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POFloating-point NaN depending on uncorrelated exception handling in C++
 

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  1. POFloating-point NaN depending on uncorrelated exception handling in C++
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    <p>Really weird:</p> <pre><code>double *data; // uncorrelated double a,b,c; double sigma = 1e-309; // denormalized number try { data = new double[10]; } // uncorrelated catch(...) { cout &lt;&lt; "error"; return 1; } a = 1/sigma; // infinite b = exp(-1/sigma); // 0 c = a * b; // NaN cout &lt;&lt; c &lt;&lt; endl; c = (1/sigma) * exp(-1/sigma); // 0 cout &lt;&lt; c &lt;&lt; endl; </code></pre> <p>Ok, the second c result could be 0 because of some optimization.</p> <p><strong>BUT</strong>: when I delete the try/catch block, the second c remains NaN! Why this different behaviour??? My compiler is VC++ 2010 Express. OS Windows 7 64-bit. I use only standard libs like iostream and cmath.</p> <p><strong>Edit</strong>: my first observation was with Debug+Win32 default settings for an empty console application. With Release+Win32 the results are: first c 0, second c NaN - no matter if try/catch present or not! Summary:</p> <pre><code> //Debug+Win32 // Release+Win32 //with try //without //with try //without c = a * b; // NaN NaN 0 0 c = (1/sigma) * exp(-1/sigma); // 0 NaN NaN NaN </code></pre> <p><strong>Edit 2</strong>: When I set the <code>/fp:strict</code> switch in C++/codegeneration, the result is same with Debug+Win32, but with Release+Win32 it changes to <code>c = a * b; // NaN</code> and <code>c = (1/sigma) * exp(-1/sigma); // 0</code> no matter if with try or not. I don't get why it stays <code>NaN+NaN</code> with Debug+Win32 and no preceding try. How to debug a program that has to be floating-point safe, when results differ dispite <code>/fp:strict</code> from Release depending on preceding try?</p> <p><strong>Edit 3</strong>: Here a full program:</p> <pre><code>// On VC++ 2010 Express in default Win32-Debug mode for empty console application. // OS: Windows 7 Pro 64-Bit, CPU: Intel Core i5. // Even when /fp:strict is set, same behaviour. // // Win32-Release mode: first c == 0, second c == NaN (independent of try) // with /fp:strict: first c == NaN, second c == 0 (also independent of try) #include &lt;iostream&gt; #include &lt;cmath&gt; using namespace std; int main() { double *data; // uncorrelated double a,b,c; double sigma = 1e-309; // denormalized number try { data = new double[10]; } // uncorrelated catch(...) { cout &lt;&lt; "error"; return 1; } a = 1/sigma; // infinite b = exp(-1/sigma); // 0 c = a * b; // NaN cout &lt;&lt; c &lt;&lt; endl; c = (1/sigma) * exp(-1/sigma); // 0 with preceding try or cout &lt;&lt; c &lt;&lt; endl; // NaN without preceding try cin.get(); return 0; } </code></pre>
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    1. COare you using optimizations or just plain debug ?
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    2. CONo optimizations active (under Project properties/Config/C++/Optimization). Just defaut settings for an empty console application with Debug+Win32. And debugging from within VC++. Even when I cout the first and second c and I run the application as exe in Windows, same effects!
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