C++11 and above

std move () : How this template function works?

January 19, 2021
Vaibhav
4,242 Comments
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The std::move( ) is a function template which perform static casts. The std::move( ) unconditionally casts its argument (an L-value or R-value) to an R-value. This template function takes a reference to an object (using a universal reference parameter) and it returns a R-value reference to the same object.

Example,

int x = 9;
int && y = std::move(x);      //Move taking L-value argument
                              //& returning R-value

int && z = std::move(4);      //Move taking R-value argument
                              //& returning R-value

auto up1 = std::make_unique<int>();
std::unique_ptr<int> up2 = std::move(up1); //Move taking unique_ptr
                                           //& returning R-value of data

std::move does not move anything but it simply cast its argument to an R-value. Please note that everything happens only in compile time.

The simplest implementation of move is as follows:

template <typename T>
typename std::remove_reference<T>::type&& move(T&& param)
{
    using ReturnType = typename std::remove_reference<T>::type&&;
    return static_cast<ReturnType>(param);
}
  • The parameter type is T&& (universal reference), which means it accepts both L-value and R-value arguments.
  • The “&&” part of the function’s return type means that std::move returns only an r-value reference.
  • The function uses typename because, we are using a datatype inside a template class T (dependent type)
using ReturnType = typename std::remove_reference<T>::type&&;

The funciton uses remove_reference because this ensures that whatever is the calling type (L-value or R-value), the return type always correspond to R-value of basic type.

The std::remove_reference template class is part of C++ library, however, we can easily implement this in following way:

template <class T>
struct remove_reference { typedef T type; };

template <class T>
struct remove_reference<const T> { typedef const T type; };

template <class T>
struct remove_reference<T&> { typedef T type; };

template <class T>
struct remove_reference<const T&> { typedef const T type; };

template <class T>
struct remove_reference<T&&> { typedef T type; };

template <class T>
struct remove_reference<const T&&> { typedef const T type; };

Code Example: Demonstration of local implementation of move function

#include <iostream>

using namespace std;

template <typename T>
typename std::remove_reference<T>::type&& my_move(T&& param)
{
    using ReturnType = typename std::remove_reference<T>::type&&;
    return static_cast<ReturnType>(param);
}

void check(int& a)      //L-value version
{
    cout << "L-value check() called" << endl;
}
void check(int&& a)      //R-value version
{
    cout << "R-value check() called" << endl;
}

int main()
{
 
 check(5);              //Calls check(&&)
 
 int w;
 check(w);              //Calls check(&)
 check(my_move(w));     //Calls check(&&)
        
  return 0;  
}

The output is:

R-value check() called                                                                                                                                                             
L-value check() called                                                                                                                                                             
R-value check() called

Main Funda: std::move( ) does not move anything, it simply cast anything to its correspoding R-value at compile time

Related Topics:

 What are the drawbacks of using enum ?
Which member functions are generated by compiler in class?
How to stop compiler from generating special member functions?
Compiler Generated Destructor is always non-virtual
How to make a class object un-copyable?
Why virtual functions should not be called in constructor & destructor ?
Explaining C++ casts
How pointer to class members are different ?
How std::forward( ) works?
What is reference collapsing?
How std::move() function works?
How delete keyword can be used to filter polymorphism
Rule of Three

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  608. 비아그라 구입비아그라구매 비아그라판매 비아그라후기 비아그라처방 비아그라판매처 비아그라가격 비아그라구입처 비아그라정품 비아그라효과 비아그파는곳 비아그라복용법 비아그라부작용 비아그라종류 비아그라사이트 비아그라구입방법 비아그라팝니다비아그라 구입비아그라구매 비아그라판매 비아그라후기 비아그라처방 비아그라판매처 비아그라가격 비아그라구입처 비아그라정품 비아그라효과 비아그파는곳 비아그라복용법 비아그라부작용 비아그라종류 비아그라사이트 비아그라구입방법 비아그라팝니다

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  615. 비아그라 파는곳,온라인 약국 비아그라,비아그라 구매,비아그라 퀵배송,비아그라 구입처 모든 상품100% 정품임을 보장하며 모든 배송은 비밀배송 퀵배송 가능합니다

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  616. 안전한 비아그라 퀵배송은 소중한 고객님의 프라이버시를 위한 비밀보장 및 배송해드립니다. 당일 배송.오후 2시 이전 입금시 100%당일 무료 배송 원칙을 합니다.

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  622. 비아그라파는곳,비아그라 파는곳,비아그라 구입,비아그라 퀵배송, 비아그라구매, 시알리스 파는​곳,,비아그라 구입방법,온라인 비아그라판매스토어,비아그라 구입,발기부전치료제,비아그라 처방전없이구입,비아그라 판매사이트,비아그라,시알리스,레비트라,팔팔정,온라인 비아그라 구매 구입 사이트 비아스토어 입니다.
    100%정품 국내 최저가로 비아스토어에서 만나 보세요!

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  625. 비아그라는 발기부전으로 어려움을 겪는 남성들에게 큰 힘이 되어주는 약품이다. 비아그라는 주로 발기부전 치료제로 이용되지만, 다른 용도로 이용하는 경우도 있다. 예를 들어, 비아그라는 폐고혈압을 치료하기 위한 목적으로도 이용된다. 또한 고산병 치료제로도 이용할 수 도 있다.

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  626. 비아그라는 발기부전의 대표적인 약이지만 복용하는 사람의 건강상태와 유전적 요인등으로 작은 부작용들이 발생할 수 있기 때문에 의사와 상담한 후 적정량의 비아그라를 복용하는 것이 바람직하다

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  629. 요요약국에서는 처방전 없이 저렴한 가격으로 100%정품을 구매를 할 수가 있습니다. 비아그라 구입을 원한다면 요요약국으로 오세요. 쉽게 100%정품을 구매 할수가 있습니다. 초보자로도 어렵지 않게 비아그라 구입을 할수가 있습니다.

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  634. 먹튀검증 검증 업계 1위 경마사이트만 소개합니다
    안전하고 경마사이트 에서 실시간경마 를 즐기실 수 있습니다.
    인터넷이 있는곳이라면 어디서든 실시간경마를 영상으로 ​편하게 베팅하세요.

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  635. 모바­일경마의 진수 최적화된 환경에서

    어디서든 간편하게 경마사이트 를 이용하세요.

    검빛 경마에서도 검증이 완료된 안전한 인터넷경마 만을 추천합니다.

    온라인경마는 반드시 믿을 수 있는곳에서 이용하여야합니다.

    일본경마사이트 까지 제공하고 있습니다.

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  636. 경마사이트는 온라인경마사이트,실시간경마사이트,인터넷경마사이트,안전한경마사이트,먹튀검증 경마사이트,사설경마사이트,한국경마,일본경마,스크린경마 등 먹튀검증 업계 1위 경마사이트만 추천하며 전세계 경마사이트 관련 정보를 공유합니다.다양한 이벤트들이 제공되고 있으며 먹튀검증 경마사이트만을 추천합니다.

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  639. 모바­일경마의 진수 최적화된 환경에서

    어디서든 간편하게 경마사이트 를 이용하세요.

    검빛 경마에서도 검증이 완료된 안전한 인터넷경마 만을 추천합니다.

    온라인경마는 반드시 믿을 수 있는곳에서 이용하여야합니다.

    일본경마사이트 까지 제공하고 있습니다.

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