7 Expressions [expr]

7.7 Constant evaluation [expr.const]

7.7.1 General [expr.const.general]

1
#
Certain contexts require expressions that satisfy additional requirements as detailed in [expr.const]; other contexts have different semantics depending on whether or not an expression satisfies these requirements.
Expressions that satisfy these requirements, assuming that copy elision ([class.copy.elision]) is not performed, are called constant expressions.
[Note 1: 
Certain constant expressions are evaluated during translation ([lex.phases]).
— end note]
2
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Recommended practice: Implementations should provide consistent results of floating-point evaluations, irrespective of whether the evaluation is performed during translation or during program execution.
[Note 2: 
Since this document imposes no restrictions on the accuracy of floating-point operations, it is unspecified whether the evaluation of a floating-point expression during translation yields the same result as the evaluation of the same expression (or the same operations on the same values) during program execution.
[Example 1: bool f() { char array[1 + int(1 + 0.2 - 0.1 - 0.1)]; // Must be evaluated during translation int size = 1 + int(1 + 0.2 - 0.1 - 0.1); // May be evaluated at runtime return sizeof(array) == size; }
It is unspecified whether the value of f() will be true or false.
— end example]
— end note]