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    <p><code>#define</code> is used to create macros in C and in C++. You can read more about it in the <a href="http://gcc.gnu.org/onlinedocs/gcc-2.95.3/cpp_1.html#SEC10" rel="noreferrer">C preprocessor documentation</a>. The quick answer is that it does a few things:</p> <ol> <li><p>Simple Macros - basically just text replacement. Compile time constants are a good example:</p> <pre><code>#define SOME_CONSTANT 12 </code></pre> <p>simply replaces the text <code>SOME_CONSTANT</code> with <code>12</code> wherever it appears in your code. This sort of macro is often used to provide conditional compilation of code blocks. For example, there might be a header included by each source file in a project with a list of options for the project:</p> <pre><code>#define OPTION_1 #define OPTION_2 #undef OPTION_3 </code></pre> <p>And then code blocks in the project would be wrapped with matching <code>#ifdef</code>/<code>#endif#</code> blocks to enable and disable those options in the finished project. Using the <code>-D</code> gcc flag would provide similar behaviour. There are strong opinions as to whether or not this method is really a good way to provide configuration for an application, however.</p></li> <li><p>Macros with arguments - allows you to make 'function-like' macros that can take arguments and manipulate them. For example:</p> <pre><code>#define SQUARE(x) ((x) * (x)) </code></pre> <p>would return the square of the argument as its result; be careful about potential order-of-operations or side-effect problems! The following example:</p> <pre><code>int x = SQUARE(3); // becomes int x = ((3) * (3)); </code></pre> <p>will works fine, but something like:</p> <pre><code>int y = SQUARE(f()); // becomes int y = ((f()) * (f())); </code></pre> <p>will call <code>f()</code> twice, or even worse:</p> <pre><code>int z = SQUARE(x++); // becomes int z = ((x++) * (x++)); </code></pre> <p>results in undefined behaviour!</p> <p>With some tools, macros with arguments can also be <a href="http://gcc.gnu.org/onlinedocs/cpp/Variadic-Macros.html" rel="noreferrer">variadic</a>, which can come in handy.</p></li> </ol> <p>As mentioned below in the comments, overuse of macros, or the development of overly complicated or confusing macros is considered bad style by many - as always, put the readability, maintainability, and debuggability of your code above 'clever' technical tricks.</p>
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    1. POWhat is the purpose of the #define directive in C++?
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    1. USCarl Norum
    1. USCarl Norum
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    1. CO+1. And since the OP asked about the "role" that `#define` plays, it should be emphasized that macros shouldn't be over-used.
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    2. COVariadic macros are standard in C99, so most modern tools should support them.
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    3. CO+1, but should add an example of how SQUARE() will be *expanded* in a few situations.
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