L-value,R-value ,Move , Copy
In C++, an expression can be categorized as either an lvalue or an rvalue. An lvalue refers to an object that has an identifiable memory address and can be on the left-hand side of an assignment operator. On the other hand, an rvalue refers to an object that does not have an identifiable memory address and can only be on the right-hand side of an assignment operator.
What is identifiable memory address?
In C++, an "identifiable memory address" refers to a memory location that can be accessed and referred to by its address. L-values represent objects that have such identifiable memory addresses because they are named variables or expressions that yield objects with memory locations. For example, you can take the address of an L-value using the address-of operator (&) and manipulate it directly.
On the other hand, R-values are temporary or unnamed objects that do not have identifiable memory addresses. They are often the result of expressions or literals. R-values are not addressable, meaning you cannot take their address using the address-of operator. The reason behind this is that R-values may not have long-term storage or may be compiler-generated temporary objects that exist only for the duration of an expression. As a result, they are not guaranteed to have a stable memory address that can be reliably accessed outside of the expression.
The inability to take the address of an R-value is one of the distinguishing characteristics that differentiate it from an L-value. This distinction is important in C++ because it allows the compiler to optimize the handling of R-values and perform efficient move operations instead of unnecessary copying. The introduction of r-value references and move semantics in C++11 provides a mechanism to exploit the temporary nature of R-values and enable more efficient resource management and object transfers.
For example:
int x = 5; // 'x' is an lvalue
int y = x + 3; // 'x + 3' is an rvalue
The main difference between an lvalue and an rvalue is that an lvalue can be modified, whereas an rvalue cannot be modified. This is because an lvalue has an identifiable memory address that can be accessed and modified, while an rvalue is just a value that can be used in an expression.
Some of the example of R Value:
9. The result of a function call that returns by value:
int square(int x) {
return x * x;
}
int main() {
int result = square(5); // result is assigned the R-value returned by square function
return 0;
}
10. Temporary objects created by the compiler for expression evaluation:
int main() {
int result = 5 * (4 + 3);
// the expression (4 + 3) is evaluated as an R-value and used to create a
// temporary object that is then used in the multiplication expression
return 0;
}
11. The result of a type cast:
int main() {
double pi = static_cast<double>(22)/7;
// the expression static_cast<double>(22) creates an R-value of type
// double, which is then used to perform the division operation and
// assign the result to pi
return 0;
}
12. The return value of a lambda expression:
int main() {
auto func = [] (int x, int y) {
return x + y;
};
int result = func(2, 3);
// the lambda expression returns an R-value of type int, which is
// assigned to result
return 0;
}
An lvalue reference is a reference to an object that has a distinct memory address and can be modified. An lvalue reference is declared using the & operator, for example int&.
A rvalue reference is a reference to a temporary object, which can be moved from. An rvalue reference is declared using the && operator, for example int&&.
The main difference between lvalue and rvalue references is that lvalue references are used to modify an object's value and rvalue references are used to move an object's value.
How about conts and enums ?
In C++, both constant variables and enumerations (enums) can be both L-values and R-values, depending on the context.
Consts :
Constant variables, declared with the const keyword, can be L-values if they are assigned a memory location and have an identifiable address. For example:
const int x = 10; // L-value
int* ptr = &x; // Valid, taking address of constant variable
R-value example with constant variable:
const int& getConstant() {
return 42;
}
int main() {
const int& r = getConstant(); // R-value
// r is a reference to the temporary R-value returned by getConstant()
// We can use the R-value in expressions, but we cannot take its address
// because it doesn't have an identifiable memory address.
return 0;
}
Enums :
Enums, on the other hand, are usually used as named constants to represent a set of integer values. Enumerations can also be used as L-values or R-values depending on the context. For example:
enum Color { RED, BLUE, GREEN }; // L-value (name of the enum type)
Color c = RED; // L-value (variable of enum type)
Color* ptr = &c; // Valid, taking address of enum variable
R-value example with enum:
enum class Color { RED, BLUE, GREEN };
Color getColor() {
return Color::RED;
}
int main() {
Color&& r = getColor(); // R-value reference
// r is an R-value reference to the temporary R-value returned by getColor()
// We can use the R-value reference in expressions, but we cannot take its
// address because the temporary object doesn't have an identifiable memory
// address.
return 0;
}
In the above examples of R value , the functions getConstant() and getColor() return temporary R-values, which are then bound to the respective references r. These references can be used in expressions, but their addresses cannot be taken since they are temporary objects without identifiable memory addresses.
