assignment operator with vector

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Copying std::vector: prefer assignment or std::copy?

I have two vectors:

And now I need to copy v1 to v2 . Is there any reason to prefer

std::copy (v1.begin(), v1.end(), v2.begin());

(or vice versa)?

• 7 Assignment operator will do the right thing. The way you've written copy , it'll mess up if v1 is larger than v2 . –  jrok Feb 20, 2013 at 10:36
• 5 If v1 is not required after the copy you could just v2.swap(v1); . –  hmjd Feb 20, 2013 at 10:38
• 1 Write what you want to do. If you want to assign one vector to another, write that. –  Alex Chamberlain Feb 20, 2013 at 10:52

6 Answers 6

Generally I would strongly prefer v2 = v1 :

• It is shorter and makes the intent more clear
• std::copy won't work if v2 doesn't have the same length as v1 (it won't resize it, so it will retain some of the old elements best case ( v2.size() > v1.size() and overwrite some random data used elsewhere in the program worst case
• If v1 is about to expire (and you use C++11) you can easily modify it to move the contents
• Performancewise assignment is unlikely to be slower then std::copy , since the implementers would probably use std::copy internally, if it gave a performance benefit.

In conclusion, std::copy is less expressive, might do the wrong thing and isn't even faster. So there isn't really any reason to use it here.

• 16 Then, what is std::copy for? –  altroware Jul 6, 2015 at 10:27
• 39 @altroware: It's for general copying from one range to another. You can't, for example, use the assignment operator to copy from a std::list to a std::vector , or from one portion of a std::vector to another portion of the same std::vector . –  Benjamin Lindley Sep 15, 2015 at 18:10
• 2 What will happen if v1 was allocated on the stack and gets destructed? Does v2 = v1 cause elements of v1 to be copied? –  James Wierzba Apr 7, 2016 at 14:57
• Which copy mechanism of the element type is called? In this case of vector<int> , are the integers copied from one vector to the other with operator= , or with int 's copy constructor? –  Gauthier Nov 9, 2016 at 8:33
• 1 @james operator= Will do a full deep copy. Idiomatic C++ uses value semantics so copying a vector copies all its elements. The exception is if it’s elements are pointers in which case the pointers are copied. After assignment, the vectors are independent. –  Ben Oct 24, 2021 at 1:26

If v2 isn't big enough you'll get a buffer overrun if you use copy as you have.

You can use a back insert iterator which will call push_back on v2 . However this could lead to multiple reallocations depending upon how big v1 is.

You're better off letting vector manage things correctly. The assignment operator does this, as does vector::assign :

I have an inkling that the assignment operator is implemented in terms of vector::assign .

• Mr. Wood, is it possible that you meant v2.assign(v1.begin(), v1.end()) rather than v2.assign(v1.begin(), v2.end())? –  Peter Schaeffer Dec 21, 2015 at 3:20

The invocation of std::copy may try to access items beyond the end of the destination vector.

Use assignment.

It's not your job to micro-optimize: that's the library writer's responsibility, and ultimately the compiler's responsibility.

You can make your code arbitrarily fast if it doesn't have to be correct.

In the case of the copy , however, it's rather doubtful whether it even is faster, and it's certainly not correct for the general case.

• 3 I agree with your statements concerning optimization, but it might be worth pointing out that the more information available to the compiler or the library, the better it can do its job. Member functions of std::vector know that they're working on an std::vector , and know how it is implemented. std::copy doesn't have this information. The conclusion is that the member functions can probably do the job better (and certainly not worse). –  James Kanze Feb 20, 2013 at 10:38

It's shorter.

std::copy is mainly meant for copying sections of containers. If you need to copy an entire container, you might as well use the copy constructor.

• If v2 isn't big enough you'll get a buffer overrun. –  Peter Wood Feb 20, 2013 at 10:36

Assignment, by far. More generally, any time the size of the vector might change, or change the entire contents of the vector, you should prefer member functions. The only time std::copy would be appropriate is if you are only replacing a small range totally within the vector.

Assignement is clearer and internally uses std::copy (or unitizalized_copy _M_allocate_and_copy depending size and capacity) or so performances are the same.

