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copied!<p>When confronted with this in the past, I've used a <code>stringstream</code> along with a manipulator that displays the current contents of the <code>stringstream</code> using <code>MessageBox</code>:</p> <pre><code>#include <windows.h> #include <sstream> #include <ostream> std::ostream &MessageBox(std::ostream &s) { std::ostringstream *st = dynamic_cast<std::ostringstream *>(&s); if (NULL != st) ::MessageBox(NULL, st->str().c_str(), "", MB_OK); return s; } </code></pre> <p>To use this, the syntax looks a fair amount like using <code>cout</code>, but with <code>MessageBox</code> replacing <code>std::endl</code>. For example:</p> <pre><code>std::ostringstream stm; stm << " blah blah blah. Value: " << 1213.1231 << MessageBox; </code></pre> <p>Edit: mostly for fnieto. In this case, the downcast really <em>is</em> necessary. The reason is fairly simple: a typical inserter receives and returns a reference to an ostream:</p> <pre><code>std::ostream &operator<<(std::ostream &os, T const &t) { // code here to insert t into os, then return os; } </code></pre> <p>This takes the original stringstream object and silently (and safely) casts it up to a simple ostream. That's fine in itself, and works fine for most inserters and manipulators, because they only interact with the <code>ostream</code> interface themselves.</p> <p>This manipulator, however, is a bit different -- it uses the <code>str()</code> member, which <code>ostream</code> doesn't define at all. For our call to <code>str()</code> to resolve and compile, we have to convert the <code>ostream &</code> to an <code>ostringstream &</code>, so the compiler is aware that the object we're working with really will have a <code>str()</code> member. </p> <p>To eliminate the downcast, we'd really only have one choice: make its parameter an <code>ostringstream &</code>. That would work as long as we never chained operators:</p> <pre><code>my_stream << x; my_stream << MessageBox; </code></pre> <p>but trying to chain those would fail:</p> <pre><code>// should be equivalent: my_stream << x << MessageBox; </code></pre> <p>Worse, the compiler's error message will probably try to tell the user something about <code>std::basic_ostream<char>::str()</code>, which isn't mentioned in the user's code at all. Worse still, most people are sufficiently accustomed to chaining or not giving identical results that it would probably take them a while to even figure out why the code sometimes worked fine, and other times failed to compile, with a completely indecipherable error message.</p>

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