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constinit.md

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execute true

What It Does

constinit specifies that a variable with static or thread-local storage duration is initialized by constant initialization. Unlike constexpr, the variable is not const-qualified and may be modified at runtime; only the initialization is guaranteed to occur at compile time.

Why It Matters

The static initialization order fiasco occurs when global variables with dynamic initializers have dependencies on each other. constinit ensures constant initialization precedes dynamic initialization, eliminating undefined behavior from initialization order dependencies.

Example

#include <array>
#include <print>

// Guaranteed compile-time initialization
constinit auto global_count = 42;
constinit auto some_array   = std::array{1, 2, 3};

// NOT constexpr - can be modified at runtime
void increment() {
    ++global_count;
}

// Error: not a constant expression
// constinit int x = some_runtime_function();

// Thread-local works too
constinit thread_local int per_thread_id = 0;

int main() {
    increment();  // global_count is now 43
    std::println("global_count: {}", global_count);
    std::println("some_array: {}", some_array);
}