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    <p>Compound types can be constructed using the declarator syntax - found in <code>[dcl.decl]</code>.</p> <p>Underlying this syntax are six fundamental constructs, within which any <code>T</code> can be substituted by any of the other constructs in the list. [In the following, <code>(T)</code> represents a list of zero or more types (which may end in '...'), and <code>&lt;T&gt;</code> represents a list of one or more types.]</p> <pre><code>T // T T* // pointer to T T&amp; // reference to T T[n] // array of size 'n' of T T C::* // pointer to C::member of type T T (T) // function taking '(T)' and returning T </code></pre> <p><strong>EDIT</strong>: The type of a class template specialization can be subsituted for any <code>T</code>:</p> <pre><code>C&lt;T&gt; // specialization of class template C with arguments '&lt;T&gt;' </code></pre> <p>There are combinations of the above that produce constructs which have special significance:</p> <pre><code>T (*)(T) // pointer to function taking '(T)' and returning T T (C::*)(T) // pointer to C::member-function taking '(T)' and returning T </code></pre> <p>Additionally, some of the constructs may be cv-qualified:</p> <pre><code>const T // const T T* const // const pointer to T T C::* const // const pointer to C::member of type T </code></pre> <p>Not all of the combinations result in valid types. According to <code>[basic.compound]</code>, only the following combinations can be used:</p> <blockquote> <p>Compound types can be constructed in the following ways:</p> <ul> <li>arrays of objects of a given type</li> <li>functions, which have parameters of given types and return void or references or objects of a given type</li> <li>pointers to void or objects or functions (including static members of classes) of a given type </li> <li>references to objects or functions of a given type</li> <li>pointers to non-static class members, which identify members of a given type within objects of a given class</li> </ul> </blockquote> <p>Additional restrictions are mentioned:</p> <blockquote> <p><code>[dcl.ptr]</code> there are no pointers to references</p> <p><code>[dcl.ref]</code> There shall be no references to references, no arrays of references, and no pointers to references</p> <p><code>[dcl.mptr]</code> A pointer to member shall not point to ... a member with reference type, or "cv void."</p> <p><code>[dcl.fct]</code> The parameter list (void) is equivalent to the empty parameter list. Except for this special case, void shall not be a parameter type. ... If the type of a parameter includes a type of the form “pointer to array of unknown bound of T” or “reference to array of unknown bound of T,” the program is ill-formed. Functions shall not have a return type of type array or function.</p> </blockquote> <p>Some of the possible constructs <strong>cannot</strong> be used as template-arguments. When you explicitly specify a set of template-arguments, the compiler must check that the template-arguments can be substituted for the template-parameters without resulting in an 'invalid type'. According to <code>[temp.deduct]\2</code>, the following constructs constitute invalid types:</p> <blockquote> <p>Type deduction may fail for the following reasons:</p> <ul> <li><p>Attempting to create an array with an element type that is void, a function type, or a reference type, or attempting to create an array with a size that is zero or negative.</p> <pre><code>template &lt;class T&gt; int f(T[5]); int I = f&lt;int&gt;(0); int j = f&lt;void&gt;(0); // invalid array </code></pre></li> <li><p>Attempting to use a type that is not a class type in a qualified name.</p> <pre><code>template &lt;class T&gt; int f(typename T::B*); int i = f&lt;int&gt;(0); </code></pre></li> <li><p>Attempting to use a type in the qualifier portion of a qualified name that names a type when that type does not contain the specified member, or if the specified member is not a type where a type is required.</p> <pre><code>template &lt;class T&gt; int f(typename T::B*); struct A {}; struct C { int B; }; int i = f&lt;A&gt;(0); int j = f&lt;C&gt;(0); </code></pre></li> <li><p>Attempting to create a pointer to reference type.</p></li> <li><p>Attempting to create a reference to a reference type or a reference to void.</p></li> <li><p>Attempting to create "pointer to member of T" when T is not a class type.</p> <pre><code>template &lt;class T&gt; int f(int T::*); int i = f&lt;int&gt;(0); </code></pre></li> <li><p>Attempting to perform an invalid conversion in either a template argument expression, or an expression used in the function declaration.</p> <pre><code>template &lt;class T, T*&gt; int f(int); int i2 = f&lt;int,1&gt;(0); // can’t conv 1 to int* </code></pre></li> <li><p>Attempting to create a function type in which a parameter has a type of void.</p></li> <li>Attempting to create a cv-qualified function type.</li> </ul> </blockquote> <p><strong>EDIT:</strong> This is based on C++03, but also applies to C++11 (which adds rvalue reference types)</p>
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    1. POWhat are the expression syntax over types C++ support?
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    1. COThe others I know about are references like `T&` , `T&&`,`T(&)(A...)` array like `T[]`
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    2. COReferences slipped my mind at first!
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    3. COThe list actually misses templates themselves. That's relevant to the question: `using t8 = tpl<tpl<int>>;`
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