Post increment operator behavior

Everything I said originally is WRONG! The point in time at which the side-affect is calculated is unspecified. Visual C++ will perform the increment after the call to foo() if test is a local variable, but if test is declared as static or global it will be incremented before the call to foo() and produce different results, although the final value of test will be correct.

The increment should really be done in a separate statement after the call to foo(). Even if the behaviour was specified in the C/C++ standard it would be confusing. You would think that C++ compilers would flag this as a potential error.

Here is a good description of sequence points and unspecified behaviour.

<----START OF WRONG WRONG WRONG---->

The "++" bit of "test++" gets executed after the call to foo. So you pass in (0,0) to foo, not (1,0)

Here is the assembler output from Visual Studio 2002:

mov ecx, DWORD PTR _i$[ebp]
push    ecx
mov edx, DWORD PTR tv66[ebp]
push    edx
call    _foo
add esp, 8
mov eax, DWORD PTR _i$[ebp]
add eax, 1
mov DWORD PTR _i$[ebp], eax

The increment is done AFTER the call to foo(). While this behavior is by design, it is certainly confusing to the casual reader and should probably be avoided. The increment should really be done in a separate statement after the call to foo()

<----END OF WRONG WRONG WRONG ---->

It's "unspecified behavior", but in practice with the way the C call stack is specified it almost always guarantees that you will see it as 0, 0 and never 1, 0.

As someone noted, the assembler output by VC pushes the right most parameter on the stack first. This is how C function calls are implemented in assembler. This is to accommodate C's "endless parameter list" feature. By pushing parameters in a right-to-left order, the first parameter is guaranteed to be the top item on the stack.

Take printf's signature:

int printf(const char *format, ...);

Those ellipses denote an unknown number of parameters. If parameters were pushed left-to-right, the format would be at the bottom of a stack of which we don't know the size.

Knowing that in C (and C++) that parameters are processed left-to-right, we can determine the simplest way of parsing and interpreting a function call. Get to the end of the parameter list, and start pushing, evaluating any complex statements as you go.

However, even this can't save you as most C compilers have an option to parse functions "Pascal style". And all this means is that the function parameters are pushed on the stack in a left-to-right fashion. If, for instance, printf was compiled with the Pascal option, then the output would most likely be 1, 0 (however, since printf uses the ellipse, I don't think it can be compiled Pascal style).