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copied!<p>Two possible reasons:</p> <ol> <li><p><strong>Assign & Check</strong></p> <p>The <code>=</code> operator (when not overriden) normally returns the value that it assigned. This is to allow statements such as <code>a=b=c=3</code>. In the context of your question, it also allows you to do something like this:</p> <pre><code>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; } </code></pre> <p>...which is equivalent to but shorter than:</p> <pre><code>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; } </code></pre> <p>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 <code>=</code> operator is actually a class with a <a href="http://www.devx.com/tips/Tip/12459" rel="nofollow noreferrer">conversion operator</a> specified for a type which can be coerced (implicitly converted) to a bool. In other words, <code>(a=b)</code> will evaulate to <code>true</code> or <code>false</code> if a is of a type which can be coerced to a boolean value.</p> <p>So the following is a similar situation to the above, except the left-hand argument to <code>=</code> is an object and not a bool:</p> <pre><code>#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 </code></pre> <p><em>Note:</em> 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 <code>&lt;</code>'s are supposed to be <'s, and there aren't supposed to be enourmous gaps between each line.</p></li> <li><p><strong>Overridden <code>=</code> operator</strong></p> <p>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 <code>=</code> operation on an object could return a boolean (if that's how the <code>=</code> operator was overridden for that object type).</p> <p>So the following code would perform the <code>=</code> operation on <code>a</code> with <code>b</code> as an argument. Then it would conditionally execute some code depending on the return value of that operation:</p> <pre><code>if (a=b){ //execute some code } </code></pre> <p>Here, <code>a</code> would have to be an object and <code>b</code> would be of the correct type as defined by the overriding of the <code>=</code> operator for objects of <code>a</code>'s type. To learn more about operator overriding, see this wikipedia article which includes C++ examples: <a href="http://en.wikipedia.org/wiki/Operator_overloading" rel="nofollow noreferrer">Wikipedia article on operator overriding</a></p></li> </ol>

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