Thread: Assignment of variables in if conditions

  1. #1
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    Assignment of variables in if conditions

    Which code snippet is more effective.

    case 1:

    Code:
     char *str;
     str = (char *) malloc(15);
     if(!str)
      return -ENOMEM;
    cases 2:
    Code:
    char *str;
    if(!(str = (char *) malloc(15)))
      return -ENOMEM
    does it effect execution time?

  2. #2
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    If so, it's negligible. I'd suggest making the decision based on your style preferences. Two things worth noting about these snippets:

    * The cast may be omitted, as void *s may be converted to other object pointer types by the = operator in C.
    * It's totally fine to use ! or ==0 to check whether str stores a null pointer, but some prefer using ==NULL in this scenario.

  3. #3
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    No, no difference at all when tested on MSVC compiler at max optimisation.

    Here's the disassembly of your first snippet:
    Code:
    char* test1(void) {
    push        ebp  
    mov         ebp,esp  
    sub         esp,0CCh  
    push        ebx  
    push        esi  
    push        edi  
    lea         edi,[ebp-0CCh]  
    mov         ecx,33h  
    mov         eax,0CCCCCCCCh  
    rep stos    dword ptr es:[edi]  
    	char *str;
    	str = (char *)malloc(15);
    mov         esi,esp  
    push        0Fh  
    call        dword ptr ds:[0A89114h]  
    add         esp,4  
    cmp         esi,esp  
    call        __RTC_CheckEsp (0A8113Bh)  
    mov         dword ptr [str],eax  
    	if (!str)
    cmp         dword ptr [str],0  
    jne         test1+3Eh (0A8140Eh)  
    	return str;
    mov         eax,dword ptr [str]  
    }
    pop         edi  
    pop         esi  
    pop         ebx  
    add         esp,0CCh  
    cmp         ebp,esp  
    call        __RTC_CheckEsp (0A8113Bh)  
    mov         esp,ebp  
    pop         ebp  
    ret
    And your second:
    Code:
    char* test2(void) {
    push        ebp  
    mov         ebp,esp  
    sub         esp,0CCh  
    push        ebx  
    push        esi  
    push        edi  
    lea         edi,[ebp-0CCh]  
    mov         ecx,33h  
    mov         eax,0CCCCCCCCh  
    rep stos    dword ptr es:[edi]  
    	char *str;
    	if (!(str = (char *)malloc(15)))
    mov         esi,esp  
    push        0Fh  
    call        dword ptr ds:[0A89114h]  
    add         esp,4  
    cmp         esi,esp  
    call        __RTC_CheckEsp (0A8113Bh)  
    mov         dword ptr [str],eax  
    cmp         dword ptr [str],0  
    jne         test2+3Eh (0A8147Eh)  
    		return str;
    mov         eax,dword ptr [str]  
    }
    pop         edi  
    pop         esi  
    pop         ebx  
    add         esp,0CCh  
    cmp         ebp,esp  
    call        __RTC_CheckEsp (0A8113Bh)  
    mov         esp,ebp  
    pop         ebp  
    ret
    The first is only 1 line longer because of the interleaved source code. Of course, I can't speak for all compilers/instruction sets, but I don't see how a compiler could mess your first example up. And as zyxwvuts said, if there is any difference on any platform it'd be negligible.


    On the subject of ! or 0 or NULL.... MISRA insists that you use NULL. In fact, you're meant to use != 0 for testing variables in conditionals, lest some poor soul be confusied..... it's an insane piece of literature. Made me laugh out loud at some points.

    I personally find your first snippet clearer. And I have a preference for NULL over 0.

  4. #4
    C++ Witch laserlight's Avatar
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    Yes, in terms of efficiency they will almost certainly be compiled to the same code. Like smokeyangel, I find the former to be more readable. As zyxwvuts noted, you should get rid of the cast of the return value of malloc (unless you are specially writing your code to be compilable as C++ too).
    Quote Originally Posted by Bjarne Stroustrup (2000-10-14)
    I get maybe two dozen requests for help with some sort of programming or design problem every day. Most have more sense than to send me hundreds of lines of code. If they do, I ask them to find the smallest example that exhibits the problem and send me that. Mostly, they then find the error themselves. "Finding the smallest program that demonstrates the error" is a powerful debugging tool.
    Look up a C++ Reference and learn How To Ask Questions The Smart Way

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