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    <p>You cannot specify a type restriction in the template type argument. However, generally, you don't have to.</p> <p>If you simply do:</p> <pre><code>template &lt;typename T&gt; addTri( T v1, T v2, T v3 ) { Vertex &amp;v1r = v1; // .... } </code></pre> <p>This will work if the function is instantiated with a derivative of Vertex. It will create an (obscure) error if <code>T &amp;</code> is <em>not</em> convertible to <code>Vertex &amp;</code>.</p> <p>If you don't even care if the types are convertible to <code>Vertex</code> as long as they have the same members, you can even skip the assignment - C++ template arguments essentially work using <a href="http://en.wikipedia.org/wiki/Duck_typing" rel="noreferrer">duck typing</a>; if you do <code>v1.x</code>, and <code>T</code> contains a member named <code>x</code>, then it will work, whatever type <code>T</code> might actually be.</p> <p>You can be a bit more sophisticated using boost's <a href="http://www.boost.org/doc/libs/1_47_0/libs/type_traits/doc/html/index.html" rel="noreferrer">type-traits library</a> and a <a href="http://www.boost.org/doc/libs/1_47_0/doc/html/boost_staticassert.html" rel="noreferrer">static assertion</a>; with this, you can start defining an assertion to make the error a bit easier to understand:</p> <pre><code>template &lt;typename T&gt; addTri( T v1, T v2, T v3 ) { BOOST_STATIC_ASSERT_MSG(boost::is_convertible&lt;T&amp;, Vertex&amp;&gt;::value, "Template argument must be a subclass of Vertex"); Vertex &amp;v1r = v1; // .... } </code></pre>
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    1. POCan T in template <typename T> use inheritance?
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    1. COAs a matter of fact, one ***can*** specify a restriction on a template parameter, for example, making use of `boost::enable_if` and `boost::is_base_and_derived`
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      1. USArmen Tsirunyan
    2. CO+1. I'm also a big fan of boost::is_base_of(), which performs a similar check.
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    3. COis_base_of looks at private bases, however, which is not what we want here
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