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copied!<p>Sorry in advance for the bad title, not sure what to call what I'm trying to do.</p> <p>Some background, if you don't care to read, skip to the next paragraph. I have a unit test class where I call assert with some condition and if it fails I output some string that was passed in. I have found that it is quite annoying to build a string to send to this if for instance I want to say <code>"Failed on index: " + i</code>. My idea is to return a <code>std::ostream</code> instead of taking a <code>std::string</code>. If the assert fails I return <code>std::cerr</code>, if the assert passes then I return a <code>std::stringstream</code>. I imagine I could do all of this just fine. I'd have to store a <code>std::stringstream</code> in my unit test class so I can return a reference.</p> <p>What I would like to do is instead of returning a standard <code>std::ostream</code> return an extended <code>std::ostream</code> that outputs <code>std::endl</code> when it's done so I don't have to remember it for each assert. In particular the idea is the following:</p> <pre><code>UnitTest("My test"); ut.assert(false) << "Hello world"; ut.assert(1 == 0) << "On the next line"; </code></pre> <p>The idea being that on destruction this new class would output the endline and that it would be destructed as soon as it was no longer used (i.e. no more << operators). So far this is what I have (I've removed some of the code in assert, and it's actually inside a class, but this is sufficient to show what's going on):</p> <pre><code>class newline_ostream : public std::ostream { public: newline_ostream(std::ostream& other) : std::ostream(other.rdbuf()){} ~newline_ostream() { (*this) << std::endl; } }; newline_ostream& assert(bool condition, std::string error) { if(!condition) { return newline_ostream(std::cerr); } return newline_ostream(std::stringstream()); } </code></pre> <p>When I try this method I get some stuff basically telling me that returning an object I just created is wrong because it's not an lvalue. When I try changing it to not return a reference it complains that there's no copy constructor (presumably this is because I'm extending <code>std::ostream</code> and it doesn't have a copy constructor).</p> <p>What I'm looking for is some method that causes the compiler to create a temporary newline_ostream that <code>assert()</code> will write it's result into that will die as soon as it's no longer used (i.e. no more << operators). Is this possible and if so how?</p>

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