So, both constant variables and enums can be L-values or R-values depending on how they are used in the code. It is not accurate to say that constant variables can only be L-values and enums can only be R-values.
An l-value is an expression that refers to an object that persists beyond the expression, such as a variable or a member of an object. An r-value is an expression that does not persist beyond the expression, such as a literal or the result of a function call.
In C++11, rvalue references were introduced to allow moving the contents of an object instead of copying them. This can be very useful for large objects, such as vectors or strings. Instead of copying the contents of these objects, which can be time-consuming, we can move the contents to a new object using an rvalue reference.
To convert an L-value to an R-value, you can use an rvalue reference. For example:
int x = 5;
int&& rvalueRef = static_cast<int&&>(x);
In this case, the static_cast creates an rvalue reference to x, which can now be treated as an R-value.
To convert an lvalue to an rvalue, you can also use the std::move() function. This function takes an lvalue reference and converts it to an rvalue reference. An rvalue reference is a new type of reference introduced in C++11 that can only bind to rvalues. By using std::move(), you are telling the compiler that you no longer need the value of the lvalue and it can be treated as an rvalue.
Note: std::move() does not actually convert an L-value to an R-value. Rather, it casts the given L-value reference to an R-value reference, effectively treating it as an R-value.
For example:
string str = "hello";
string&& rvalueRef = move(str); // 'str' is now treated as an rvalue
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Copy/Assignment VS Move
Copy constructor and assignment operator are used to perform deep copy when an object contains pointer members that point to dynamically allocated memory. If a shallow copy is performed, two objects will end up sharing the same memory, which can cause issues like double-free, dangling pointers, or memory leaks.
The default copy constructor and assignment operator provided by the compiler perform shallow copy, i.e., they copy the values of the member variables of the source object into the corresponding member variables of the destination object. If a deep copy is required, a user-defined copy constructor and assignment operator can be implemented to perform the required copy.
Similarly, On the other hand, the objective of the move constructor and move assignment operator is to transfer the resources (such as memory) owned by an object to another object. It is used to optimize the performance of an application by reducing the amount of data copying that is needed. For example, when returning objects from a function, or when using containers such as vectors or lists. move constructor and move assignment operator are useful in cases where the object contains a large amount of data, and copying the data is an expensive operation. By using move semantics, the data can be efficiently transferred from the source object to the destination object, avoiding the overhead of copying.
In short, the objective behind copy and assignment operator is to avoid shallow copy and perform deep copy, whereas the objective behind move constructor and move assignment operator is to transfer the ownership of the resources held by the source object to the destination object in an efficient manner.
The move constructor and move assignment operator make use of rvalue references to move the contents of one object to another. By doing so, they can avoid unnecessary copying of data and improve performance.
The move constructor is a constructor that takes an rvalue reference to another object of the same type and moves its contents to the new object. It can be invoked implicitly when a temporary object is created, or explicitly using the std::move() function.
Copy Constructor:
A copy constructor is a special type of constructor that is used to create a new object by copying the values of an existing object. The syntax for a copy constructor is as follows:
class MyClass {
public:
MyClass(const MyClass& obj) {
// Copy constructor code here
}
};
The copy constructor takes an object of the same class as its parameter, which is then used to create a new object. The copy constructor is called automatically when a new object is created using an existing object, for example, when an object is passed as a parameter to a function by value.
Assignment Operator:
The assignment operator is used to assign the values of one object to another object of the same class. The syntax for an assignment operator is as follows:
class MyClass {
public:
MyClass& operator=(const MyClass& obj) {
// Assignment operator code here
}
};
The assignment operator takes an object of the same class as its parameter, which is then used to assign the values of one object to another object. The assignment operator is called automatically when an object is assigned a new value.
move constructor
The move constructor is a constructor that takes an rvalue reference to another object of the same type and moves its contents to the new object. It can be invoked implicitly when a temporary object is created, or explicitly using the std::move() function.
