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assign vector without copy

How to Construct C++ Objects Without Making Copies

Today’s guest post is written by guest author Miguel Raggi. Miguel is a Computer Science and Math professor at UNAM, Mexico’s largest university. He loves clean, expressive, performant C++ code (and strives to convince students to write it in this way!). Miguel is the author of discreture , an open source C++ library to efficiently generate combinatorial objects, such as combinations, partitions, set partitions, and many more. Interested to write on Fluent C++ too? Check out the guest posting area.

C++ references are a powerful but tricky tool: used correctly, they can improve performance with little impact on the clarity of code. But used badly, they can hide performance issues, or even send a peaceful program into the realm of undefined behaviour.

In this post, we will explore how to use the various references of C++ to minimize copies when constructing an object that holds a value, and how in some cases we can even reach zero copies.

This article assumes that you’re familiar with move semantics, lvalue, rvalue and forwarding references. If you’d like to be refreshed on the subject, you can take a look at lvalues, rvalues and their references .

Copying from an lvalue, moving from an rvalue

Let’s imagine we have a TextBox class that holds a string, maybe to edit and display.

We want to be able to construct a TextBox  by passing it a std::string , and make a copy only when necessary. That is, when we pass it an lvalue. But when we pass it an rvalue, we would like to only move from that rvalue and into text_ .

One way to go about this is to create two constructors:

The first one takes an lvalue reference (no copy), and copies it into text_ (one copy).

The second one takes an rvalue reference (no copy) and moves it into text_ (no copy).

To make this class simpler, we can merge those two constructors into one:

What’s going on here? If we pass it an lvalue, the copy constructor of `std::string` gets called to construct the text  parameter (one copy), then text is moved into text_ (no copy).

And if we pass it an rvalue, the move constructor of std::string gets called to construct the text   parameter (no copy), and then text is moved into text_ (no copy).

Referencing an lvalue, moving from an rvalue

But what if we don’t need to modify or own the object that is passed to us? This is often the case with helper or connecting classes.

Then we really just need a reference or pointer to the object, not a full copy. For example, if we have a class called TextDisplayer whose main purpose is to display some text to the window, we would like to do something like this:

And this sometimes works fine. Except that it has an error just waiting to happen.

Consider the following three construction contexts:

Oops. Versions two and three have undefined behavior lying in wait, because the references that displayer2 and displayer3 hold are now invalid, since they were destroyed right after the constructors finish.

What we really want is for TextDisplayer to hold a reference if we are given an lvalue (that we assume will keep on existing) or alternatively, hold (and own) the full string if given an rvalue (and acquire it by moving from it).

In either case, there is no reason to make a copy, so we would like to avoid it if possible. We will see how to do just that.

Forwarding references

So how do we make a class that holds a reference if given an lvalue, but moves (and owns) when given rvalues?

This is where forwarding references come in. We wish create a template T which will be deduced as:

  • An lvalue reference if given an lvalue
  • Not a reference if given an rvalue

Fortunately, some really smart people already thought of this and gave us reference collapsing.  Here is how we would like to use it to make our wrapper that never makes a copy.

Note: in real code we would choose a more descriptive name for T , such as String . We could also add a static_assert that std::remove_cvref<T> should be std::string .

(As pointed out by FlameFire and John Lynch in the comments section, the template parameter T in the constructor is not a forwarding reference, contrary to what the first version of this article was suggesting. However, we shall make use of forwarding references below in the deduction guide and helper function.)

If we pass an lvalue reference to the constructor of TextDisplayer , T is deduced to be an std::string& , so no copies are made. And if we pass an rvalue reference, T is deduced to be an std::string , but it’s moved in (as T is moveable in our case), so there are no copies made either.

Making the call site compile

Unfortunately, the following doesn’t compile:

It gives the following error (with clang)

Strangely, using the rvalue version does compile and work (in C++17):

The problem when passing an lvalue is that constructor type deduction is done in two steps. The first step is to deduce the type for class template parameters (in our case, T ) and instantiate the class. The second step is to pick a constructor, after the class has been instantiated. But once T is deduced to be a std::string, it can’t choose the constructor taking a parameter of type std:string&& . Perhaps surprisingly, the constructor chosen in the second step doesn’t have to be the one used for template parameter deduction.

