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| <title>Using the GNU Compiler Collection (GCC): Name lookup</title> |
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| <a name="Name-lookup_002c-templates_002c-and-accessing-members-of-base-classes"></a> |
| <h4 class="subsection">11.7.2 Name lookup, templates, and accessing members of base classes</h4> |
| |
| <a name="index-base-class-members"></a> |
| <a name="index-two_002dstage-name-lookup"></a> |
| <a name="index-dependent-name-lookup"></a> |
| |
| <p>The C++ standard prescribes that all names that are not dependent on |
| template parameters are bound to their present definitions when parsing |
| a template function or class.<a name="DOCF5" href="#FOOT5"><sup>5</sup></a> Only names that are dependent are looked up at the point |
| of instantiation. For example, consider |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample"> void foo(double); |
| |
| struct A { |
| template <typename T> |
| void f () { |
| foo (1); // <span class="roman">1</span> |
| int i = N; // <span class="roman">2</span> |
| T t; |
| t.bar(); // <span class="roman">3</span> |
| foo (t); // <span class="roman">4</span> |
| } |
| |
| static const int N; |
| }; |
| </pre></div> |
| |
| <p>Here, the names <code>foo</code> and <code>N</code> appear in a context that does |
| not depend on the type of <code>T</code>. The compiler will thus require that |
| they are defined in the context of use in the template, not only before |
| the point of instantiation, and will here use <code>::foo(double)</code> and |
| <code>A::N</code>, respectively. In particular, it will convert the integer |
| value to a <code>double</code> when passing it to <code>::foo(double)</code>. |
| </p> |
| <p>Conversely, <code>bar</code> and the call to <code>foo</code> in the fourth marked |
| line are used in contexts that do depend on the type of <code>T</code>, so |
| they are only looked up at the point of instantiation, and you can |
| provide declarations for them after declaring the template, but before |
| instantiating it. In particular, if you instantiate <code>A::f<int></code>, |
| the last line will call an overloaded <code>::foo(int)</code> if one was |
| provided, even if after the declaration of <code>struct A</code>. |
| </p> |
| <p>This distinction between lookup of dependent and non-dependent names is |
| called two-stage (or dependent) name lookup. G++ implements it |
| since version 3.4. |
| </p> |
| <p>Two-stage name lookup sometimes leads to situations with behavior |
| different from non-template codes. The most common is probably this: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample"> template <typename T> struct Base { |
| int i; |
| }; |
| |
| template <typename T> struct Derived : public Base<T> { |
| int get_i() { return i; } |
| }; |
| </pre></div> |
| |
| <p>In <code>get_i()</code>, <code>i</code> is not used in a dependent context, so the |
| compiler will look for a name declared at the enclosing namespace scope |
| (which is the global scope here). It will not look into the base class, |
| since that is dependent and you may declare specializations of |
| <code>Base</code> even after declaring <code>Derived</code>, so the compiler can’t |
| really know what <code>i</code> would refer to. If there is no global |
| variable <code>i</code>, then you will get an error message. |
| </p> |
| <p>In order to make it clear that you want the member of the base class, |
| you need to defer lookup until instantiation time, at which the base |
| class is known. For this, you need to access <code>i</code> in a dependent |
| context, by either using <code>this->i</code> (remember that <code>this</code> is of |
| type <code>Derived<T>*</code>, so is obviously dependent), or using |
| <code>Base<T>::i</code>. Alternatively, <code>Base<T>::i</code> might be brought |
| into scope by a <code>using</code>-declaration. |
| </p> |
| <p>Another, similar example involves calling member functions of a base |
| class: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample"> template <typename T> struct Base { |
| int f(); |
| }; |
| |
| template <typename T> struct Derived : Base<T> { |
| int g() { return f(); }; |
| }; |
| </pre></div> |
| |
| <p>Again, the call to <code>f()</code> is not dependent on template arguments |
| (there are no arguments that depend on the type <code>T</code>, and it is also |
| not otherwise specified that the call should be in a dependent context). |
| Thus a global declaration of such a function must be available, since |
| the one in the base class is not visible until instantiation time. The |
| compiler will consequently produce the following error message: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample"> x.cc: In member function `int Derived<T>::g()': |
| x.cc:6: error: there are no arguments to `f' that depend on a template |
| parameter, so a declaration of `f' must be available |
| x.cc:6: error: (if you use `-fpermissive', G++ will accept your code, but |
| allowing the use of an undeclared name is deprecated) |
| </pre></div> |
| |
| <p>To make the code valid either use <code>this->f()</code>, or |
| <code>Base<T>::f()</code>. Using the <samp>-fpermissive</samp> flag will also let |
| the compiler accept the code, by marking all function calls for which no |
| declaration is visible at the time of definition of the template for |
| later lookup at instantiation time, as if it were a dependent call. |
| We do not recommend using <samp>-fpermissive</samp> to work around invalid |
| code, and it will also only catch cases where functions in base classes |
| are called, not where variables in base classes are used (as in the |
| example above). |
| </p> |
| <p>Note that some compilers (including G++ versions prior to 3.4) get these |
| examples wrong and accept above code without an error. Those compilers |
| do not implement two-stage name lookup correctly. |
| </p> |
| |
| <div class="footnote"> |
| <hr> |
| <h4 class="footnotes-heading">Footnotes</h4> |
| |
| <h3><a name="FOOT5" href="#DOCF5">(5)</a></h3> |
| <p>The C++ standard just uses the |
| term “dependent” for names that depend on the type or value of |
| template parameters. This shorter term will also be used in the rest of |
| this section.</p> |
| </div> |
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