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POC++ polymorphism with references
 

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    <p>I've been writing some classes in C++ that operate on files. When I started, I made all my constructors accept one <code>std::string</code> argument that was interpreted as a filename, because that was quick and easy.</p> <p>The only functions specific <code>std::fstream</code> that any of the classes use is <code>.open()</code>. I figured that since <code>std::ofstream</code> and <code>std::ostringstream</code> both derive from <code>std::ostream</code> and I was only using <code>std::ostream</code>'s functions, that it would be much better to have each constructor take an <code>std::ostream</code> or <code>std::istream</code> argument as appropriate, instead of keeping the classes bound to files.</p> <p>A great textbook example of why polymorphism is useful. GCC is pretty unhappy about it though. This is a simplification of my code:</p> <pre><code>#include &lt;iostream&gt; #include &lt;fstream&gt; class Foo { public: Foo(std::istream&amp;); std::istream&amp; input; }; Foo::Foo(std::istream&amp; is) { this-&gt;input = is; } int main() { std::ifstream is("test.txt"); Foo foo(is); } </code></pre> <p>Which, when compiled with <code>g++ test.cpp</code>, produces the following colorful assortment of errors:</p> <pre><code>test.cpp: In constructor ‘Foo::Foo(std::istream&amp;)’: test.cpp:10:1: error: uninitialized reference member ‘Foo::input’ [-fpermissive] In file included from /usr/include/c++/4.7/ios:43:0, from /usr/include/c++/4.7/ostream:40, from /usr/include/c++/4.7/iostream:40, from test.cpp:1: /usr/include/c++/4.7/bits/ios_base.h: In member function ‘std::basic_ios&lt;char&gt;&amp; std::basic_ios&lt;char&gt;::operator=(const std::basic_ios&lt;char&gt;&amp;)’: /usr/include/c++/4.7/bits/ios_base.h:791:5: error: ‘std::ios_base&amp; std::ios_base::operator=(const std::ios_base&amp;)’ is private In file included from /usr/include/c++/4.7/ios:45:0, from /usr/include/c++/4.7/ostream:40, from /usr/include/c++/4.7/iostream:40, from test.cpp:1: /usr/include/c++/4.7/bits/basic_ios.h:64:11: error: within this context In file included from /usr/include/c++/4.7/iostream:41:0, from test.cpp:1: /usr/include/c++/4.7/istream: In member function ‘std::basic_istream&lt;char&gt;&amp; std::basic_istream&lt;char&gt;::operator=(const std::basic_istream&lt;char&gt;&amp;)’: /usr/include/c++/4.7/istream:56:11: note: synthesized method ‘std::basic_ios&lt;char&gt;&amp; std::basic_ios&lt;char&gt;::operator=(const std::basic_ios&lt;char&gt;&amp;)’ first required here test.cpp: In constructor ‘Foo::Foo(std::istream&amp;)’: test.cpp:11:16: note: synthesized method ‘std::basic_istream&lt;char&gt;&amp; std::basic_istream&lt;char&gt;::operator=(const std::basic_istream&lt;char&gt;&amp;)’ first required here </code></pre> <p>I have trouble sifting through all the template arguments but it looks like it doesn't like <code>this-&gt;input = is;</code>. It's important for each class to be able to access the stream as a member, because various different member functions all have to be able to see it. I figured GCC would probably be nicer about a pointer, but to me, pointers to classes look suspiciously like dynamically allocated memory, and I only want to raise those suspicions if they're true. What's the right way to go about this?</p>
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