In which case is if(a=b) a good idea?

Question

Possible Duplicate:
Inadvertent use of = instead of ==

C++ compilers let you know via warnings that you wrote,

if( a = b ) { //...

And that it might be a mistake that you certainly wanted to write:

if( a == b ) { //...

But is there a case where the warning should be ignored, because it's a good way to use this "feature"? I don't see any code clarity reason possible, so is there a case where it’s useful?

Answer 1

Two possible reasons:

1. Assign & Check

   The = operator (when not overriden) normally returns the value that it assigned. This is to allow statements such as a=b=c=3. In the context of your question, it also allows you to do something like this:

   bool global;//a global variable
   
   //a function
   int foo(bool x){
   
      //assign the value of x to global
      //if x is equal to true, return 4
      if (global=x)
          return 4;
   
      //otherwise return 3
      return 3;
   }
   

   ...which is equivalent to but shorter than:

   bool global;//a global variable
   
   //a function
   int foo(bool x){
   
      //assign the value of x to global
      global=x;
   
      //if x is equal to true, return 4
      if (global==true)
          return 4;
   
      //otherwise return 3
      return 3;
   }
   

   Also, it should be noted (as stated by Billy ONeal in a comment below) that this can also work when the left-hand argument of the = operator is actually a class with a conversion operator specified for a type which can be coerced (implicitly converted) to a bool. In other words, (a=b) will evaulate to true or false if a is of a type which can be coerced to a boolean value.

   So the following is a similar situation to the above, except the left-hand argument to = is an object and not a bool:

   #include <iostream>
   using namespace std;
   
   class Foo {
   public:
       operator bool (){ return true; }
       Foo(){}
   };
   
   int main(){
       Foo a;
       Foo b;
   
       if (a=b)
           cout<<"true";
       else
           cout<<"false";
   }
   
   //output: true 
   

   Note: At the time of this writing, the code formatting above is bugged. My code (check the source) actually features proper indenting, shift operators and line spacing. The <'s are supposed to be <'s, and there aren't supposed to be enourmous gaps between each line.

2. Overridden = operator

   Since C++ allows the overriding of operators, sometimes = will be overriden to do something other than what it does with primitive types. In these cases, the performing the = operation on an object could return a boolean (if that's how the = operator was overridden for that object type).

   So the following code would perform the = operation on a with b as an argument. Then it would conditionally execute some code depending on the return value of that operation:

   if (a=b){
      //execute some code
   }
   

   Here, a would have to be an object and b would be of the correct type as defined by the overriding of the = operator for objects of a's type. To learn more about operator overriding, see this wikipedia article which includes C++ examples: Wikipedia article on operator overriding

Answer 2

while ( (line = readNextLine()) != EOF) {
    processLine();
}

Answer 3

You could use to test if a function returned any error:

if (error_no = some_function(...)) {
    // Handle error
}

Assuming that some_function returns the error code in case of an error. Or zero otherwise.

Answer 4

This is a consequence of basic feature of the C language:

The value of an assignment operation is the assigned value itself.

The fact that you can use that "return value" as the condition of an if() statement is incidental.

By the way, this is the same trick that allows this crazy conciseness:

void strcpy(char *s, char *t)
{
    while( *s++ = *t++ );
}

Of course, the while exits when the nullchar in t is reached, but at the same time it is copied to the destination s string.

Whether it is a good idea, usually not, as it reduce code readability and is prone to errors.

Answer 5

Although the construct is perfectly legal syntax and your intent may truly be as shown below, don't leave the "!= 0" part out.

if( (a = b) != 0 ) {
   ...
}

The person looking at the code 6 months, 1 year, 5 years from now, at first glance, is simply going to believe the code contains a "classic bug" written by a junior programmer and will try to "fix" it. The construct above clearly indicates your intent and will be optimized out by the compiler. This would be especially embarrassing if you are that person.

Your other option is to heavily load it with comments. But the above is self-documenting code, which is better.

Lastly, my preference is to do this:

a = b;
if( a != 0 ) {
   ...
}

This is about a clear as the code can get. If there is a performance hit, it is virtually zero.

Answer 6

I know that with this syntax you can avoid putting an extra line in your code, but I think it takes away some readability from the code.

This syntax is very useful for things like the one suggested by Steven Schlansker, but using it directly as a condition isn't a good idea.

