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POHow do I make a C++ macro behave like a function?
 

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  1. POHow do I make a C++ macro behave like a function?
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    <p>Let's say that for some reason you need to write a macro: <code>MACRO(X,Y)</code>. <strong>(Let's assume there's a good reason you can't use an inline function.)</strong> You want this macro to emulate a call to a function with no return value.</p> <hr> <h3>Example 1: This should work as expected.</h3> <pre><code>if (x &gt; y) MACRO(x, y); do_something(); </code></pre> <h3>Example 2: This should not result in a compiler error.</h3> <pre><code>if (x &gt; y) MACRO(x, y); else MACRO(y - x, x - y); </code></pre> <h3>Example 3: This should <em>not</em> compile.</h3> <pre><code>do_something(); MACRO(x, y) do_something(); </code></pre> <hr> <p>The naïve way to write the macro is like this:</p> <pre><code>#define MACRO(X,Y) \ cout &lt;&lt; "1st arg is:" &lt;&lt; (X) &lt;&lt; endl; \ cout &lt;&lt; "2nd arg is:" &lt;&lt; (Y) &lt;&lt; endl; \ cout &lt;&lt; "Sum is:" &lt;&lt; ((X)+(Y)) &lt;&lt; endl; </code></pre> <p>This is a very bad solution which fails all three examples, and I shouldn't need to explain why.</p> <p><strong>Ignore what the macro actually does, that's not the point.</strong></p> <hr> <p>Now, the way I most often see macros written is to enclose them in curly braces, like this:</p> <pre><code>#define MACRO(X,Y) \ { \ cout &lt;&lt; "1st arg is:" &lt;&lt; (X) &lt;&lt; endl; \ cout &lt;&lt; "2nd arg is:" &lt;&lt; (Y) &lt;&lt; endl; \ cout &lt;&lt; "Sum is:" &lt;&lt; ((X)+(Y)) &lt;&lt; endl; \ } </code></pre> <p>This solves example 1, because the macro is in one statement block. But example 2 is broken because we put a semicolon after the call to the macro. This makes the compiler think the semicolon is a statement by itself, which means the else statement doesn't correspond to any if statement! And lastly, example 3 compiles OK, even though there is no semicolon, because a code block doesn't need a semicolon.</p> <hr> <p>Is there a way to write a macro so that it pass all three examples?</p> <hr> <p><i>Note: I am submitting my own answer as part of the <a href="https://stackoverflow.com/questions/162246/is-stack-overflow-appropriate-for-sharing-knowledge-tips-tricks-and-interesting">accepted way of sharing a tip</a>, but if anyone has a better solution feel free to post it here, it may get more votes than my method. :)</i></p>
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    1. COSee, also: [Why are there sometimes meaningless do/while and if/else statements in C/C++ macros?](http://stackoverflow.com/questions/154136/why-are-there-sometimes-meaningless-dowhile-and-ifelse-statements-in-cc-macros)
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    2. COMacros are __not__ type safe. Which is why you should instead opt for inline functions, which are.
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    3. CO@b1naryatr0phy I think the messiness of macros is more the issue, [why not macros](http://www.iso-9899.info/wiki/Why_not_macros).
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