Why output differs in C and Java in the expression m++ + (++m) [duplicate]

Question

Consider:

int m = 2, n;

n = m++ + (++m);

In C output is:

m = 4, n = 4;

In Java output is:

m = 4, n = 5;

How does this happen?

Scevour

which compiler do you use? the same code gives m=4 and n=6 for me (Visusal Studio 2010) – Morningglory

It's because you can't change the same variable twice in a statement without sequencing point. That causes undefined behavior in C. – Masera

Answer 1

It can differ because C does not allow a correct program to contain such an expression - C does not define the behaviour of such a program. This gives C compilers wide latitude in how they interpret such expressions.

Java more tightly constrains implementations by defining the expected behaviour of expressions like this.

(The rule that this breaks in C is that an expression may not modify the value of an object more than once without an intervening sequence point).

Bower

"C does not allow a correct program to contain such an expression." Yes it does; it just doesn't specify its behaviour. Not the same thing. – Basalt

@EJP: Such a program is not correct. – Bower

if such a program is not correct the compiler shouldn't compile it. – Basalt

@EJP: A conforming compiler is allowed to reject the program in the example. However, C compilers are not required to diagnose this particular error because doing so can be impossible at compile-time: consider (*p)++ + (*q)++), which is OK as long as p != q. – Bower