For example, for move constructor:
class MyClass {
public:
MyClass(MyClass&& other) {
// Move the contents of 'other' to 'this'
}
};
MyClass obj1;
MyClass obj2(std::move(obj1)); // Invoke the move constructor explicitly
move assignment operator:
The move assignment operator is an overloaded operator that takes an rvalue reference to another object of the same type and moves its contents to the current object. It can be invoked implicitly when assigning a temporary object to an object of the same type, or explicitly using the std::move() function.
For example, for move assignment operator:
class MyClass {
public:
MyClass& operator=(MyClass&& other) {
// Move the contents of 'other' to 'this'
return *this;
}
};
MyClass obj1, obj2;
obj2 = move(obj1); // Invoke the move assignment operator explicitly
The move constructor or move assignment operator is invoked when an object is moved, i.e., when it is being transferred from one object to another object. This can happen in a few different ways:
The compiler determines whether to use the move constructor or the copy constructor based on the type of the object being moved. If the object being moved is an r-value, the move constructor is used. If it is an l-value, the copy constructor is used. The type of the object is determined based on the expression that creates it.
As discussed earlier to convert an l-value to an r-value, you can use the std::move() function. std::move() takes an l-value reference as its argument and returns an r-value reference. This does not actually move the object, but instead allows the object to be treated as an r-value, which can be useful for invoking the move constructor or move assignment operator. However, it is important to note that using std::move() on an object that is still needed can lead to unexpected behavior, such as double deletion.
Example demonstrating the use of move constructor, move assignment operator, copy constructor and assignment operator overloading in C++:
#include <cstring>
#include <iostream>
using namespace std;
class MyString {
public:
// Default constructor
MyString() : m_str(nullptr), m_len(0) {}
// Parameterized constructor
MyString(const char* str) : m_len(strlen(str)) {
m_str = new char[m_len + 1];
strcpy(m_str, str);
}
// Copy constructor
MyString(const MyString& other) : m_len(other.m_len) {
m_str = new char[m_len + 1];
strcpy(m_str, other.m_str);
}
// Move constructor
MyString(MyString&& other) noexcept {
m_len = other.m_len;
m_str = other.m_str;
other.m_len = 0;
other.m_str = nullptr;
}
// Destructor
~MyString() {
delete[] m_str;
}
// Copy assignment operator overloading
MyString& operator=(const MyString& other) {
if (this != &other) {
delete[] m_str;
m_len = other.m_len;
m_str = new char[m_len + 1];
strcpy(m_str, other.m_str);
}
return *this;
}
// Move assignment operator overloading
MyString& operator=(MyString&& other) noexcept { // noexcept means it will not throw any exceptions.
if (this != &other) {
delete[] m_str;
m_len = other.m_len;
m_str = other.m_str;
other.m_len = 0;
other.m_str = nullptr;
}
return *this;
}
// Getter method for string length
size_t length() const {
return m_len;
}
// Getter method for string
const char* c_str() const {
return m_str;
}
private:
char* m_str; // pointer to character array holding string
size_t m_len; // length of string
};
int main() {
// Example of move constructor
MyString str1("Hello");
MyString str2 = move(str1);
if(str1.c_str() == nullptr) {
cout << "str1 = nullptr " << ", length = " << str1.length() << endl;
}
// cout << "str1 = " << str1.c_str() << ", length = " << str1.length() << endl; // Output: str1 = , length = 0
cout << "str2 = " << str2.c_str() << ", length = " << str2.length() << endl; // Output: str2 = Hello, length = 5
// Example of move assignment operator
MyString str3("World");
MyString str4("C++");
cout << "Before move assignment: str3 = " << str3.c_str() << ", length = " << str3.length() << endl; // Output: Before move assignment: str3 = World, length = 5
cout << "Before move assignment: str4 = " << str4.c_str() << ", length = " << str4.length() << endl; // Output: Before move assignment: str4 = C++, length = 3
str3 = move(str4);
cout << "After move assignment: str3 = " << str3.c_str() << ", length = " << str3.length() << endl; // Output: After move assignment: str3 = C++, length = 3
if(str4.c_str() == nullptr) {
cout << "After move assignment: str4 = nullptr " << ", length = " << str4.length() << endl;
}
// Example of copy constructor
MyString str5("Hello");