We would then need to construct it like this:

which is not very elegant (but nonetheless works).

Let’s see two ways of solving this: The way before C++17 and the C++17 way.

Before C++17, we can create a helper function similar to make_unique or any of the make_*   functions, whose main purpose was to overcome the pre-C++17 limitation that the compiler can’t deduce class templates using constructors.

In C++17 we got automatic deduction for class templates using constructors . But we also got something else that comes along with it: deduction guides.

In short, deduction guides are a way to tell the compiler how to deduce class templates when using a constructor, which is why we are allowed to do this:

and it will deduce the value type of the std::vector from the value type of the iterators.

So we need to provide a deduction guide for our constructor. In our case, it consists in adding the following line:

This allows us to write the following code:

and both cases compile. More importantly, they never , for any reason, make a copy of the string. They either move or reference the original.

Making it const

One thing that we lost from the original implementation of TextDisplayer which simply saved a reference, was the constness of the std::string reference. After all, we don’t want to risk modifying the original std::string that the caller trusted us with! We should store a const reference when given an lvalue, not a reference.

It would be nice to simply change the declaration of the member variable text_ to something like:

The const is effective when we are given rvalues, and decltype(text_) will be const std::string. But when given lvalues, decltype(text_) turns out to be std::string& . No const . Bummer.

The reason is that T is a reference, so const applies to the reference itself, not to what is referenced to . which is to say, the const does nothing, since every reference is already constant, in the sense that, unlike pointers, it can’t “point” to different places. This is the phenomenon described in The Formidable Const Reference That Isn’t Const .

We can work around this issue with a bit of template magic. In order to add const to the underlying type of a reference, we have to remove the reference, then add const to it, and then take a reference again:

Now we have to ask T whether it is a reference or not, and if so, use constTref . If not, use const T .

And finally, we can just declare text_ as follows:

The above works in both cases (lvalues and rvalues), but is ugly and not reusable. As this is a blog about expressive code, we should strive to make the above more readable. One way is to  add some extra helpers that can be reused: const_reference , which gives a const reference to a type (be it a reference or not), and add_const_to_value , which acts as std::add_const on normal types and as const_reference on references.

And so our TextDisplayer class can now be declared like this:

Isn’t there a risk of invalidating our references?

It’s difficult (but possible) to invalidate our reference to the string. If we hold the string (when given an rvalue), there is no way for it to be invalidated. And when given an lvalue, if both the lvalue and TextDisplayer live in stack memory, we know the lvalue string will outlive the TextDisplayer , since the TextDisplayer was created after the string, which means the TextDisplayer will be deleted before the string. So we’re good in all those cases.

But some more elaborate ways of handing memory in client code could lead to dangling references. Allocating a TextDisplayer on the heap, for example, as in new TextDisplayer(myLvalue) , or getting it from a std::unique_ptr , leaves the possibility of the TextDisplayer outliving the lvalue it is referring to, which would cause undefined behaviour when we try to use it.

One way to work around this risk would be disable operator new on TextDisplayer , to prevent non-stack allocations. Furthermore, as is always the danger when holding pointers or references, making copies of TextDisplayer could also lead to issues and should also be forbidden or redefined.

Finally, I guess we might still manually delete the string before TextDisplayer goes out of scope. It shouldn’t be the common case, but I don’t think there is anything we can do about that. But I’ll be happy to be proven wrong in the comments section. Bonus points if your solution doesn’t involve std::shared_ptr or any other extra free store allocations.

You may also like

  • Lvalues, rvalues and their references
  • Getting the Benefits of Strong Typing in C++ at a Fraction of the Cost
  • 10 Techniques That Will Make You Understand Other People’s Code Better

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std:: vector ::assign

Return value, iterator validity, exception safety.

std::vector:: assign

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 assign() method

  • since C++20
  • since C++11
  • until C++11

Replaces the contents of the container with the contents of another.

(1) Replaces the contents with count copies of value value .

(2) Replaces the contents with copies of those in the range [ first, last ) .

