better-enums/doc/tutorial/9-constexpr.md

Compile-time usage

When used with $cxx11, Better Enums are generated entirely during compilation. All the data is available for use by your own constexpr functions. The examples in this tutorial aren't very useful, but look at the demos at the bottom of the main page to get an idea of what can be done. Here, you will see the basics.

#include <iostream>

// The reason for this is explained below.
#ifndef BETTER_ENUMS_CONSTEXPR_TO_STRING
#define BETTER_ENUMS_CONSTEXPR_TO_STRING
#endif

<em>#include <enum.h></em>

<em>BETTER_ENUM</em>(<em>Channel</em>, <em>int</em>, <em>Red</em> = <em>1</em>, <em>Green</em> = <em>2</em>, <em>Blue</em> = <em>3</em>)

<em>constexpr</em> Channel      channel = <em>Channel::_from_integral(2)</em>;
<em>constexpr</em> int          value = <em>channel._to_integral()</em>;

<em>constexpr</em> const char   *name = <em>channel._to_string()</em>;
<em>constexpr</em> Channel      parsed = <em>Channel::_from_string("Red")</em>;

All of the above are computed during compilation. The reason for the macro definition at the top of the file is explained on the opt-in features page. Basically, it makes _to_string constexpr, but slows down compilation.

You can also do things such as:

<em>constexpr size_t length</em>(<em>const char *s</em>, <em>size_t index = 0</em>)
{
    return <em>s[index] == '\0'</em> ? <em>index</em> : <em>length(s, index + 1)</em>;
}

<em>constexpr</em> size_t    <em>length_of_name_of_second_constant</em> =
    <em>length(Channel::_names()[1])</em>;

int main()
{
    std::cout << <em>length_of_name_of_second_constant</em> << std::endl;

    return 0;
}

Which prints "5", the length of "Green". That 5 was also computed during compilation.

%% description = Better Enums can be used entirely at compile time in C++11. All conversion functions are available for constexpr functions or templates.