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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)
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member functions

• vector::assign
• vector::back
• vector::begin
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• C++11 vector::cbegin
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• vector::clear
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• C++11 vector::crend
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• 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 ::operator=

Return value, iterator validity, exception safety.

cppreference.com

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

Replaces the contents of the container.

[ edit ] Parameters

[ edit ] return value, [ edit ] complexity, [ edit ] exceptions, [ edit ] notes.

After container move assignment (overload (2) ), unless element-wise move assignment is forced by incompatible allocators, 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.rev.reqmts]/17 , and a more direct guarantee is under consideration via LWG issue 2321 .

[ edit ] Example

The following code uses operator = to assign one std::vector to another:

[ edit ] See also

• conditionally noexcept
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Vectors are same as dynamic arrays with the ability to resize itself automatically when an element is inserted or deleted, with their storage being handled automatically by the container.  

This operator is used to assign new contents to the container by replacing the existing contents.  It also modifies the size according to the new contents.

Syntax :  

Examples:  

Errors and Exceptions 1. If the containers are of different types, an error is thrown.  2. It has a basic no exception throw guarantee otherwise.

Time Complexity – Linear O(N)

Output: 

This operator is used to reference the element present at position given inside the operator. It is similar to the at() function, the only difference is that the at() function throws an out-of-range exception when the position is not in the bounds of the size of vector, while this operator causes undefined behavior .

Syntax :   

Examples: 

Errors and Exceptions 1. If the position is not present in the vector, it shows undefined behavior.  2. It has a no exception throw guarantee otherwise. 

Time Complexity – Constant O(1)

Application   Given a vector of integers, print all the integers present at odd positions. 

Algorithm   1. Run a loop till the size of the vector.  2. Check if the position is not divisible by 2, then print the element at that position.

  Let us see the differences in a tabular form -:

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std::vector:: operator=

Replaces the contents of the container.

[ edit ] Parameters

[ edit ] return value, [ edit ] complexity, [ edit ] example.

The following code uses to assign one std:: vector to another:

[ edit ] See also

• conditionally noexcept

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Vector with move constructor and move assignment operator [closed]

Please verify whether the code for a move constructor and a move assignment operator is correct.

• memory-management

• \$\begingroup\$ This isn't that kind of code review; take the tour , see How to Ask . If you aren't certain it works, test it. \$\endgroup\$ –  jonrsharpe Apr 2, 2017 at 8:54

Default constructor

Stop trying to save space like that. THe point of good coding is to make it readable by humans. The computer can read any style so try and make it more maintainable by making it readable.

I would write it like this:

Sure you can use nullptr for the zero sized array. But this seems like a premature optimization. You can make the rest of your code less complicated by never having a null m_pInt . If there is never a null then your code does not need to check for it.

Also currently you have different behavior for:

Constructor

Sure this works. But std::cout is not the only stream you way want to print too

So I would pass a stream to print() . It can default to std::cout . Then you should add an operator<< . As the normal way of printing something is via << .

Note: prefer "\n" to std::endl . The difference is a stream flush. The stream will already flush itself when required. So you adding extra flushes is only going to cause the flushes to be less optimal.

Copy Constructor

Move constructor.

Yes that works. But you should also mark the move constructor as noexcept . The standard containers have optimizations that can be applied if they know tour class will not throw when being moved. Otherwise they have to fallback to copying to provide the strong exception guarantee.

The standard technique is to use swap though. It makes it look and behave just like move assignment. See below.

Copy Assignment

Yes the test for self assignment looks like a good optimization.

BUT its not. Self assignment happens so rarely (in fact basically never in real code) that what you are doing is pesimizing the normal flow and as a result will make your code slower. You do need to cope with self assignment but because it is so rare you don't need to worry that it is not the optimal path.

The second issue I have here is that you destroy the local data before you have a copy of the new data ready.

If something goes wrong in the rest of your code then you will be unable to roll back the state and thus can not provide the strong exception guarantee. When copying an object it should happen in three distict phases.

