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POAny way to make parameterized user defined literals?
 

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  1. POAny way to make parameterized user defined literals?
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    <p>A little while ago I had an idea about "parameterized" user-defined literals and was wondering if there is any way to do this in the current C++ standard.</p> <p>Basically, the idea is to have a user-defined literal whose behaviour can be tweaked according to some parameters. As a simple example, I chose a "fixed-point" literal which turns a floating-point number into an integer; the parameter is the precision in terms of the number of decimal places.</p> <p>This is just an exercise for now, since I'm not sure how or if this would be useful in a real application.</p> <p>My first idea went something like this:</p> <pre class="lang-c prettyprint-override"><code>namespace fp_impl { constexpr int floor(long double n) { return n; } constexpr int pow10(int exp) { return exp == 0 ? 1 : 10 * pow10(exp - 1); } template&lt;int i&gt; constexpr int fixed_point(long double n) { return floor(n * pow10(i)); } namespace fp2 { constexpr int operator"" _fp (long double n) { return fixed_point&lt;2&gt;(n); } } namespace fp4 { constexpr int operator"" _fp (long double n) { return fixed_point&lt;4&gt;(n); } } } template&lt;int prec&gt; struct fp; template&lt;&gt; struct fp&lt;2&gt; { namespace lit = fp2; }; template&lt;&gt; struct fp&lt;4&gt; { namespace lit = fp4; }; int main() { { using namespace fp&lt;2&gt;::lit; std::cout &lt;&lt; 5.421_fp &lt;&lt; std::endl; // should output 542 } { using namespace fp&lt;4&gt;::lit; std::cout &lt;&lt; 5.421_fp &lt;&lt; std::endl; // should output 54210 } } </code></pre> <p>However, it doesn't compile because namespace aliases are not permitted at class scope. (It also has a problem with requiring you t manually define every version of <code>operator"" _fp</code>.) So I decided to try something with macros:</p> <pre class="lang-c prettyprint-override"><code>namespace fp { namespace detail { constexpr int floor(long double n) { return n; } constexpr int pow10(int exp) { return exp == 0 ? 1 : 10 * pow10(exp - 1); } template&lt;int i&gt; constexpr int fixed_point(long double n) { return floor(n * pow10(i)); } } } #define SPEC(i) \ namespace fp { \ namespace precision##i { \ constexpr int operator"" _fp(long double n) { \ return fp::detail::fixed_point&lt;i&gt;(n); \ } \ } \ } SPEC(2); SPEC(4); #undef SPEC #define fp_precision(i) namespace fp::precision##i int main() { { using fp_precision(2); std::cout &lt;&lt; 5.421_fp &lt;&lt; std::endl; } { using fp_precision(4); std::cout &lt;&lt; 5.421_fp &lt;&lt; std::endl; } } </code></pre> <p>This works, though it still has the requirement of using the <code>SPEC()</code> macro for every precision you ever want to use. Of course, some preprocessor trickery could be used to do this for every value from, say, 0 to 100, but I'm wondering if there could be anything more like a template solution, where each one is instantiated as it is needed. I had a vague idea of using an operator"" declared as a friend function in a template class, though I suspect that won't work either.</p> <p>As a note, I did try <code>template&lt;int i&gt; constexpr int operator"" _fp(long double n)</code>, but it seems this is not an allowed declaration of a literal operator.</p>
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    1. UScelticminstrel
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      1. POAny way to make parameterized user defined literals?
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    1. COI don't really think there is a better alternative than your macro solution since template literals do not work. Unless you use another preprocessor or write your own instead of using the standard one. But I doubt it's worth it...
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      1. POAny way to make parameterized user defined literals?
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    2. COWhy not just use a `constexpr` fuction where you want non-default values?
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      1. POAny way to make parameterized user defined literals?
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    3. COI'm not sure I understand how that applies to this problem.
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      1. UScelticminstrel
 

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