Answer 7

This isn't actually a deliberate feature of C, but a consequence of two other features:

Assignment returns the assigned value

This is useful for performing multiple assignments, like a = b = 0, or loops like while ((n = getchar()) != EOF).

Numbers and pointers have truth values

C originally didn't have a bool type until the 1999 standard, so it used int to represent Boolean values. Backwards compatibility requires C and C++ to allow non-bool expressions in if, while, and for.

So, if a = b has a value and if is lenient about what values it accepts, then if (a = b) works. But I'd recommend using if ((a = b) != 0) instead to discourage anyone from "fixing" it.

Answer 8

A common example where it is useful might be:

do {
 ...
} while (current = current->next);

Answer 9

You should explicitly write the checking statement in a better coding manner, avoiding the assign & check approach. Example:

if ((fp = fopen("filename.txt", "wt")) != NULL) {
    // Do something with fp
}

Answer 10

while( (l = getline()) != EOF){
        printf("%s\n", l);
}

This is of course the simplest example, and there are lots of times when this is useful. The primary thing to remember is that (a = true) returns true, just as (a = false) returns false.

Answer 11

void some( int b ) {
    int a = 0;
    if(  a = b ) {
       // or do something with a
       // knowing that is not 0
    } 
    // b remains the same 
 }

Answer 12

But is there a case where the warning should be ignored because it's a good way to use this "feature"? I don't see any code clarity reason possible so is there a case where its useful?

The warning can be suppressed by placing an extra parentheses around the assignment. That sort of clarifies the programmer's intent. Common cases I've seen that would match the (a = b) case directly would be something like:

if ( (a = expression_with_zero_for_failure) )
{
    // do something with 'a' to avoid having to reevaluate
    // 'expression_with_zero_for_failure' (might be a function call, e.g.)
}
else if ( (a = expression2_with_zero_for_failure) )
{
    // do something with 'a' to avoid having to reevaluate
    // 'expression2_with_zero_for_failure'
}
// etc.

As to whether writing this kind of code is useful enough to justify the common mistakes that beginners (and sometimes even professionals in their worst moments) encounter when using C++, it's difficult to say. It's a legacy inherited from C and Stroustrup and others contributing to the design of C++ might have gone a completely different, safer route had they not tried to make C++ backwards compatible with C as much as possible.

Personally I think it's not worth it. I work in a team and I've encountered this bug several times before. I would have been in favor of disallowing it (requiring parentheses or some other explicit syntax at least or else it's considered a build error) in exchange for lifting the burden of ever encountering these bugs.

Answer 13

Preamble

Note that this answer is about C++ (I started writing this answer before the tag "C" was added).

Still, after reading Jens Gustedt's comment, I realized it was not the first time I wrote this kind of answer. Truth is, this question is a duplicate of another, to which I gave the following answer:

Inadvertent use of = instead of ==

So, I'll shamelessly quote myself here to add an important information: if is not about comparison. It's about evaluation.

This difference is very important, because it means anything can be inside the parentheses of a if as long as it can be evaluated to a Boolean. And this is a good thing.

Now, limiting the language by forbidding =, where all other operators are authorized, is a dangerous exception for the language, an exception whose use would be far from certain, and whose drawbacks would be numerous indeed.

For those who are uneasy with the = typo, then there are solutions (see Alternatives below...).

About the valid uses of if(i = 0) [Quoted from myself]

The problem is that you're taking the problem upside down. The "if" notation is not about comparing two values like in some other languages.

The C/C++ if instruction waits for any expression that will evaluate to either a Boolean, or a null/non-null value. This expression can include two values comparison, and/or can be much more complex.

For example, you can have:

if(i >> 3)
{
   std::cout << "i is less than 8" << std::endl
}

Which proves that, in C/C++, the if expression is not limited to == and =. Anything will do, as long as it can be evaluated as true or false (C++), or zero non-zero (C/C++).

About valid uses

Back to the non-quoted answer.

The following notation:

if(MyObject * p = findMyObject())
{
   // uses p
}

enables the user to declare and then use p inside the if. It is a syntactic sugar... But an interesting one. For example, imagine the case of an XML DOM-like object whose type is unknown well until runtime, and you need to use RTTI:

void foo(Node * p_p)
{
    if(BodyNode * p = dynamic_cast<BodyNode *>(p_p))
    {
        // this is a <body> node
    }
    else if(SpanNode * p = dynamic_cast<SpanNode *>(p_p))
    {
        // this is a <span> node
    }
    else if(DivNode * p = dynamic_cast<DivNode *>(p_p))
    {
        // this is a <div> node
    }
    // etc.
}

RTTI should not be abused, of course, but this is but one example of this syntactic sugar.

