Operator precedence for “<<” and “++” in VS2008 with optimization

问题

I'm stuck with a weird VS2008 C++ issue, that looks like operator precedence is not respected.

My question is what is the output of this:

int i = 0;  
std::cout << ((i != 0) ? "Not zero " : "zero ") << ++i << std::endl;  

Normally the ++ has precedence over the <<, right? Or is the << considered like a function call giving it a higher precedence than the ++? What is the 100% correct standard answer to this?

To check, I created a new empty project (VS2008 console app), pasted only this code in the main and here are the results:

Debug|Win32: “zero 1”  
Release|Win32: “zero 1”  
Debug|x64: “zero 1”  
Release|x64: “Not zero 1”

Btw, the following example produces the exact same results:

i = 0;  
printf("%s %d\n", ((i != 0) ? "Not zero" : "zero"), ++i);  

And also changing the type of optimization in release has no effect, but disabling optimization outputs “zero 1” like other configurations.

回答1:

This is nothing to do with operator precedence.
You are using << which is syntactic sugar for a function call:

std::cout << ((i != 0) ? "Not zero " : "zero ") << ++i << std::endl; 

// Equivalent too:

operator<<(operator<<(operator<<(std::cout, ((i != 0) ? "Not zero " : "zero ")), ++i), std::endl);

The only rule here is that a parameter must be fully evaluated before the function is called. There are no restrictions on what order the parameters are evaluated or even if their evaluation is interleaved with calls (or even partially evaluated).

Interpretation 1:

1) ((i != 0) ? "Not zero " : "zero "))
2) ++i
3) operator<<(std::cout, (1));
4) operator<<((3), (2));
5) operator<<((4), std::endl);

Interpretation 2:

1) ++i
2) ((i != 0) ? "Not zero " : "zero "))
3) operator<<(std::cout, (2));
4) operator<<((3), (1));
5) operator<<((4), std::endl);

Interpretation 3:

1) ((i != 0) ? "Not zero " : "zero "))
2) operator<<(std::cout, (1));
3) ++i
4) operator<<((2), (3));
5) operator<<((4), std::endl);

Looking at interpretation 1 as a reference:
The rules that must be applied:

 A) (1) happens before (3)
 B) (2) happens before (4)
 C) (3) happens before (4)
 D) (4) happens before (5)

来源：https://stackoverflow.com/questions/3463261/operator-precedence-for-and-in-vs2008-with-optimization