CODEWITHSUNDEEP
c++standards
Rob Harvey
Rob Harvey

Reputation: 13

Error in the standards?

§5/4 C++ standard

i = 7, i++, i++;  // i becomes 9
i = ++i + 1;  //the behavior is unspecified

That should be changed to 

i = 7, i++, i++;  // the behavior is undefined
i = ++i + 1;  //the behavior is undefined

right?

Upvotes: 1

Views: 183

Answers (4)

Prasoon Saurav
Prasoon Saurav

Reputation: 92854

i = 7, i++, i++; // i becomes 9

is perfectly fine. Operator = has higher precedence than ,so the expression is equivalent to 

(i = 7), i++, i++;

which is perfectly well defined behaviour because , is a sequence point.

As far as 

i = ++i + 1; //the behavior is unspecified

is concerned the behaviour is undefined in C++03 but well defined in C++0x. If you have the C++0x draft you can check out in section 1.9/15

i = i++ + 1; // the behavior is undefined

Upvotes: 6

Konrad Rudolph
Konrad Rudolph

Reputation: 545508

That should be changed to

i = 7, i++, i++;  // the behavior is undefined

Why? The standard is correct, this shouldn’t be changed, and this behaviour is well-defined.

A comma (,) introduces a sequence point into the calculation so the order of execution is defined.

Upvotes: 2

CB Bailey
CB Bailey

Reputation: 791411

Yes, please see this defect report: http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#351 .

Clarification: the example is wrong but your 'fix' is incorrect for the first statement. The first statement is correctly commented in the standard. It is only the second comment that is inaccurate.

Upvotes: 7

Peter Alexander
Peter Alexander

Reputation: 54270

No, the standard is right. The comma operator guarantees that any side effects of previous operands are completed before evaluating the next.

These guarantees are provided by sequence points, which the comma operator (as well as && and ||) are.

Note that you are correct on the wording change for the second statement. It is undefined, not unspecified.

Upvotes: 4

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