The behavior is undefined

This overload participates in overload resolution only if InputIt satisfies LegacyInputIterator .

This overload has the same effect as overload (1) if InputIt is an integral type.

(3) Replaces the contents with the elements from the initializer list ilist .

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

Parameters ​

  • count - the new size of the container
  • value - the value to initialize elements of the container with
  • first , last - the range to copy the elements from
  • ilist - initializer list to copy the values from

Return value ​

Complexity ​.

  • (1) Linear in count - O(count) .
  • (2) Linear in distance between first and last - O(std::distance(firs,t last)) .
  • (3) Linear in ilist.size() - O(ilist.size()) .

Exceptions ​

  • Return value

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Get a slice or a sub-vector from a vector in C++

In this quick article, we’ll explore to get a slice or a sub-vector from a given vector in C++.

1. Using Range Constructor

The idea is to get input iterators to the starting and ending position in the vector and pass them to the range constructor of the vector class to get a sub-vector.

Download    Run Code

Output: 2 2 4 6

2. Using std::copy function

We can also use the standard algorithm std::copy for copying part of a vector to another vector, as shown below:

  Before calling std::copy , make sure the destination vector has enough space to accommodate all elements. If this is not the case, prefer using std::back_insert iterator, which will call push_back() on the specified vector, as shown below:

That’s all about getting a slice or a sub-vector from a vector in C++.

Extract a subvector from a vector in C++

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An array has a fixed size when it is declared. But if we want a dynamic array whose size can vary at runtime then we use vector. Elements are inserted in vector from end using push_back(). Like arrays, elements in vector are also stored contiguously and they can traversed using iterators. Iterators return pointers to elements.

In this article, we will explore different methods to copy a vector in C++ and all covered methods use deep copy that the memory address of new copy is different from the original vector.

Ways of copying a vector into other

1. iterative method :-.

In this method a loop runs over the old vector and elements are inserted in new vector using push_back().This function inserts elements in vector from back.

Output :- (If inputs are 10,20,30,40)

Old vector elements are : 10 20 30 40 New vector elements are : 10 20 30 40

Complexity :- As the loop traverses all the elements once so complexity is O(n) where n is number of elements in old vector.

2. Assignment operator(=) :-

In this method elements from old vector are inserted into new vector by assigning new vector to old vector. This does not happen in arrays.

Output :- (If inputs are 1,2,3,4) Old vector elements are : 1 2 3 4 New vector elements are : 1 2 3 4

Complexity :- Linear time complexity as elements have to traversed once to assign them to new vector.

3. Passing vector as constructor :-

At the time of declaration of vector, passing old vector as an argument of constructor, copies the elements of passed vector into the newly declared vector.

Output :- Old vector elements are : 1 2 3 4 New vector elements are : 1 2 3 4

Complexity :- O(n) as elements are traversed once when old vector is passed as argument to constructor.

4. Using inbuild functions :-

  • copy(first_iterator, last_iterator, back_inserter()) :-

This function takes 3 arguments, the first iterator of old vector, the last iterator of old vector and third is back_inserter function to insert values from back. We can also pass first iterator + number of elements in place of last iterator and last iterator of new vector in place of back_inserter() if we want to copy a certain portion of elements starting from first element of old vector.

Output :- Old vector elements are : 1 2 3 4 New vector elements of v2 are : 1 2 3 4 New vector elements of v3 are : 1 2 3 0

Complexity :- O(n) as copy() takes linear time to traverse elements. But begin(),end() and back_inserter() takes constant time.

  • assign(first_iterator, last_iterator) :-

This function takes 2 arguments, first iterator to old vector and last iterator to old vector. It then assigns values of old vector to new vector.

Complexity :- O(n) as assign() takes O(n) time where n is the number of elements in old vector.

Apart from these there are more methods in C++ STL(Standard Template Library) which allows copying of vectors in different manners. They are :-

  • copy_n(iterator_source, num, iterator_dest) :

By using this method we can choose how many elements have to be copied to new vector. It takes 3 arguments :-

iterator_source : The pointer to the beginning of source vector, from where elements have to be started copying. num : Integer specifying how many numbers would be copied to destination vector starting from iterator_source. If a negative number is entered, no operation is performed. iterator_dest : The pointer to the beginning of destination vector, to where elements have to be started copying.

