to the last element in the vector (reverse beginning). It moves from last to first element
• crend() – Returns a constant reverse iterator pointing to the theoretical element preceding the first element in the vector (considered as reverse end)

• size() – Returns the number of elements in the vector.
• max_size() – Returns the maximum number of elements that the vector can hold.
• capacity() – Returns the size of the storage space currently allocated to the vector expressed as number of elements.
• resize(n) – Resizes the container so that it contains ‘n’ elements.
• empty() – Returns whether the container is empty.
• shrink_to_fit() – Reduces the capacity of the container to fit its size and destroys all elements beyond the capacity.
• reserve() – Requests that the vector capacity be at least enough to contain n elements.

Element access

• reference operator [g] – Returns a reference to the element at position ‘g’ in the vector
• at(g) – Returns a reference to the element at position ‘g’ in the vector
• front() – Returns a reference to the first element in the vector
• back() – Returns a reference to the last element in the vector
• data() – Returns a direct pointer to the memory array used internally by the vector to store its owned elements.

• assign() – It assigns new value to the vector elements by replacing old ones
• push_back() – It push the elements into a vector from the back
• pop_back() – It is used to pop or remove elements from a vector from the back.
• insert() – It inserts new elements before the element at the specified position
• erase() – It is used to remove elements from a container from the specified position or range.
• swap() – It is used to swap the contents of one vector with another vector of same type. Sizes may differ.
• clear() – It is used to remove all the elements of the vector container
• emplace() – It extends the container by inserting new element at position
• emplace_back() – It is used to insert a new element into the vector container, the new element is added to the end of the vector

The time complexity for doing various operations on vectors is-

• Random access – constant O(1)
• Insertion or removal of elements at the end – constant O(1)
• Insertion or removal of elements – linear in the distance to the end of the vector O(N)
• Knowing the size – constant O(1)
• Resizing the vector- Linear O(N)  

All Member Functions of std::vector

Following is the list of all member functions of std::vector class in C++:

• How to Check if a Vector Contains a Given Element in C++?

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• <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>
• list::~list

member functions

• list::assign
• list::begin
• C++11 list::cbegin
• C++11 list::cend
• list::clear
• C++11 list::crbegin
• C++11 list::crend
• C++11 list::emplace
• C++11 list::emplace_back
• C++11 list::emplace_front
• list::empty
• list::erase
• list::front
• list::get_allocator
• list::insert
• list::max_size
• list::merge
• list::operator=
• list::pop_back
• list::pop_front
• list::push_back
• list::push_front
• list::rbegin
• list::remove
• list::remove_if
• list::resize
• list::reverse
• list::splice
• list::unique

non-member overloads

• relational operators (list)
• swap (list)

std:: list ::assign

Return value

Iterator validity

Exception safety.

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stl vector assign vs insert

I understand the semantics of the 2 operations , assign- erases before replacing with supplied values. insert - inserts values at specified location(allocates new memory if necessary).

Apart from this is there any reason to preffer one over the other? Or to put it another way, is there any reason to use assign instead of insert.

• 5 Maybe you can rescue this question by asking this instead: "Assign vs insert on empty STL vector , any difference?" –  Emile Cormier Commented Jan 22, 2010 at 17:59
• Fair enough. Thats what i had in mind. –  Pradyot Commented Jan 22, 2010 at 18:01
• 12 years and still not revised. tsk tsk tsk –  Max Power Commented Jul 7, 2022 at 19:26

4 Answers 4

assign and insert are only equivalent if the vector is empty to begin with. If the vector is already empty, then it's better to use assign , because insert would falsely hint to the reader that there are existing elements to be preserved.

If you wish to invoke the semantics of assign , call assign - if you wish to invoke the semantics of insert , call insert. They aren't interchangeable.

As for calling them on an empty vector, the only difference is that you don't need to supply an iterator to insert at when you call assign . There may be a performance difference, but that's implementation specific and almost certainly negligable.

assign() will blow away anything that's already in the vector , then add the new elements. insert() doesn't touch any elements already in the vector .

Other than that, if the vector you are modifying starts out empty, there is little difference.

insert() will insert the elements at a particular position. It can be a single value, or multiple values using first and last iterators. Its like inserting more elements in front of a position.

assign() will just assign the values starting at beginning till end as per input agruments "InputIterator first and InputIterator last ". This will remove all existing elements and instead populate the vector with new values as per range provided in its arguments.

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