7 Expressions [expr]

7.7 Constant evaluation [expr.const]

7.7.1 General [expr.const.general]

1
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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 is not performed, are called constant expressions.
[Note 1: 
Constant expressions can be evaluated during translation.
— end note]
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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]