C++ constexpr vs consteval vs concept

 constexpr (Compile-time OR Runtime)

Can run at compile-time OR runtime:

constexpr int add(int a, int b) {

    return a + b;

}

int x = add(5, 3);           // Runtime: called during program execution

constexpr int y = add(5, 3); // Compile-time: computed during compilation

Flexible: Compiler decides when to evaluate

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consteval (MUST be Compile-time)

FORCES compile-time evaluation:

consteval int add(int a, int b) {

    return a + b;

}

constexpr int y = add(5, 3); // ✓ OK: Compile-time

int x = add(5, 3);           // ✗ ERROR: Must be compile-time!

Strict: ALWAYS evaluated during compilation

---

concept (Type Constraint)

Defines requirements for template parameters:

template <typename T>

concept IsNumber = std::is_arithmetic_v<T>;

template <IsNumber T>  // ← T must satisfy IsNumber

void print(T value) { ... }

print(5);      // ✓ OK: int is a number

print("hi");   // ✗ ERROR: string is not a number

---

Key Differences:

Feature	constexpr	consteval	concept
Purpose	Compile/runtime function	Compile-only function	Type constraint
When	Flexible	Always compile-time	Compile-time check
Returns	Value	Value	true/false (constraint)
Used for	Calculations	Guaranteed compile-time	Template requirements