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POStatic assertion if possible, dynamic assertion otherwise?
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  1. POStatic assertion if possible, dynamic assertion otherwise?
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    copied!<p>Let's say I have a template function that takes an integer and a const reference to an instance of type T. Now depending on the integer, only some T's are acceptible, otherwise an exception is thrown at runtime.</p> <p>If all uses of this function would use constant integers, it would be possible to make the int a template parameter and use a static assertion to check if it is acceptable. So instead of <code>func(1,c)</code> one would use <code>func&lt;1&gt;(c)</code> and would gain compile-time type checking. Is there any way to write <code>func(1,c)</code> and still keep the compile-time check, while also being able to write <code>func(i,c)</code> and use a dynamic assertion? The goal is to make it transparent to the developer. It would simply be great to add this safety without bothering the developers about things like compile-time constants. They'd probably only remember that <code>func(1,c)</code> always works and use that, avoiding the check.</p> <p><strong>How can I define a function with a static assertion whenever possible and a dynamic assertion otherwise?</strong></p> <hr> <p>The following code shows the solution for GCC by <strong>Ivan Shcherbakov</strong>:</p> <pre><code>#include &lt;iostream&gt; #include &lt;cassert&gt; template&lt;typename T&gt; void __attribute__((always_inline)) func(const int&amp; i, const T&amp; t); void compile_time_error_() __attribute__((__error__ ("assertion failed"))); template&lt;&gt; void __attribute__((always_inline)) func(const int&amp; i, const float&amp; t) { do { if (i != 0) { if (__builtin_constant_p(i)) compile_time_error_(); std::cerr &lt;&lt; "assertion xzy failed" &lt;&lt; std::endl; exit(1); } } while (0); func_impl&lt;float&gt;(i,t); } </code></pre> <p>This will only allow the combination of i=0 and T=float. For other combinations a good way would be creating a Macro that produces the code of <code>template&lt;&gt; func(const int&amp; i, const T&amp; t)</code> with T and i != 0 replaced.</p>
 

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