Lvalues vs Rvalues
In C++, every expression is either an lvalue or an rvalue. Understanding the distinction is crucial for mastering move semantics. The way C++ treats these categories of expressions determines how resources are managed, especially when optimizing for performance and memory usage.
An lvalue (locator value) refers to an object that occupies some identifiable location in memory (i.e., it has an address). Variables and dereferenced pointers are lvalues.
An rvalue (read value) is a temporary object or value that does not have a persistent memory address. Literals and the result of most expressions are rvalues.
main.cpp
12345678910111213141516171819202122232425#include <iostream> int getValue() { return 42; } int main() { int x = 10; // x is an lvalue int y = x; // x is an lvalue, y is an lvalue int* p = &x; // &x is an lvalue expression (address of x) int z = getValue(); // getValue() returns an rvalue // lvalues can appear on the left of assignment x = 20; // OK: x is an lvalue // rvalues cannot appear on the left of assignment // 10 = x; // Error: 10 is an rvalue // Demonstrate with std::cout std::cout << "x: " << x << std::endl; std::cout << "z: " << z << std::endl; // Temporary object (rvalue) std::cout << "getValue(): " << getValue() << std::endl; }
In the code above, observe how variables are lvalues (they can appear on the left side of an assignment), while temporary results and literals are rvalues (they cannot be assigned to).
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Can you explain the difference between lvalues and rvalues with examples?
How do move semantics relate to lvalues and rvalues?
Why is understanding lvalues and rvalues important for performance in C++?
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In C++, every expression is either an lvalue or an rvalue. Understanding the distinction is crucial for mastering move semantics. The way C++ treats these categories of expressions determines how resources are managed, especially when optimizing for performance and memory usage.
An lvalue (locator value) refers to an object that occupies some identifiable location in memory (i.e., it has an address). Variables and dereferenced pointers are lvalues.
An rvalue (read value) is a temporary object or value that does not have a persistent memory address. Literals and the result of most expressions are rvalues.
main.cpp
12345678910111213141516171819202122232425#include <iostream> int getValue() { return 42; } int main() { int x = 10; // x is an lvalue int y = x; // x is an lvalue, y is an lvalue int* p = &x; // &x is an lvalue expression (address of x) int z = getValue(); // getValue() returns an rvalue // lvalues can appear on the left of assignment x = 20; // OK: x is an lvalue // rvalues cannot appear on the left of assignment // 10 = x; // Error: 10 is an rvalue // Demonstrate with std::cout std::cout << "x: " << x << std::endl; std::cout << "z: " << z << std::endl; // Temporary object (rvalue) std::cout << "getValue(): " << getValue() << std::endl; }
In the code above, observe how variables are lvalues (they can appear on the left side of an assignment), while temporary results and literals are rvalues (they cannot be assigned to).
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