Want to win a PS4? Go Premium and enter to win our High-Tech Treats giveaway. Enter to Win

x
Solved

# Comma operator and order or precedence

Posted on 2013-12-11
Medium Priority
297 Views
Ah hello.

I am sure I am having a silly moment here, but I am trying to understand the comma operator in C++ and "operator precedence" rules (which I believe are a general maths thing, hence me posting this in the Maths TA too:)).

From what I have read about the comma operator, we have two facts

1) The result of it is the right hand value, so for example a,b would yield b.
2) It has the lowest operator precedence.  I read this means "it's always the last one to bind to an expression", but I don't quite get that bit.

So, if we have

a = (b , c)

the evaulation sequence would first be (b,c) which would yield c, hence a would be assigned c.  But I am struggling with

a = b , c

This is apparently equivalent to

(a = b) , c

The result of this expression is b.

Now, I know we have to evaluate the brackets first, which results in b, so we then have

b, c

Q1) From rule 1 above, why is the result hence not c?  Clearly something is incorrect in what I deduced above, but I cannot see it...

Q2) Can someone clarify fact 2) above and what "it's always the last one to bind to an expression" means?

TIA
0
[X]
###### Welcome to Experts Exchange

Add your voice to the tech community where 5M+ people just like you are talking about what matters.

• Help others & share knowledge
• Earn cash & points
• 3
• 2

LVL 31

Expert Comment

ID: 39710942

1. This is because the comma seperates the two statements, so the expression a = b , c is equivalent to ( a = b ) , c

One maybe important fact: In C++ it's possible to override comma-operators which may lead to confusion since if it is overridden it starts acting like a function call which means there's no order defined in which left- and rightside expressions are executed. For this you can find a sample at i.e. http://madebyevan.com/obscure-cpp-features/

Hope that helps,

ZOPPO
0

LVL 19

Author Comment

ID: 39710950
Thanks ZOPPO, but what you say I have already observed.  The result of

( a = b ) , c

is (and this is where I might be wrong!)

b, c

which is why I cannot see why the overall answer is not c!
0

LVL 31

Accepted Solution

Zoppo earned 800 total points
ID: 39710959
Yes, that's correct, but the c is never assigned to anything. You have two different statements, "a = b" and "c", so a is equal to b afterward, no matter in which order the statements are evaluated.

Maybe it's clear when you check this:

x = ( a = b, c );

This results in 'a = b' and 'x = c'.
0

LVL 19

Author Closing Comment

ID: 39710972

"but the c is never assigned to anything"

plus the lovely little example perfectly explain this.

Thank you!
0

LVL 31

Expert Comment

ID: 39710979
You're welcome, I'm glad I could help :o)

Have a nice day,

best regards,

ZOPPO
0

## Featured Post

Question has a verified solution.

If you are experiencing a similar issue, please ask a related question

When writing generic code, using template meta-programming techniques, it is sometimes useful to know if a type is convertible to another type. A good example of when this might be is if you are writing diagnostic instrumentation for code to generatâ€¦
Basic understanding on "OO- Object Orientation" is needed for designing a logical solution to solve a problem. Basic OOAD is a prerequisite for a coder to ensure that they follow the basic design of OO. This would help developers to understand the bâ€¦
The viewer will learn how to use the return statement in functions in C++. The video will also teach the user how to pass data to a function and have the function return data back for further processing.
The viewer will be introduced to the technique of using vectors in C++. The video will cover how to define a vector, store values in the vector and retrieve data from the values stored in the vector.
###### Suggested Courses
Course of the Month8 days, 7 hours left to enroll