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
• 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 must be 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.

References