MyString str6(str5);
cout << "str5 = " << str5.c_str() << ", length = " << str5.length() << endl; // Output: str5 = Hello, length = 5
cout << "str6 = " << str6.c_str() << ", length = " << str6.length() << endl; // Output: str6 = Hello, length = 5
// Example of copy assignment operator
MyString str7("World");
MyString str8("C++");
cout << "Before copy assignment: str7 = " << str7.c_str() << ", length = " << str7.length() << endl; // Output: Before copy assignment: str7 = World, length = 5
cout << "Before copy assignment: str8 = " << str8.c_str() << ", length = " << str8.length() << endl; // Output: Before copy assignment: str8 = C++, length = 3
str7 = str8;
cout << "After copy assignment: str7 = " << str7.c_str() << ", length = " << str7.length() << endl; // Output: After copy assignment: str7 = C++, length = 3
cout << "After copy assignment: str8 = " << str8.c_str() << ", length = " << str8.length() << endl; // Output: After copy assignment: str8 = C++, length = 3
return 0;
}
/*
amit@DESKTOP-9LTOFUP:~/OmPracticeC++$ ./a.out
str1 = nullptr , length = 0
str2 = Hello, length = 5
Before move assignment: str3 = World, length = 5
Before move assignment: str4 = C++, length = 3
After move assignment: str3 = C++, length = 3
After move assignment: str4 = nullptr , length = 0
str5 = Hello, length = 5
str6 = Hello, length = 5
Before copy assignment: str7 = World, length = 5
Before copy assignment: str8 = C++, length = 3
After copy assignment: str7 = C++, length = 3
After copy assignment: str8 = C++, length = 3
*/
In the above program statement MyString(MyString&& other) noexcept, noexcept is a specifier indicating that the move constructor MyString(MyString&& other) is declared as non-throwing. This means that the function guarantees that it will not throw any exceptions.
This is useful because it allows the compiler to generate more efficient code, since it does not need to generate additional exception handling code. It also provides additional safety guarantees, as the caller can rely on the function not throwing an exception and can avoid writing additional exception handling code.
However, it's important to note that noexcept is not a magic bullet that guarantees no exceptions will ever be thrown. It's still possible for exceptions to be thrown if there is a bug in the code or if an external condition (such as a lack of memory) prevents the operation from completing successfully. Therefore, it's important to use noexcept judiciously and ensure that it accurately reflects the behavior of the function.
Advantages of Copy Constructor/Assignment Overloading:
Disadvantages of Copy Constructor/Assignment Overloading:
Advantages of Move Constructor/Move Operator Overloading: :
Disadvantages of Move Constructor/Move Operator Overloading :
Limitations of both Copy and Move Operations:
In summary, move constructor and move assignment operator provide a way to efficiently transfer the ownership of resources from one object to another, without the overhead of making a deep copy. This can lead to significant performance improvements, especially when working with large objects or resources like dynamic memory, file handles, or network sockets.
To enable move semantics, a class should define a move constructor and a move assignment operator. The move constructor should transfer the ownership of the resources from the source object to the destination object, while the move assignment operator should release the resources of the destination object and transfer the ownership of the resources from the source object.
When working with move semantics, it is important to keep in mind that the source object is usually left in a valid but unspecified state. Therefore, the object should not be used after it has been moved from, unless it has been explicitly reset or re-initialized.
Finally, the std::move() function can be used to convert an lvalue into an rvalue reference, which is necessary for invoking the move constructor or move assignment operator. However, it is important to note that std::move() does not actually move anything; it simply casts the lvalue to an rvalue reference, allowing the move constructor or move assignment operator to be called.
Thanks for reading till end , please comment if you have any!
Senior Software Engineering Manager at Indeed.com
1yL/R values on their own are more of a pre C++11 concept. Since we are now at C++23 might be worth expanding this to include all the value categories (G/R/L/X/PR) and how expressions map.