Mutable--keyword

[Butcher3Years]

🔄 C++ Mutable Keyword: Const Methods + Lambda Mastery

Mutable in Classes (Const Methods)

mutable lets specific data members change even in const objects/methods:

class Player {
private:
    int health{100};
    mutable int cacheHits{0};  // Cache/counter - logically const
    
public:
    int getHealth() const {    // Promised not to change *logical* state
        ++cacheHits;           // ✅ Allowed: doesn't affect health
        return health;
    }
};

## 🐑 C++ Lambda Mutable: Value vs Reference Capture Deep Dive

### Capture by Value (Default: const copy)
```cpp
int counter = 0;

// ❌ Regular value capture = IMMUTABLE copy
auto lambda1 = [counter]() { 
    // counter = 10;  // ERROR: captured copy is const
    return counter;
};

// ✅ mutable = MODIFIABLE copy  
auto lambda2 = [counter]() mutable {
    counter = 10;      // Modifies lambda's INTERNAL copy only
    return counter;    // Returns 10 on every call after first
};

Capture by Reference (Always mutable)

int counter = 0;

// ✅ Reference = Direct access to original variable
auto lambda3 = [&counter]() { 
    counter = 20;      // Modifies ORIGINAL variable
    return counter;    // Reflects external changes
};