Output :- The new vector elements entered using copy_n() : 1 2 3 4 0 0

Complexity :- Best case of O(1) if no traversal is required. Worst case of O(n) if all elements from old vector are copied to new vector.

copy_if() : This function copies elements according to result of a “condition“ which is provided with the help of a 4th argument, a function returning a boolean value. This function takes 4 arguments, 3 of them similar to copy() and additional function, which when returns true, a number is copied, else number is not copied.

copy_backwards() : This function starts copying elements into the destination vector from backwards and keeps on copying till all numbers are not copied. The copying starts from the “iterator_dest” but in backward direction. It also takes similar arguments as copy().

Output :- The new vector elements entered using copy_if() : 1 3 5 0 0 0 The new vector elements entered using copy_backward() : 0 1 2 3 4 0

Complexity :- Best case of O(1) if no traversal required. Worst case of O(n) as in case of copy() function.

All the ways discussed above implements deep copy which means the location address of old and new vector are different. So, when values of old vector are changed, it does not reflect in new vector.

copy_if() takes how many arguments?

It is same as copy() and one extra argument is for the function returning boolean value according to condition (adsbygoogle = window.adsbygoogle || []).push({}); (adsbygoogle = window.adsbygoogle || []).push({}); riya deb.

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cppreference.com

Copy elision.

Omits copy and move (since C++11) constructors, resulting in zero-copy pass-by-value semantics.

[ edit ] Explanation

[ edit ] non-mandatory copy /move (since c++11) elision.

Under the following circumstances, the compilers are permitted, but not required to omit the copy and move (since C++11) construction of class objects even if the copy /move (since C++11) constructor and the destructor have observable side-effects. The objects are constructed directly into the storage where they would otherwise be copied/moved to. This is an optimization: even when it takes place and the copy /move (since C++11) constructor is not called, it still must be present and accessible (as if no optimization happened at all), otherwise the program is ill-formed:

  • In a return statement , when the operand is the name of a non-volatile object with automatic storage duration, which isn't a function parameter or a catch clause parameter, and which is of the same class type (ignoring cv-qualification ) as the function return type. This variant of copy elision is known as NRVO, "named return value optimization."

When copy elision occurs, the implementation treats the source and target of the omitted copy /move (since C++11) operation as simply two different ways of referring to the same object, and the destruction of that object occurs at the later of the times when the two objects would have been destroyed without the optimization (except that, if the parameter of the selected constructor is an rvalue reference to object type, the destruction occurs when the target would have been destroyed) (since C++11) .

Multiple copy elisions may be chained to eliminate multiple copies.

[ edit ] Notes

Copy elision is the only allowed form of optimization (until C++14) one of the two allowed forms of optimization, alongside allocation elision and extension , (since C++14) that can change observable side-effects. Because some compilers do not perform copy elision in every situation where it is allowed (e.g., in debug mode), programs that rely on the side-effects of copy/move constructors and destructors are not portable.

[ edit ] Example

Possible output:

[ edit ] Defect reports

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

[ edit ] See also

  • copy initialization
  • copy constructor
  • move constructor
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Home » C++ STL

Different ways to copy a vector in C++

C++ STL | Copying a vector : Learn - different ways to copy a vector in C++ STL, Shallow copy vs Deep copy, Different copy methods, etc. Submitted by Radib Kar , on July 07, 2020

In this article, we are going to see how to copy a vector into another vector in various ways? We will also learn about shallow copy and deep copy , their differences, and consequences while checking each of the vector copy methods.

Before starting with the methods for copying a vector into the other, let's discuss what shallow copy and deep copy are. What are differences b/w them, and why do we need two different terms to classify the copy process.

