std::move and std::forward in c++
The std::move and std::forward functions in the C++ programming language are two important functional features or valuable C++ program utilities introduced in C++11. Both std::move and std::forward are derived from the C++ concept of rvalue references and move semantics. However, the use and purpose of both std::move and std::forward function methods are defined differently in C++.
std::move function concept in C++.
The std::move() function method in C++ is used to treat a class object as an rvalue, allowing it to be easily moved from a source location to a target location instead of copying the class’s program resources.
Syntax of the std::move function.
std::move(object);
Example of the std::move function.
#include <iostream>
#include <utility>
using namespace std;
int main()
{
string message1 = “Vcanhelpsu Edtech Platform”;
string message2 = std::move(message1);
cout << message2 << endl;
return 0;
}
It’s output.
Vcanhelpsu Edtech Platform
In this example.
string message2 = std::move(message1);
This allows you to use the move constructor method of the string.
Key points about the std::move function.
- Remember, in a C++ program, the std::move() function doesn’t actually move anything by itself.
- The std::move() function casts the actual class data object, converting your expression to an x-value. So that a move constructor or move assignment operator can be selected.
The std::move() function in a C++ program is better understood as follows.
std::move(object)
↓
“C++ users should consider this class program object movable”
↓
Now the move operation can be applied to this object.
What happens to the original object in C++?
After the std::move() function applies the operation.
string message1 = “Vcanhelpsu Edtech Platform”;
string message2 = std::move(message1);
Here, message1 is in a valid but unspecified program state.
Here, you should not assume that message1 still contains the text value information “Vcanhelpsu Edtech Platform”.
Since, C++ users can safely apply allowed operations to a valid class program object, such as assigning a new value to a variable object.
message1 = “Edtech”;
The std::forward function concept in C++.
The std::forward() function method in C++ programs is used to avoid the value category of a program argument, especially in generic class functions and templates.
std::forward in simple terms.
The std::forward method in a C++ program allows a program argument to remain static as an lvalue or rvalue right value until it is passed to another function in the program.
The std::forward method is an essential feature for perfect forwarding in a C++ program, specifically.
Why is std::forward needed in a C++ program?
Imagine a user encountering this condition in a C++ program:
void task(int& p)
{
cout << “this is an lvalue expression”;
}
void task(int&& p)
{
cout << “this is an rvalue expression”;
}
Now here we create a class program template.
template <typename T>
void wrapper(T&& element)
{
task(element);
}
Here, suppose we use the calling method.
int p = 7;
wrapper(p);
wrapper(14);
This example is what C++ users might expect:
this is an lvalue expression
this is an rvalue expression
But without the std::forward function method, the program element inside the function is treated as an lvalue because it has a name defined.
Because of this, both calls in the current program can be selected as the lvalue version.
Using the std::forward method in a C++ program.
In this example, we could write it like this.
template <typename T>
void wrapper(T&& element)
{
process(std::forward<T>(element));
}
Now here:
int p = 7;
wrapper(p);
wrapper(14);
Its output.
This is an lvalue expression
This is an rvalue expression
Here, std::forward<T>(element) holds the original program value category as secure.
Main difference between std::move vs std::forward
| std::move function method | std::forward function method |
| std::move function used toConverts an expression to an rvalue/xvalue define in program | std::forward functionConditionally used to preserve the original value by its default category |
| std::move function used when you want to move any program data or value object | Where std::forward function mainly used in generic/template code |
| std::move function Does not preserve whether the original was lvalue expression | std::forward function Preserves lvalue/rvalue by its default nature |
| std::move function basic used when Common in move constructors and move assignment data element value expression | std::forward function basic used in Common in perfect program value data element forwarding method |
| std::move function Usually written as – std::move(p) | std::forward function Usually written as – std::forward<T>(p) |
Example of a simple std::move function method in a C++ program.
So, let’s use a basic std::move function method in a C++ program.
string text1 = “Vcanhelpsu”;
string text2 = std::move(text1);
Here it means.
“The C++ program has stopped using the string variable text1 in its current state, and has allowed you to move or transfer its resources.”
std::forward function method.
template <typename T>
void wrapper(T&& element)
{
process(std::forward<T>(element));
}
Here it means.
In this example, std::forward “forwards a value into the same value category in which it was originally received.”
A perfect forwarding in a C++ program.
A perfect program data value argument forwarding in a C++ program means passing a class program argument to a template function, regardless of whether it was originally an lvalue expression or an rvalue expression.
A perfect forwarding follows this common pattern in C++.
template <typename T>
void wrapper(T&& element)
{
process(std::forward<T>(element));
}
Here, the T&& ampersand address operator in this example is known as a forwarding reference when a T is extracted from it, and std::forward<T>() preserves the original value category.
This technique is widely used in the C++ standard library, including functions like std::make_unique and std::make_shared.
An easy way to remember the std::move and std::forward functions in C++ programming.
std::move function method.
std::move(p)
↓
“Here we treat the p variable as a movable object.”
std::forward function method.
std::forward<T>(p)
↓
“Here, forward the p variable exactly as it was sent at the beginning.”
std::move and std::forward functions in one line.
In C++ programming, the std::move function method is used to move a program-declared variable value. Similarly, the std::forward function method is used to avoid value categories when forwarding program object values.
