better-enums/doc/tutorial/2-conversions.md

Conversions

Let's begin by including enum.h and declaring our enum:

#include <cassert>
#include <iostream>

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

<em>BETTER_ENUM</em>(<em>Channel</em>, <em>int</em>, <em>Cyan</em> = <em>1</em>, <em>Magenta</em>, <em>Yellow</em>, <em>Black</em>)

We now have an int-sized enum with four constants.

There are three groups of conversion functions: for strings, case-insensitive strings, and integers. They all follow the same pattern, so I'll explain the string functions in detail, and the rest can be understood by analogy.

$internal_toc

Strings

There are three functions:

1. ._to_string
2. ::_from_string
3. ::_from_string_nothrow

int main()
{
    <em>Channel     channel = Channel::Cyan</em>;
    std::cout << <em>channel._to_string()</em> << " ";

As you'd expect, the code above prints "Cyan".

If channel is invalid — for example, if you simply cast the number "42" to Channel — then the result of to_string is undefined.

---

    channel = <em>Channel::_from_string("Magenta")</em>;
    std::cout << channel._to_string() << " ";

This is also straightforward. If you pass a string which is not the name of a declared value, _from_string throws std::runtime_error.

---

If you don't want an exception, there is _from_string_nothrow:

    <em>better_enums::optional<Channel></em> maybe_channel =
        <em>Channel::_from_string_nothrow("Yellow")</em>;

    if (<em>!maybe_channel</em>)
        std::cout << "error";
    else
        std::cout << <em>maybe_channel-></em>_to_string() << " ";

This returns an optional value, in the style of boost::optional or the proposed std::optional.

What that means for the above code is:

• if the conversion succeeds, maybe_channel converts to true and *maybe_channel is the converted value of type Channel,
• if the conversion fails, maybe_channel converts to false.

In $cxx11, you can use auto to avoid writing out the optional type:

    <em>auto</em>        maybe_channel = <em>Channel::_from_string_nothrow("Yellow")</em>;
    if (<em>!maybe_channel</em>)
        std::cout << "error";
    else
        std::cout << <em>maybe_channel-></em>_to_string() << " ";

Case-insensitive strings

The "_nocase" string conversions follow the same pattern, except for the lack of a "to_string_nocase".

1. ::_from_string_nocase

2. ::_from_string_nocase_nothrow

   channel = Channel::_from_string_nocase("cYaN"); std::cout << channel._to_string() << " ";

   maybe_channel = Channel::_from_string_nocase_nothrow("rEeD"); assert(!maybe_channel);

Integers

And, it is similar with the representation type int:

1. ._to_integral

2. ::_from_integral

3. ::_from_integral_nothrow

4. ::_from_integral_unchecked

   channel = Channel::Cyan; std::cout << channel._to_integral() << " ";

   channel = Channel::_from_integral(2); std::cout << channel._to_string() << " ";

   maybe_channel = Channel::_from_integral_nothrow(0); assert(!maybe_channel);

That prints "1 Magenta".

_from_integral_unchecked is a no-op unchecked cast of integers to enums, so use it carefully.

    channel = <em>Channel::_from_integral_unchecked(0)</em>;
    // <em>Invalid</em> - better not to try converting it to string!

Validity checking

For completeness, Better Enums also provides three validity checking functions, one for each of the groups of conversions — string, case-insensitive string, and integer:

    assert(<em>Channel::_is_valid(3)</em>);
    assert(<em>Channel::_is_valid("Magenta")</em>);
    assert(<em>Channel::_is_valid_nocase("cYaN")</em>);

---

Almost done.

There is one unfortunate wrinkle. You cannot convert a literal constant such as Channel::Cyan directly to, for example, a string. You have to prefix it with +:

    std::cout << (<em>+Channel::Cyan</em>)._to_string();

This is due to some type gymnastics in the implementation of Better Enums. The reference has a full explanation.

---

    std::cout << std::endl;
    return 0;
}

%% description = Better Enums conversion functions. Converting to string, from string, to int, from int, and validation, both case-sensitive and case-insensitive. Exception-throwing and non-throwing variants presented.