Shallow copy vs Deep copy

A vector is essentially an object which has some memory allocation upon execution. Now say you have defined and declared some vector A and you want to copy the content of A into another vector B . Now, there can be two cases, one case is we define a new vector B , but doesn't allocate any new memory for that, rather just link up to the vector A similar like vector B is pointing to the same location of vector A . Thus it has the same copy of vector A . This is known as a shallow copy. If we make any changes to vector A even after our copy operation is done, vector B will have the same changes which are not intended.

On the other hand, when we create a new location for vector B and copy contents from vector A , then it's known as deep copy . If we make any change to vector A after the copy operation it won't be reflected in vector B which is of course intended.

While discussing the copy method we will detail whether it's a shallow copy or deep copy and why so that you never make any mistake using the copy method and guess it's a deep copy ! (yes we often intend to have a deep copy normally).

Different copy methods

1) copy content iteratively.

Here we simply copy the contents of vector A into vector B iteratively. Vector B is defined as a new location, so it's of course a deep copy. We have verified that too by changing elements after our copy operation.

Example code:

2) Copy content recursively

Here we simply copy the contents of vector A into vector B recursively. Vector B is defined as a new location, so it's of course a deep copy. We have verified that too by changing elements after our copy operation.

3) Using assignment operator "=" (overwriting the current contents)

Another way to copy a vector into another is by using the assignment operator. Yes, it works perfectly! Don't try this at all for static array!. The reason behind why such assignment operator works because it simply overwrites the current members if any available, else assigns the value from where it's being copied. Below is an example and we can see, it's a deep copy.

Note: In Java, assignment operator does a shallow copy only.

4) Using copy constructor

We can also pass the vector A as a constructor to vector B which will invoke copy constructor and serve a deep copy .

5) std::copy() function

There is another method using the std function copy() . Though it's less used you should know a variety of functions the C++ standard library has. And yes, it does a deep copy. The copy function takes three argument

The first one is: iterator from where copy will be started: beginning of vector A

The second one is: iterator to where copy will be ended: the end of vector A

The third one is: output iterator which points to destination vector: beginning of Vector B

So the syntax is,

A very important note about the above code:

Check here I have defined vector B as vector<int> B(5) , but if you look in earlier codes you will find I had defined like vector<int> B

Now the question is why I made the change this time. Is there any reason or I just did that! Okay, let's say we do the same as before.

Then the code will be:

Oops! The output is segmentation fault! . Why? Probably now you get the point. Whenever we write vector<int> B the vector has no idea about how many elements will be there and it doesn't allocate memory for elements at all. But in our copy function there is the output iterator which tries to traverse vector B, but fails as no memory allocated for elements. That's why segmentation fault and we need to allocate memory, rather say define the vector along with its elements. That's why we need vector<int> B(5) . Go through my article on vector initialization methods to know in details about this. Oops, one little mistake can lead you to segmentation fault and you may just get insane to find the bug! Nice lesson I hope.

6) vector::assign() function

Vector has an inbuilt function too to copy contents from other content. It's named assign() function. It's again a deep copy method.

The syntax is like below,

Okay, that's all. So, we found all the copy methods actually do a good job as they all do deep copy and we can use any one of them blindly. Do comment and let us know which copy method you would prefer and why?

Related Tutorials

  • Create and initialize a vector using different ways in C++ STL
  • Access vector elements using for each loop in C++ STL
  • Different ways to access elements from a Vector in C++ STL
  • vector::size() function with example in C++ STL
  • vector::max_size() function with example in C++ STL
  • vector::resize() function with example in C++ STL
  • vector::capacity() function with example in C++ STL
  • vector::empty() function with example in C++ STL
  • vector::reserve() function with example in C++ STL
  • vector::shrink_to_fit() function with example in C++ STL
  • vector::swap() function with Example in C++ STL
  • Push and print elements in an integer vector in C++ STL
  • Push and print elements in a float vector in C++ STL
  • Check vector is empty or not | C++ STL
  • Copy a vector to another in C++
  • Copy a vector to another by using vector.assign() function in C++
  • Erase elements in C++ Vector using vector::erase()
  • Find largest and smallest elements in a vector | C++ STL
  • Insert elements in vector using vector::insert() | C++ STL
  • C++ STL sort function to sort an array or vector
  • Appending a vector to a vector in C++ STL
  • Difference between size and capacity of a vector in C++ STL
  • Minimum and maximum elements of a vector in C++ STL
  • How to find the maximum/largest element of a vector in C++ STL?
  • How to find the minimum/smallest element of a vector in C++ STL?
  • How to reverse vector elements in C++ STL?
  • How to find the sum of elements of a vector in C++ STL?
  • How to join two vectors in C++ STL?
  • How to find common elements between two Vectors using in C++ STL?
  • How to copy array elements to a vector in C++ STL?
  • Changing a particular element of a vector in C++ STL
  • 2D vector in C++ STL with user defined size
  • How to check whether an element exists in a vector in C++ STL?