So your Copy assignment should look like this:

If you look carefully at those three stages. Stage 1 looks like the constructor and stage 3 looks like the destructor and stage 2 looks like a standard swap function so we can simplify the above to exactly that:

This is called the copy and swap idiom.

Move Assignment

Again the pesimizing test for self assignment.

The standard move assignment is to swap the source and the destination. This has a couple of benefits.

• You don't call delete (and thus don't invoke the destructor). Thus it is potentially faster.
• Because you did not delete the data there is an opportunity for it to be reused.
• If the source is going out of scope it will invoke its destructor and destroy your data but it will be done after the completion of your object thus giving you strong exception guarantee. Thus allowing you to make your assignment operator noexcept.

Standard Move Assignment

I wrote a series of posts about all this.

Vector - Resource Management Allocation Vector - Resource Management Copy Swap Vector - Resize Vector - Simple Optimizations Vector - the Other Stuff

• \$\begingroup\$ Hi. What do you mean by "Stop trying to save space like that." regarding the code MyVector():m_Size(0), m_pInt(nullptr) { } ? What would you write instead if this was the only constructor? \$\endgroup\$ –  krismath May 1, 2017 at 21:20
• 2 \$\begingroup\$ @krismath: One initialization per line. The point is to write readable code not try and squeeze as many operations onto a single line as you can. Updating answer with how I would write it. \$\endgroup\$ –  Martin York May 1, 2017 at 23:03
• \$\begingroup\$ MyVector() could be noexcept if you allow for m_pInt being nullptr . That's worth the hassle easily. \$\endgroup\$ –  Deduplicator May 1, 2017 at 23:16
• 1 \$\begingroup\$ @Deduplicator Not sure a noexcept default constructor buys you anything. Are there any optimizations in the STL this will enable? If it does not gain you anything then why add the extra complexity to the rest of your code. Especially when it adds two different meanings for basically the same declaration MyVector x; /* nullptr but size 0*/ and MyVector y(0); /* Not nullptr but size 0*/ \$\endgroup\$ –  Martin York May 1, 2017 at 23:31
• \$\begingroup\$ Fantastic answer. 1. I wonder why didn't you add m_pInt = nullptr in the destructor after deleting it. Omission? 2. In the ctor: MyVector(int x = 0) : m_size(0) . I believe you wanted to say m_size(x) here right? 3. I always tend to use {} instead of () for initialization of objects because it prevents widening and is more readable (doesn't look like a function call). \$\endgroup\$ –  KeyC0de Oct 19, 2018 at 3:47

Not the answer you're looking for? Browse other questions tagged c++ c++11 memory-management vectors or ask your own question .

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What are vectors and how to do vector assignment in R?

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Vectors data structures in R consist of order collection of numbers, characters, or strings. There are two major types of vectors :

• Atomic or homogeneous vectors
• Recursive or heterogeneous vectors.

To create a vector named x , with values namely 10, 20, 30, 3.5 and 3.8 , use c() function in R command:

Note : The c() function will return a vector.

Assignment in R

There are various methods or operators in R to perform assignment operations. Let’s discuss some techniques to perform this operation on vectors.

Using <- operator

This assignment operator( <- ) consists of two characters:

• < (less than)

These two characters occur side by side, while the tip points to a variable will receive an assignment value. The assignment of <- operator is also possible in reverse direction -> .

Let’s look at the code below:

Explanation

• Line 3 : We create a vector using the c() function and assign it to the x variable using <- operator.
• Line 4 : We print the vector x on console.
• Line 6 : We create a vector using the c() function and assign it to y variable using -> reversed operator.
• Line 7 : We print vector y to the console.

Using = operator

Equal to ( = ) operator is also used for value assignment. So, we can perform the above operation like:

Let’s understand the equal = operator with the help of a code example:

• Line 2 : We create a vector x using the c() function and equal to = operator.
• Line 3 : We print x to the console.

Using assign()

Assignment can also be made by using the assign() method. It’s an equivalent method of assignment as above.

In the example below, we use the assign() function in detail:

• Line 2 : We use the assign() function for assignment second argument value to first one.
• Line 3 : We print the vector x to the console.

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