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copied!<h2>Preamble</h2> <p>Note that this answer is about C++ (I started writing this answer before the tag "C" was added).</p> <p>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:</p> <p><a href="https://stackoverflow.com/questions/399792/inadvertent-use-of-instead-of/400644#400644"><a href="https://stackoverflow.com/questions/399792/inadvertent-use-of-instead-of/400644#400644">Inadvertent use of = instead of ==</a></a></p> <p>So, I'll shamelessly quote myself here to add an important info: <b><code>if</code> is not about comparison. It's about evaluation.</b></p> <p>This difference is very important because it means anything can be inside the parentheses of a <code>if</code> as long as it can be evaluated to a boolean. <b>And this is a good thing.</b></p> <p>Now, limiting the language by forbidding <code>=</code>, 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.</p> <p>For those who are unneasy with the <code>=</code> typo, then there are solutions (see <b>Alternatives</b> below...).</p> <h2><a href="https://stackoverflow.com/questions/399792/inadvertent-use-of-instead-of/400644#400644">About the valid uses of if(i = 0) [Quoted from myself]</a></h2> <p>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.</p> <p>The C/C++ <code>if</code> 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.</p> <p>For example, you can have:</p> <pre><code>if(i >> 3) { std::cout << "i is less than 8" << std::endl } </code></pre> <p>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++).</p> <h2>About valid uses</h2> <p>Back to the non-quoted answer.</p> <p>The following notation:</p> <pre><code>if(MyObject * p = findMyObject()) { // uses p } </code></pre> <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:</p> <pre><code>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. } </code></pre> <p>RTTI should not be abused, of course, but this is but one example of this syntactic sugar.</p> <p>Another use would be to use what is called C++ Variable Injection. In Java, there is this cool keyword:</p> <pre><code>synchronized(p) { // Now, the Java code is synchronized using p as a mutex } </code></pre> <p>In C++, you can do it, too. I don't have the exact code in mind (nor the exact DDJ's article where I discovered it), but this simple define should be enough for demonstration purposes:</p> <pre><code>#define synchronized(lock) \ if (auto_lock lock_##__LINE__(lock)) synchronized(p) { // Now, the C++ code is synchronized using p as a mutex } </code></pre> <p>(Note that this macro is quite primitive, and should not be used as is in production code. The real macro uses a <code>if</code> and a <code>for</code>. See <b>sources</b> below for a more correct implementation).</p> <p>This is the same way, mixing injection with <code>if</code> and <code>for</code> declaration, you can declare a primitive foreach macro (if you want an industrial-strength foreach, use boost's).</p> <h2>About your typo problem</h2> <p>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.</p> <p>For example, this code won't compile for multiple reasons:</p> <pre><code>if( NULL = b ) // won't compile because it is illegal // to assign a value to r-values. </code></pre> <p>Or even better:</p> <pre><code>const T a ; // etc. if( a = b ) // Won't compile because it is illegal // to modify a constant object </code></pre> <p>This is why in my code, <code>const</code> is one of the most used keyword you'll find. Unless I REALLY want to modify a variable, it is declared <code>const</code> and thus, the compiler protects me from most errors, including the typo error that motivated you to write this question.</p> <p>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?</p> <h2>Conclusion</h2> <p>As shown in the examples above, there are multiple valid uses for the feature you used in your question.</p> <p>My own code is a magnitude cleaner and clearer since I use the code injection enabled by this feature:</p> <pre><code>void foo() { // some code LOCK(mutex) { // some code protected by a mutex } FOREACH(char c, MyVectorOfChar) { // using c } } </code></pre> <p>... 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).</p> <h2>Interesting sources</h2> <p>I finally found the articles I've had read on variable injection. Here we go !!!</p> <ul> <li><a href="http://www.drdobbs.com/184401723?pgno=6" rel="nofollow noreferrer">http://www.drdobbs.com/184401723?pgno=6</a></li> <li><a href="http://www.drdobbs.com/184401728" rel="nofollow noreferrer">http://www.drdobbs.com/184401728</a></li> <li><a href="http://www.drdobbs.com/184401752" rel="nofollow noreferrer">http://www.drdobbs.com/184401752</a></li> </ul> <h2>Alternatives</h2> <p>If one fears being victim of the <code>=</code>/<code>==</code> typo, then perhaps using a macro could help:</p> <pre><code>#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 ; } </code></pre> <p>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)</p>

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