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How to copy elements of an Array in a Vector in C++

An Array is a collection of items stored at contiguous memory locations. The idea is to store multiple items of the same type together.

Array

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.

Following are the different ways to copy elements from an array to a vector:

Method 1: Naive Solution Traverse the complete array and insert each element into the newly assigned vector using the push_back() function. Below is the implementation of the above approach:

Method 2: Range-based Assignment during Initialization In C++ , the Vector class provides a constructor which accepts a range, so to create a vector from array elements, pass the pointer to the first and last position of the range as the argument during the vector initialization that needs to be copied to the vector i.e, (arr, arr+N).

Below is the implementation of the above approach:

Note that Iterators used to point at the memory addresses of STL containers can also be used.

  • std::begin(arr)- Iterator to the first element of an array.
  • std::end(arr)- Iterator to the one after the last element of an array.
vector<int> v(begin(arr), end(arr));

Method 3: Using Inbuilt Function Insert(position, first_iterator, last_iterator): The insert() is a built-in function in C++ STL that inserts new elements before the element at the specified position, effectively increasing the container size by the number of elements inserted. Instead of a single value, a range can also be passed as arguments.

Method 4: Using Inbuilt Function Copy(first_iterator, last_iterator, back_inserter()): This is another way to copy array elements into a vector is to use the inbuilt copy function. This function takes 3 arguments, an iterator to the first element of the array, an iterator to the last element of the array, and the back_inserter function to insert values from the back.

Method 5: Using Inbuilt Function Assign( first_iterator, last_iterator ): The vector::assign() function can be used to assign values to a new vector or an already existing vector. It can also modify the size of the vector if necessary. It takes the iterator to the first and last position as arguments.

Method 6: Using Inbuilt Function Transform(first_iterator, last_iterator, back_insert(), function): The std::transform() function takes 4 arguments, an iterator to the first element of the array, an iterator to the last element of the array, the back_inserter function to insert values from the back and a user-defined function which can be used to modify all the elements of the array i.e, perform a unary operation, convert lower case characters to upper case, etc.

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COMMENTS

  1. c++

    Assigning vectors without copying them Ask Question Asked 6 years, 10 months ago Modified 6 years, 10 months ago Viewed 3k times 2 I have two std::vector with several std::unordered_set inside. However, one of the vector will be replacing the other throughout the execution, like this:

  2. std::vector<T,Allocator>::assign

    count - the new size of the container value - the value to initialize elements of the container with first, last - the range to copy the elements from

  3. Preventing Object Copy in C++ (3 Different Ways)

    There are three ways to achieve this : Keeping the Copy Constructor and Copy assignment operator as private in the class. Below is the C++ implementation to illustrate how this can be done. #include <iostream> using namespace std; class Base { int x; public: Base () { } Base (int y): x (y) { } private: // Copy constructor

  4. How to Construct C++ Objects Without Making Copies

    One way to go about this is to create two constructors: class TextBox { public: explicit TextBox (const std::string& text) : text_ (text) {} explicit TextBox (std::string&& text) : text_ (std::move (text)) {} private: std::string text_; }; The first one takes an lvalue reference (no copy), and copies it into text_ (one copy).

  5. ::assign

    <vector> std:: vector ::assign C++98 C++11 Assign vector content Assigns new contents to the vector, replacing its current contents, and modifying its size accordingly. C++98 C++11 In the range version (1), the new contents are elements constructed from each of the elements in the range between first and last, in the same order.

  6. 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 to 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 ...

