ConstKeywords.md

Const keywords

🧠 C++ Cheatsheet: constexpr vs const vs constinit

📌 Comparison Table

Feature	const	constexpr	constinit
Mutability	❌ No	❌ No	✅ Yes
Compile-time evaluation	❌ Not guaranteed	✅ Always	✅ Initialization only
Where it can be used	Anywhere	Anywhere (with restrictions)	Global or static variables only
Usable in static_assert	❌ No	✅ Yes	❌ No
Avoids runtime initialization	❌ No	✅ Yes	✅ Yes
Usable in templates	❌ No	✅ Yes	❌ No

---

🟦 const

• Declares an immutable value
• May be initialized at runtime
• Not necessarily known at compile time

const int a = runtime_func(); // OK
a = 10;                       // ❌ Error: const

---

🟩 constexpr

• Value is guaranteed to be evaluated at compile time
• Variable is immutable
• Can be used in static_assert, templates, array sizes, etc.

constexpr int square(int x) { return x * x; }

constexpr int a = square(5);
static_assert(a == 25);

int arr[a]; // OK

---

🟨 constinit

• Declares that a variable will be initialized at compile time
• Value can be mutable
• Only valid for global or static variables
• Prevents the "static initialization order fiasco"

constinit int counter = 0;
counter = 42;  // OK — not const

constinit int invalid; // ❌ Error: must be initialized

// ❌ Invalid inside functions:
void foo() {
    constinit int local = 1; // Error
}

---

💣 The "static initialization order fiasco"

// A.cpp
Logger loggerA("A");

// B.cpp
extern Logger loggerA;
Logger loggerB("B", &loggerA); // might use an uninitialized loggerA

🔐 Solution:

• Use constinit
• Or use a function-local static variable:

Logger& getLogger() {
    static Logger instance("A"); // lazy and safe
    return instance;
}

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✅ When to use what?

You need...	Use
A constant value known to the compiler	constexpr
An immutable value, not necessarily compile-time	const
A mutable value initialized at compile-time	constinit