Constexpr Function

Documentation Index

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constexpr int calculate_factorial(int n) {
    int result = 1;
    for (int i = 1; i <= n; ++i) {
        result *= i;
    }
    return result;
}

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Core Mechanics

• Dual Execution Context: A constexpr function does not guarantee compile-time execution. It only indicates that compile-time execution is requested and potentially possible. To force compile-time evaluation, the result must be assigned to a constexpr variable or used in a context that requires a compile-time constant (e.g., template arguments, array bounds). To strictly guarantee compile-time execution without relying on the calling context, C++20 introduced the consteval specifier (immediate functions).
• Implicit Inline: Every constexpr function is implicitly an inline function. Consequently, its definition must be visible in the translation unit where it is called.
• Type Flexibility (C++23): As of C++23, there are no restrictions on the return type or parameter types of a constexpr function declaration. Prior to C++23, these were strictly required to be Literal Types (e.g., scalar types, references, or classes with a trivial destructor and at least one constexpr constructor).
• Unconditional Runtime Degradation (C++23): As of C++23, a constexpr function is perfectly well-formed even if no possible invocation could ever result in a constant expression (it simply degrades to a runtime function). Prior to C++23, if a function lacked at least one valid compile-time execution path, the program was ill-formed, no diagnostic required (IFNDR).

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Evolution of Definition Constraints

• C++11: The function body must consist of exactly one return statement. No local variable declarations, loops, or conditional statements (if/switch) are permitted.
• C++14: Permits multiple return statements, initialized local variables, mutation of local variables, and control flow structures (if, switch, for, while).
• C++20: Permits transient dynamic memory allocation (new and delete, provided memory is allocated and deallocated within the same compile-time evaluation context), try and catch blocks, virtual function calls, changing the active member of a union, and uninitialized variables.
• C++23: Permits goto statements, labels, static variables, and thread_local variables. It fully permits the declaration and use of static constexpr variables inside the function. Crucially, it removes the requirement that parameter/return types must be Literal Types and removes the IFNDR rule for functions lacking a valid compile-time execution path.

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Constant Evaluation Restrictions (Execution Path)

1. Call non-constexpr functions.
2. Read or modify mutable state outside its scope. It can read variables outside its scope, but only if those variables are themselves usable in constant expressions (e.g., reading a global constexpr variable or a const integral variable is permitted).
3. Evaluate a goto statement or label. They may exist in the function, but the compile-time control flow must not reach them.
4. Evaluate the initialization of a static or thread_local variable, unless that variable is declared static constexpr.
5. Read from an uninitialized variable. The variable may be declared without initialization, but it must be assigned a value before it is read.
6. Perform a reinterpret_cast or cast from void*.
7. Evaluate a throw expression. (try/catch blocks can exist, but throwing an exception halts constant evaluation).
8. Evaluate operations on non-literal types. Even though C++23 allows non-literal types in the signature, attempting to evaluate them at compile time will fail.

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Syntax Visualization: Compile-Time vs Runtime vs Consteval

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

// C++20: Guarantees compile-time execution
consteval int strict_square(int x) {
    return x * x;
}

int main() {
    // Evaluated at compile-time. 
    // The compiler replaces this with `constexpr int a = 25;`
    constexpr int a = square(5); 

    // Evaluated at runtime.
    // The compiler generates standard assembly for a function call.
    int dynamic_val = 10;
    int b = square(dynamic_val); 
    
    // Evaluated at compile-time.
    // strict_square(dynamic_val) would result in a compilation error.
    int c = strict_square(5);
    
    return 0;
}