[c++]code executon

Hello everybody, my first post on this forum.

I am starting to learning c++ language, i got myself a msv c++ 2010 express edition, downloaded/bookmarked lots of tutorials here on the internet so i have some question/s that might sound stupid to you that i want to ask, anyway, here it goes:
consider following code (pseudo)

Code:

---

if(expression1)
{
    if(expression2)
    {
        //some complicated task
    }
}

---

Does second if statement got evaluated if first is false?

I start debugging and looked into disassembly (Debug/Windows/Disassembly), compare two chars, first if statement:
[code]
01071548 movsx eax,byte ptr [a]
0107154C movsx ecx,byte ptr [b]
01071550 cmp eax,ecx
01071552 jne main+8Bh (107158Bh) <-- about this
[code]
I am not familiar with assembly, i want to ask about last instruction what does it do?

no, the second if statement is not evaluated

It depends on how the compiler handles it, taking optimizations into account.

...the first if statement is what executes the second

Quote:

---

Originally Posted by iiSoMeGuY 7x

no, the second if statement is not evaluated

---

It could be.

e.g.

Code:

---

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

---

could be better assembled as:

Code:

---

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

---

Let say that in second if stement, right operand (literal constant in your case) is given by
executing complicated function, something like:

Code:

---

if( blah )
{
    if( var2 == HeavyFunction() )
    {
        //some complicated task
    }
}

---

Quote:

---

Originally Posted by borko1980

Let say that in second if stement, right operand (literal constant in your case) is given by
executing complicated function, something like:

Code:

---

if( blah )
{
    if( var2 == HeavyFunction() )
    {
        //some complicated task
    }
}

---

---

Probably not. I just wanted to point out that it was conceivably possible for the compiler to optimize things away. The best way to tell is to actually look at the compiled code.

Quote:

---

...look at the compiled code.

---

In case of my compiler, where?

Quote:

---

Originally Posted by borko1980

In case of my compiler, where?

---

In a debugger or disassembler.

The whole point of my question is that i want to avoid evaluation of second if statement if first is false, because second is very expensive.By some human/normal logic "iiSoMeGuY 7x" should be right but as you said it depends on the compiler, so now i don't know if this is good coding practice to continue with?
How/where (if possible) to see assembly of compiled code?

Well, you should be able to trust your compiler to do the right thing. It wouldn't be an optimization if the compiler chose to evaluate a lengthy boolean that wasn't necessary.

OK, here is quick sample from msv:
C++

Code:

---

#include <iostream>

int main()
{
        const char a = 'a';
        char b, c;
        std::cin >> b;
        std::cin >> c;

        if(a == b)
        {
                std::cout << "a = b" << std::endl;
                if(b == c)
                {
                        std::cout << "b = c" << std::endl;
                }
        }

        std::cin.get();
        return 1;
}

---

disassembly, i putted breakpoint into std::cin.get() line and input "z" as char so entire if statements should be avoided

Code:

---

const char a = 'a';
00F6151E  mov        byte ptr [a],61h 
        char b, c;
        std::cin >> b;
00F61522  lea        eax,[b] 
00F61525  push        eax 
00F61526  mov        ecx,dword ptr [__imp_std::cin (0F6A32Ch)] 
00F6152C  push        ecx 
00F6152D  call        std::operator>><char,std::char_traits<char> > (0F61055h) 
00F61532  add        esp,8 
        std::cin >> c;
00F61535  lea        eax,[c] 
00F61538  push        eax 
00F61539  mov        ecx,dword ptr [__imp_std::cin (0F6A32Ch)] 
00F6153F  push        ecx 
00F61540  call        std::operator>><char,std::char_traits<char> > (0F61055h) 
00F61545  add        esp,8 

        if(a == b)
00F61548  movsx      eax,byte ptr [b] 
00F6154C  cmp        eax,61h 
00F6154F  jne        main+0B3h (0F615B3h) 
        {
                std::cout << "a = b" << std::endl;
00F61551  mov        esi,esp 
00F61553  mov        eax,dword ptr [__imp_std::endl (0F6A330h)] 
00F61558  push        eax 
00F61559  push        offset string "a = b" (0F67898h) 
00F6155E  mov        ecx,dword ptr [__imp_std::cout (0F6A334h)] 
00F61564  push        ecx 
00F61565  call        std::operator<<<std::char_traits<char> > (0F61168h) 
00F6156A  add        esp,8 
00F6156D  mov        ecx,eax 
00F6156F  call        dword ptr [__imp_std::basic_ostream<char,std::char_traits<char> >::operator<< (0F6A338h)] 
00F61575  cmp        esi,esp 
00F61577  call        @ILT+435(__RTC_CheckEsp) (0F611B8h) 
                if(b == c)
00F6157C  movsx      eax,byte ptr [b] 
00F61580  movsx      ecx,byte ptr [c] 
00F61584  cmp        eax,ecx 
00F61586  jne        main+0B3h (0F615B3h) 
                {
                        std::cout << "b = c" << std::endl;
00F61588  mov        esi,esp 
00F6158A  mov        eax,dword ptr [__imp_std::endl (0F6A330h)] 
00F6158F  push        eax 
00F61590  push        offset string "b = c" (0F67830h) 
00F61595  mov        ecx,dword ptr [__imp_std::cout (0F6A334h)] 
00F6159B  push        ecx 
00F6159C  call        std::operator<<<std::char_traits<char> > (0F61168h) 
00F615A1  add        esp,8 
00F615A4  mov        ecx,eax 
00F615A6  call        dword ptr [__imp_std::basic_ostream<char,std::char_traits<char> >::operator<< (0F6A338h)] 
00F615AC  cmp        esi,esp 
00F615AE  call        @ILT+435(__RTC_CheckEsp) (0F611B8h) 
                }
        }

---

As i read this it only shows what each expression in c++ does in assembly but not what is actually executed, does above means that its executing second if statement?

No it doesn't. The compiler can't know what b or c will be at the time it compiles, so it behaves exactly as you would expect using the same logic as iiSoMeGuY 7x.

OK. Thanks greatly on your help.

Quote:

---

Originally Posted by borko1980

Does second if statement got evaluated if first is false?

---

No, and it also doesn't get executed when you do

Code:

---

if (statement1 && statement2) {
    // do something
}

---

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

Quote:

---

Originally Posted by borko1980

I am not familiar with assembly, i want to ask about last instruction what does it do?

---

There's no need to learn assembly in order to learn C++. The assembly outputted by your compiler can differ according to the compiler, compiler version and compiler settings used to compile the code. What shouldn't differ is the observable behavior of your program, to the extent that your program is written in standard C++.

What I explained above is guaranteed by standard C++. So we can even give statement2 side-effects and rely on the fact that it will not be executed when statement1 is false.

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.

Quote:

---

Originally Posted by D_Drmmr

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.

Thanks everybody for clearing this up.

Quote:

---

Originally Posted by Chris_F

It could be.

e.g.

Code:

---

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

---

could be better assembled as:

Code:

---

//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:

Code:

---

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.)

Quote:

---

Originally Posted by borko1980

Quote:

---

...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.

Quote:

---

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.

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&#180;re combined by && or ||. This guarantees that the expressions are evaluated from left to right, making sure that code like

Code:

---

if( ptr && ptr->func() )
{
  ...
}

---

does not produce UB when ptr is 0.