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PONaN ASCII I/O with Visual C++
 

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    <p>I want to read and write NaN values from/into text files using iostream and Visual C++. When writing a NaN value, i get <code>1.#QNAN</code>. But, reading it back outputs <code>1.0</code> .</p> <pre><code>float nan = std::numeric_limits&lt;float&gt;::quiet_NaN (); std::ofstream os("output.txt"); os &lt;&lt; nan ; os.close(); </code></pre> <p>The output is <code>1.#QNAN</code> .</p> <pre><code>std::ifstream is("output.txt"); is &gt;&gt; nan ; is.close(); </code></pre> <p><code>nan</code> equals <code>1.0</code>.</p> <p><strong>Solution</strong></p> <p>Finally, as suggested by awoodland, I've come up with this solution. I chose "nan" as a string representation of a NaN. Both &lt;&lt; and >> operators are overridden.</p> <pre><code>using namespace ::std; class NaNStream { public: NaNStream(ostream&amp; _out, istream&amp; _in):out(_out), in(_in){} template&lt;typename T&gt; const NaNStream&amp; operator&lt;&lt;(const T&amp; v) const {out &lt;&lt; v;return *this;} template&lt;typename T&gt; const NaNStream&amp; operator&gt;&gt;(T&amp; v) const {in &gt;&gt; v;return *this;} protected: ostream&amp; out; istream&amp; in; }; // override &lt;&lt; operator for float type template &lt;&gt; const NaNStream&amp; NaNStream::operator&lt;&lt;(const float&amp; v) const { // test whether v is NaN if( v == v ) out &lt;&lt; v; else out &lt;&lt; "nan"; return *this; } // override &gt;&gt; operator for float type template &lt;&gt; const NaNStream&amp; NaNStream::operator&gt;&gt;(float&amp; v) const { if (in &gt;&gt; v) return *this; in.clear(); std::string str; if (!(in &gt;&gt; str)) return *this; if (str == "nan") v = std::numeric_limits&lt;float&gt;::quiet_NaN(); else in.setstate(std::ios::badbit); // Whoops, we've still "stolen" the string return *this; } </code></pre> <p>A minimal working example: a finite float and a NaN are written into a stringstream and then read back.</p> <pre><code>int main(int,char**) { std::stringstream ss; NaNStream nis(ss, ss); nis &lt;&lt; 1.5f &lt;&lt; std::numeric_limits&lt;float&gt;::quiet_NaN (); std::cout &lt;&lt; ss.str() &lt;&lt; std::endl; // OUTPUT : "1.5nan" float a, b; nis &gt;&gt; a; nis &gt;&gt; b; std::cout &lt;&lt; a &lt;&lt; b &lt;&lt; std::endl; // OUTPUT : "1.51.#QNAN" } </code></pre>
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    1. COThe question should be, "How to do formatted I/O with NaN". I suppose. Good question.
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    2. COhttp://pubs.opengroup.org/onlinepubs/007904975/functions/scanf.html says `If the fprintf() family of functions generates character string representations for infinity and NaN (a symbolic entity encoded in floating-point format) to support IEEE Std 754-1985, the fscanf() family of functions shall recognize them as input.` for whatever that's worth.
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    3. CO@Moo: if that's true, then at least we know now that iostreams don't use `fscanf` :-) In any case that'd be a ridiculous thing to rely on, since the textual representation of NaN varies from compiler to compiler, and probably from state to state, too. Maybe it's just not possible to *read* NaN. (There isn't even a literal for it, coming to think of it.)
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