Order of Evaluation

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Evaluation of Expressions

Evaluation of each expression includes:

• value computations
  • calculation of value that is returned by expression
• initiation of side effects
  • access(read/write) to an object
  • modifiation of object
  • calling I/O function
  • calling a function that does on of the above

Ordering

"sequenced-before" is an asymmetric, transitive, pair-wise relationship between evaluations within the same thread.

If A is sequenced before B, then evaluation of A will be complete before evaluation of B begins.

If A is not sequenced before B and B is not sequenced before A, then two possibilities exist:

• evaluations of A and B are unsequenced: they may be performed in any order and may overlap (within a single thread of execution, the compiler may interleave the CPU instructions that comprise A and B)
• evaluations of A and B are indeterminately sequenced: they may be performed in any order but may not overlap: either A will be complete before B, or B will be complete before A. The order may be the opposite the next time the same expression is evaluated.

Rules

For rules about how expressions are sequenced look HERE

Undefined Behavior

• if a side effect on a scalar object is unsequenced relative to another side effect on the same scalar object
  • i = ++i + i++;
• if a side effect on a scalar object is unsequenced relative to a value computation using the value of the same scalar object
  • auto n = ++i + i;