C programming unary operator precedence

Question

   #include <stdio.h>

   int main()
    {
     int c=10,b;
     b=++c+++c;
     printf("%d",b);
     return 0;
    }

Could someone please let me know,why it is throwing compilation error?

This code has a lot of problems but the issue you seem to want to know about is the [maximal munch rule](http://stackoverflow.com/q/5341202/1708801) — Shafik Yaghmour, Apr 14 '15 at 16:33
-1: are you compiling in C or C++? They are different languages. -1: What is the compilation error? Saying there is an error is less useful than saying the error. -1: Asking about `+++` as a general rule without sufficient research effort shown. +1: actual reproducible example. Verdict: -2. Feel free to improve your question and @Yakk me and I'll change my vote. — Yakk - Adam Nevraumont, Apr 14 '15 at 16:33

score 3 · Answer 1 · answered Apr 14 '15 at 16:35

The gibberish is tokenised as

++ c ++ + c

and parsed as

((++c)++) + c

This tries to increment the rvalue yielded by ++c, which isn't allowed. You can only increment an lvalue (or a class type, in C++).

Even if it were allowed, this would give undefined behaviour: you'd have an unsequenced modification and use of the value of c.

score 0 · Answer 2 · edited May 23 '17 at 11:43

0

The compilation error would be solved if you put a space in ++c + ++c

b = ++c + ++c;

But that just fixes the compilation error. What you are doing is Undefined Behavior. Trying to change the value of c more than once in the same expression using ++ will lead to undefined Behavior.

Read Why are these constructs (using ++) undefined behavior?

edited May 23 '17 at 11:43

Community

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answered Apr 14 '15 at 16:34

Arun A S

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Except this is undefined behaviour. For example, I get 23 with Clang and 24 with GCC when compiling as C++, along with a warning by both. – chris Apr 14 '15 at 16:35
@chris , Yes, it is Undefined Behavior. Never said it wasn't. – Arun A S Apr 14 '15 at 16:36

C programming unary operator precedence

2 Answers2