C++ Constant Initialization | `constexpr`, `constinit`, and Static Order

What is constant initialization?

Constant initialization initializes an object with a value fixed at compile time (constant expression). Contrast with dynamic initialization that runs before main but executes runtime code.

Works with constexpr functions and constexpr if; distinguish from value initialization and zero initialization.

constexpr int x = 10;   // constant initialization
int y = compute();      // dynamic initialization (runtime)

Why it matters

• Performance: no runtime init cost for that object
• Safety: avoids cross-TU static initialization order hazards for data that can stay constexpr
• Optimization: lets the compiler embed values directly

constexpr variables

Typical uses: array bounds, switch cases, non-type template arguments, lookup tables.

constinit (C++20)

constinit guarantees constant initialization for static/thread_local objects, but unlike constexpr the object may still be mutable at runtime (unless also const).

Compare:

	constexpr	constinit
Init time	compile-time	compile-time
Mutate at runtime	no	yes (if not const)
Where	many contexts	static/thread_local only

Initialization order recap

Static storage duration objects: typically zero-init, then constant init where applicable, then dynamic init for the rest.

Cross-TU dynamic order is unspecified—design so globals do not depend on each other’s dynamic init.

Pitfalls & practice

• Do not expect constexpr from non-constexpr functions.
• Use constinit to lock in compile-time initialization when refactor might otherwise introduce dynamic init accidentally.
• Use inline constexpr in headers to avoid ODR violations.

FAQ

See Korean article Q&A—topics include performance, order phases, constinit vs constexpr, and learning resources (cppreference — constant initialization).

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Related posts

• constexpr functions
• constexpr if
• Value initialization
• Zero initialization

Keywords

C++, constant initialization, constexpr, constinit, C++20.

See also

• Constexpr series
• Advanced constexpr