[c++]code executon

[GCDEF]

There wouldn't be much point to an if statement if things inside its block got executed whether it was true or not. This isn't a case of "probably not" or it depends on optimizations, it definitely doesn't. It would probably easier to verify what your code is doing with the debugger than to try to figure out the assembly.

[nuzzle]

If statement1 is false, statement2 will not be executed. Similarly for || (logical OR).

Just to be clear.

In the || case, statement2 will not be executed if statement1 is true.

The short-cut rule states that the right-hand side will not be evaluated if the evaluation of the left-hand side is sufficient to determine the outcome of the whole expression.

[borko1980]

Thanks everybody for clearing this up.

[Eri523]

It could be.

e.g.

if(var1 == 42)
{
    if(var2 == 42)
    {
        do_something()
    }
}

could be better assembled as:

//pseudo 
mov reg, var1
and reg, var2
cmp reg, 42
je _do_something

Definitely not. The two are not even equivalent. Your assembly snippet would take the "true" route if all bits set in both variables give the value of 42 (== 0x2A). That would be equivalent to the following C++ snippet:

if ((var1 & var2) == 42) {
  // ...
}

It also would violate the short circuit evaluation rule as pointed out by D_Drmmr which could be fatal if var2 was replaced by something that has side effects. (But maybe you meant that as an optimization that would not be done it var2 had side effects, so my objection would be less relevant.)

...look at the compiled code.

In case of my compiler, where?

FYI, VC++ has an option to generate an assembly listing of the compiled code for you. You'll find it in the project properties under C/C++ -> Output Files. IIRC the generated listing file will have the file name extension .cod.

[Chris_F]

But maybe you meant that as an optimization that would not be done it var2 had side effects, so my objection would be less relevant.

That's more or less what I meant. I realize it's a bad example. It's more likely that the if statements and variables would simply be removed all together through optimization anyway.

[GNiewerth]

Not sure how other compilers deal with that issue, but MS compilers handle the individual expressions in an if clause as sequence points if they´re combined by && or ||. This guarantees that the expressions are evaluated from left to right, making sure that code like

if( ptr && ptr->func() )
{
   ...
}

does not produce UB when ptr is 0.