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POCalling a C++ namespace-qualified function within a template function in another namespace
 

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  1. POCalling a C++ namespace-qualified function within a template function in another namespace
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    <p>If I have a default function 'foo' defined in a namespace in one header file:</p> <pre><code>//DefaultFoo.h namespace DefaultFooNamespace { template &lt;typename T&gt; void foo(T&amp; x){/*...*/} } </code></pre> <p>and overloads of foo defined in another namespace in one or more other header files, e.g., FooOverloads.h:</p> <pre><code>//FooOverloads.h namespace FooOverloadsNamespace { template &lt;typename T&gt; void foo(std::vector&lt;T&gt;&amp; x){/*...*/} void foo(std::string&amp; x){/*...*/} //etc. } </code></pre> <p>and I have another function 'bar' which calls foo after bringing both DefaultFooNamespace and FooOverloadsNamespace namespaces into scope (note that I #include only DefaultFoo.h in Bar.h, as DefaultFoo.h contains a single function in a namespace which is intended never to be extended, unlike FooOverloadsNamespace which will be extended by the addition of additional overloads):</p> <pre><code>//Bar.h #include "DefaultFoo.h" namespace BarNamespace { template &lt;typename T&gt; void bar(T&amp; x) { //... do stuff then call foo using DefaultFooNamespace::foo; using FooOverloadsNamespace::foo; foo(x); } } </code></pre> <p>'bar' won't compile unless I either make sure FooOverloads.h is #include'd before a #include of Bar.h, or I make sure that FooOverloads.h is #include'd in Bar.h, or alternatively I provide a declaration of a 'foo' function for a dummy class in FooNamespace and #include that in Bar.h, e.g.</p> <pre><code>//Dummy.h: struct DummyClass { private: DummyClass(){} }; namespace FooNamespace { inline void foo(DummyClass&amp; x); //purely to open up namespace } //Bar.h #include "Dummy.h" </code></pre> <p>Is there any way around this, such that I can define bar and avoid having to create redundant code to open up FooNamespace in Bar?</p>
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    1. USMichael Price
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    1. COwhat is Foo.h? what is App.h? Are you saying that `using FooOverloadsNamespace::foo;` fails to compile unless `FooOverloadsNamespace::foo` has been declared? Because that's how C++ works. Why can't you `#include "FooOverloads.h"` in Bar.h? Include what you use.
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      1. POCalling a C++ namespace-qualified function within a template function in another namespace
      1. USJonathan Wakely
    2. COSorry, edited to fix some of misnamed items. But basically what I am saying is that the FooOverloadsNamespace is intended to be extensible, and may be spread over several header files, but I have to arbitrarily select one of them for #inclusion in Bar.h.
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    3. COThat's not going to work. For the call `foo(x)` in `bar` to find the right overload it has to have seen that overload. You can't just include one overload, then expect `using FooOverloadsNamespace::foo` to magically make all overloads in all headers available.
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      1. POCalling a C++ namespace-qualified function within a template function in another namespace
      1. USJonathan Wakely
 

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