Problem with understanding Operators
Author
My C++ instructor has recently been explaining the unary prefix and postfix operators (mainly ++ and --)
It shows out that: ++a + ++a
is equivalent to
(++a,
++a,
a + a)
so if a = 1
it returns 6
I tried it on Borland and VC++6.0 and it worked as he said
However I thought it would return 5, as in it adds one to a, (a=2), it adds one to the next a (a=3) so the expression will be 5
++a + ++a
++1 + ++a
2 + ++a
2 + ++2
2 + 3
5
Am desperate, and I know it has to do with precedence
The statement ++a + ++a is undefined, as it involves mutation of a value twice between sequence points.
The compiler is free to spit out whatever it feels like, really.
The compiler is free to spit out whatever it feels like, really.
Quote:Original post by arithma
Am desperate, and I know it has to do with precedence
Speaking strictly from an operator precedence point of view, pre-increment has higher precedence than addition, so both increments will be done before the addition. Where things get really weird is with post-increment: what are the values of a and b after the following statements?
int b, a = 1;b = a++ + a++;This is why some languages don't have increment operators, like Python.
Author
The replies are still inconclusive: who is right
Does ++a + ++a have a defined behavior indorsed by the standard, or is it left to the compiler to decide upon it
Does ++a + ++a have a defined behavior indorsed by the standard, or is it left to the compiler to decide upon it
Quote:Original post by arithma
The replies are still inconclusive: who is right
What do you mean? Only one person has posted about this.
Author
Ooops,
I didn't see that the same person was posting as I rushed into viewing the reply
However, can I conclude that the standard doesnot impose the order-of-precedence point-of-view?
I didn't see that the same person was posting as I rushed into viewing the reply
However, can I conclude that the standard doesnot impose the order-of-precedence point-of-view?
Quote:Original post by arithma
Ooops,
I didn't see that the same person was posting as I rushed into viewing the reply
However, can I conclude that the standard doesnot impose the order-of-precedence point-of-view?
Since the + operator is not a sequence point, the operands could be evaluated in any order.
Quote:Original post by arithma
However, can I conclude that the standard doesnot impose the order-of-precedence point-of-view?
It has nothing to do with order of operations. As Oluseyi has stated, multiple modifications of a single variable in between sequence points is undefined. It's not supported by the standard, and is compiler-implementation defined.
Read more about it here.
This topic is closed to new replies.
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