Constant expressions

Several varieties of expressions are known as constant expressions

Preprocessor constant expression

The expression following #if or #elif must expand to

• operators other than assignment, increment, decrement, function-call, or comma whose arguments are preprocessor constant expressions
• integer constants
• character constants
• the special preprocessor operator defined

Character constants, when evaluated in #if-expressions, may be interpreted in the source character set, the execution character set, or some other implementation-defined character set.

Integer constant expression

An integer constant expression is an expression that consists only of

• operators other than assignment, increment, decrement, function-call, or comma, except that cast operators can only cast arithmetic types to integer types unless they are part of an operand to a sizeof, _Alignof(since C11) or typeof/typeof_unqual(since C23) operator.
• integer constants
• enumeration constants
• character constants
• floating constants, but only if they are immediately used as operands of casts to integer type
• sizeof operators whose operands are not VLA(since C99)

Integer constant expressions are evaluated at compile time. The following contexts require expressions that are known as integer constant expressions:

• The size of a bit-field.
• The value of an enumeration constant
• The case label of a switch statement
• The size of a non-VLA(since C99) array
• Integer to pointer implicit conversion.

Static initializer

Expressions that are used in the initializers of objects with static and thread_local storage duration or declared with the constexpr storage-class specifier(since C23) must be either string literals or expressions that may be one of the following

Unlike with integer constant expressions, static initializer expressions are not required to be evaluated at compile time; the compiler is at liberty to turn such initializers into executable code which is invoked prior to program startup.

static int i = 2 || 1 / 0; // initializes i to value 1

The value of a floating-point static initializer is never less accurate than the value of the same expression executed at run time, but it may be better.

Floating-point constant expressions

Arithmetic constant expressions of floating-point types that are not used in static initializers are always evaluated as-if during run-time and are affected by the current rounding (if FENV_ACCESS is on) and report errors as specified in math_errhandling.

void f(void)
{
#pragma STDC FENV_ACCESS ON
    static float x = 0.0/0.0; // static initializer: does not raise an exception
    float w[] = { 0.0/0.0 }; // raises an exception
    float y = 0.0/0.0; // raises an exception
    double z = 0.0/0.0; // raises an exception
}

Notes

If an expression evaluates to a value that is not representable by its type, it cannot be used as a constant expression.

Implementations may accept other forms of constant expressions. However, these constant expressions are not considered as integer constant expressions, arithmetic constant expressions, or address constant expressions, and thus cannot be used in the contexts requiring these kinds of constant expressions. For example, int arr[(int)+1.0]; declares a VLA.

References

• C23 standard (ISO/IEC 9899:2023):
• 6.6 Constant expressions (p: TBD)
• C17 standard (ISO/IEC 9899:2018):
• 6.6 Constant expressions (p: 76-77)
• C11 standard (ISO/IEC 9899:2011):
• 6.6 Constant expressions (p: 106-107)
• C99 standard (ISO/IEC 9899:1999):
• 6.6 Constant expressions (p: 95-96)
• C89/C90 standard (ISO/IEC 9899:1990):
• 3.4 CONSTANT EXPRESSIONS

See also