C++11 and above

std move () : How this template function works?

January 19, 2021
Vaibhav
3,755 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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