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    <pre><code>template &lt;int number1&gt; typename boost::enable_if_c&lt; (number1 &gt;= 10) &gt;::type reportErrorIfLessThan10() { // ... } </code></pre> <p>The above <code>enable_if</code>, without the _c because we have a plain bool, looks like this:</p> <pre><code>template&lt;bool C, typename T = void&gt; struct enable_if { typedef T type; }; template&lt;typename T&gt; struct enable_if&lt;false, T&gt; { }; </code></pre> <p><a href="http://en.wikipedia.org/wiki/Boost_C%2B%2B_Libraries" rel="nofollow noreferrer">Boost</a>'s <code>enable_if</code> takes not a plain bool, so they have another version which has a _c appended, that takes plain bools. You won't be able to call it for <code>number1</code> &lt; 10. <em>SFINAE</em> will exclude that template as possible candidates, because <code>enable_if</code> will not expose a type <code>::type</code> if the condition evaluates to <code>false</code>. If you want, for some reason, test it in the function, then if you have the <a href="http://en.wikipedia.org/wiki/C%2B%2B0x" rel="nofollow noreferrer">C++1x</a> feature available, you can use <code>static_assert</code>:</p> <pre><code>template &lt;int number1&gt; void reportErrorIfLessThan10() { static_assert(number &gt;= 10, "number must be &gt;= 10"); } </code></pre> <p>If not, you can use BOOST_STATIC_ASSERT:</p> <pre><code>template &lt;int number1&gt; void reportErrorIfLessThan10() { BOOST_STATIC_ASSERT(number &gt;= 10); } </code></pre> <p>The only way to display a descriptive message is using static_assert, though. You can more or less simulate that, using types having names that describe the error condition:</p> <pre><code>namespace detail { /* chooses type A if cond == true, chooses type B if cond == false */ template &lt;bool cond, typename A, typename B&gt; struct Condition { typedef A type; }; template &lt;typename A, typename B&gt; struct Condition&lt;false, A, B&gt; { typedef B type; }; struct number1_greater_than_10; } template &lt;int number1&gt; void reportErrorIfLessThan10() { // number1 must be greater than 10 sizeof( typename detail::Condition&lt; (number1 &gt;= 10), char, detail::number1_greater_than_10 &gt;::type ); } </code></pre> <p>It prints this here:</p> <blockquote> <p>error: invalid application of 'sizeof' to incomplete type 'detail::number1_greater_than_10' </p> </blockquote> <p>But I think the very first approach, using <code>enable_if</code> will do it. You will get an error message about an undeclared <code>reportErrorIfLessThan10</code>.</p>
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