# C++ Value categories

**[Value categories](https://en.cppreference.com/w/cpp/language/value_category)** classify expressions based on whether they can be used to obtain an object's address and whether they're about to be destroyed. Lvalues have persistent identity and can be referenced; rvalues (temporaries or results of `std::move`) are about to disappear. This distinction enables move semantics and perfect forwarding.

Understand value categories to predict which overloads are chosen and how move semantics behave.

## Example

This example shows how value categories determine which constructor or assignment operator is called.

```cpp
// compile: g++ -std=c++17 -o values values.cpp
// run: ./values
// description: value categories determine move vs copy semantics

#include <iostream>

class Widget {
public:
    Widget() { std::cout << "Default\n"; }
    Widget(const Widget&) { std::cout << "Copy\n"; }
    Widget(Widget&&) { std::cout << "Move\n"; }
};

int main() {
    Widget a;              // lvalue: default constructor
    Widget b = a;          // lvalue: copy
    Widget c = Widget();   // rvalue: move (or elision)
    
    Widget d = std::move(a);  // move: cast lvalue to rvalue
    
    return 0;
}
```