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POderived basic_ostream: "using" keyword and ambiguous overload for operator <<
 

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  1. POderived basic_ostream: "using" keyword and ambiguous overload for operator <<
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    <p>I have a derived basic_ostream class and an inline modifier (similar to setw). My stream class should also inherit all the operator &lt;&lt; behavior from its parent. I get different compiler errors depending on whether I use the "using" keyword or not:</p> <pre><code>#include &lt;iostream&gt; struct modifier { }; template &lt;typename C, typename T=std::char_traits&lt;C&gt; &gt; struct mystream : public std::basic_ostream&lt;C, T&gt; { // this is where the trouble is using std::basic_ostream&lt;C, T&gt;::operator &lt;&lt;; inline mystream &amp; operator &lt;&lt; (const modifier &amp; mod) { // ...custom behavior... return *this; } }; int main() { mystream&lt;char&gt; foo; modifier m; foo &lt;&lt; "string"; // this fails if the using is present foo &lt;&lt; 123; // this fails if the using is absent foo &lt;&lt; m; } </code></pre> <p>When I put the using directive in, the compiler is confused about the "string" output, and if I comment it out, it gets confused about the integer 123 output, in both cases giving me "error: ambiguous overload for 'operator&lt;&lt;'". I have the problem with both g++ 4.2.1 and g++4.8. What's the right way forward here?</p>
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    1. USJohn S
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    1. COWhy are you deriving from `basic_ostream`? What are you doing that a non-member `operator <<` cannot do?
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    2. COI commented below--my stream has a lot of custom behavior, including its own streambuf, etc., and that operator needs to change the state of the stream.
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    3. COUse `xalloc` and `iword` / `pword`, they are specifically designed for maintaining custom stream state. You *can* derive from `basic_ostream` if you want to construct something with your own `streambuf` (`fstream` does just that), just don't add any `operator<<` members to it.
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