Another use would be to use what is called C++ variable injection. In Java, there is this cool keyword:

synchronized(p)
{
   // Now, the Java code is synchronized using p as a mutex
}

In C++, you can do it, too. I don't have the exact code in mind (nor the exact Dr. Dobb's Journal's article where I discovered it), but this simple define should be enough for demonstration purposes:

#define synchronized(lock) \
   if (auto_lock lock_##__LINE__(lock))

synchronized(p)
{
   // Now, the C++ code is synchronized using p as a mutex
}

(Note that this macro is quite primitive, and should not be used as is in production code. The real macro uses a if and a for. See sources below for a more correct implementation).

This is the same way, mixing injection with if and for declaration, you can declare a primitive foreach macro (if you want an industrial-strength foreach, use Boost's).

About your typo problem

Your problem is a typo, and there are multiple ways to limit its frequency in your code. The most important one is to make sure the left-hand-side operand is constant.

For example, this code won't compile for multiple reasons:

if( NULL = b ) // won't compile because it is illegal
               // to assign a value to r-values.

Or even better:

const T a ;

// etc.

if( a = b ) // Won't compile because it is illegal
            // to modify a constant object

This is why in my code, const is one of the most used keyword you'll find. Unless I really want to modify a variable, it is declared const and thus, the compiler protects me from most errors, including the typo error that motivated you to write this question.

But is there a case where the warning should be ignored because it's a good way to use this "feature"? I don't see any code clarity reason possible so is there a case where its useful?

Conclusion

As shown in the examples above, there are multiple valid uses for the feature you used in your question.

My own code is a magnitude cleaner and clearer since I use the code injection enabled by this feature:

void foo()
{
    // some code

    LOCK(mutex)
    {
       // some code protected by a mutex
    }

    FOREACH(char c, MyVectorOfChar)
    {
       // using 'c'
    }
}

... which makes the rare times I was confronted to this typo a negligible price to pay (and I can't remember the last time I wrote this type without being caught by the compiler).

Interesting sources

I finally found the articles I've had read on variable injection. Here we go!!!

• FOR_EACH and LOCK (2003-11-01)
• Exception Safety Analysis (2003-12-01)
• Concurrent Access Control & C++ (2004-01-01)

Alternatives

If one fears being victim of the =/== typo, then perhaps using a macro could help:

#define EQUALS ==
#define ARE_EQUALS(lhs,rhs) (lhs == rhs)

int main(int argc, char* argv[])
{
   int a = 25 ;
   double b = 25 ;

   if(a EQUALS b)
      std::cout << "equals" << std::endl ;
   else
      std::cout << "NOT equals" << std::endl ;

   if(ARE_EQUALS(a, b))
      std::cout << "equals" << std::endl ;
   else
      std::cout << "NOT equals" << std::endl ;

   return 0 ;
}

This way, one can protect oneself from the typo error, without needing a language limitation (that would cripple language), for a bug that happens rarely (i.e., almost never, as far as I remember it in my code).

Answer 14

Never!

The exceptions cited don't generate the compiler warning. In cases where the compiler generates the warning, it is never a good idea.

Answer 15

There's an aspect of this that hasn't been mentioned: C doesn't prevent you from doing anything it doesn't have to. It doesn't prevent you from doing it because C's job is to give you enough rope to hang yourself by. To not think that it's smarter than you. And it's good at it.

Answer 16

RegEx sample

RegEx r;

if(((r = new RegEx("\w*)).IsMatch()) {
   // ... do something here
}
else if((r = new RegEx("\d*")).IsMatch()) {
   // ... do something here
}

Assign a value test

int i = 0;
if((i = 1) == 1) {
   // 1 is equal to i that was assigned to a int value 1
}
else {
   // ?
}

Answer 17

My favourite is:

if (CComQIPtr<DerivedClassA> a = BaseClassPtr)
{
...
}
else if (CComQIPtr<DerivedClassB> b = BaseClassPtr)
{
...
}