  7. std::vector::assign

    std::vector:: assign. From cppreference.com ... initializer list to copy the values from Type requirements -InputIt must meet the requirements of InputIterator. Complexity. 1) linear in count. 2) linear in distance between first and last. 3) linear in ilist. size Example.

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

    Constructs the container with the copy of the contents of. The allocator is obtained as if by calling std::allocator_traits<allocator_type>select_on_container_copy_construction(other.get_allocator()) . (since C++11) Constructs the container with the copy of the contents of , using as the allocator. class template argument deduction, only the ...

  9. vector<...>::assign() method

    Parameters . count - the new size of the container; value - the value to initialize elements of the container with; first, last - the range to copy the elements from; ilist - initializer list to copy the values from; Return value (none) Complexity (1) Linear in count - O(count). (2) Linear in distance between first and last - O(std::distance(firs,t last)). (3) Linear in ilist.size() - O(ilist ...

  10. vector :: assign() in C++ STL

    vector:: assign () is an STL in C++ which assigns new values to the vector elements by replacing old ones. It can also modify the size of the vector if necessary. The syntax for assigning constant values: vectorname.assign (int size, int value) Parameters: size - number of values to be assigned value - value to be assigned to the vectorname

  11. Get a slice or a sub-vector from a vector in C++

    In this quick article, we'll explore to get a slice or a sub-vector from a given vector in C++. 1. Using Range Constructor. The idea is to get input iterators to the starting and ending position in the vector and pass them to the range constructor of the vector class to get a sub-vector. 1.

  12. std::vector<T,Allocator>::operator=

    1) Copy assignment operator. Replaces the contents with a copy of the contents of other . If std::allocator_traits<allocator_type>::propagate_on_container_copy_assignment::value is true, the allocator of *this is replaced by a copy of other. If the allocator of *this after assignment would compare unequal to its old value, the old allocator is ...

  13. c++

    179 This question already has answers here : Copying std::vector: prefer assignment or std::copy? (6 answers) Closed 3 years ago. I am trying to efficiently make a copy of a vector. I see two possible approaches: std::vector<int> copyVecFast1(const std::vector<int>& original) { std::vector<int> newVec; newVec.reserve(original.size());

  14. Different ways to copy a vector in C++

    // C++ code to demonstrate copy of vector by constructor method. #include<iostream> #include<vector> //to include vector container in code using namespace std; int main () { // Initializing vector with values without taking inputs from user vector<int> v1 {1, 2, 3, 4}; // Declaring new vector and passing old vector as argument to constructor v...

  15. Ways to copy a vector in C++

    Method 1: Iterative method. This method is a general method to copy, in this method a loop is used to push_back () the old vector elements into the new vector. They are deeply copied CPP #include<iostream> #include<vector> using namespace std; int main () { vector<int> vect1 {1, 2, 3, 4}; vector<int> vect2; for (int i=0; i<vect1.size (); i++)

  16. Copy elision

    Non-mandatory copy /move (since C++11) elision. Under the following circumstances, the compilers are permitted, but not required to omit the copy and move (since C++11) construction of class objects even if the copy /move (since C++11) constructor and the destructor have observable side-effects. The objects are constructed directly into the storage where they would otherwise be copied/moved to.

  17. How do you copy the contents of an array to a std::vector in C++

    To cut a long story short Method 4, using vector::insert, is the best for bsruth's scenario. Here are some gory details: Method 1 is probably the easiest to understand. Just copy each element from the array and push it into the back of the vector. Alas, it's slow.

  18. Different ways to copy a vector in C++

    And yes, it does a deep copy. The copy function takes three argument. The first one is: iterator from where copy will be started: beginning of vector A. The second one is: iterator to where copy will be ended: the end of vector A. The third one is: output iterator which points to destination vector: beginning of Vector B.

  19. How to copy elements of an Array in a Vector in C++

    Following are the different ways to copy elements from an array to a vector: Method 1: Naive Solution Traverse the complete array and insert each element into the newly assigned vector using the push_back () function. Below is the implementation of the above approach: C++ #include <bits/stdc++.h> using namespace std; int main () {