better-enums/doc/tutorial/5-safety.md

Scope and safety

This tutorial shows some of the safety features of Better Enums: scope, how to control conversions, and the lack of a default constructor.

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Scope

You have probably noticed by now that Better Enums are scoped: when you declare

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

<em>ENUM</em>(<em>Channel</em>, <em>int</em>, <em>Red</em> = <em>1</em>, <em>Green</em>, <em>Blue</em>)

you don't get names such as Red in the global namespace. Instead, you get Channel, and Red is accessible as Channel::Red. This is no big deal in $cxx11, which has enum class. In $cxx98, however, this typically requires effort. Better Enums brings scope uniformly to both variants. So, despite the above declaration, you can safely declare

<em>ENUM</em>(<em>Node</em>, <em>char</em>, <em>Red</em>, <em>Black</em>)

and everything will work as expected.

int main()
{
    assert((+<em>Channel::Red</em>)._to_integral() <em>!=</em> (+<em>Node::Red</em>)._to_integral());

Implicit conversion

A major complaint in $cxx98 is that enums are implicitly convertible to integers. Unfortunately, that is also true of Better Enums, and I haven't found a way to forbid the conversions and still have switch case checking.

Better Enums can be made as safe as enum class in $cxx11, however. If your compiler supports enum class and you define BETTER_ENUMS_STRICT_CONVERSION before including enum.h, the following code will not compile:

    Channel     channel = Channel::Red;
    int         n = channel;

The reason this is not enabled by default is explained in the reference page on strict conversions.

Default constructor

Better Enums don't have a default constructor, for three reasons.

• Better Enums is a library that can't know what your application would like the default value to be for each enum, or whether you even want one.
• I chose not to leave the default value undefined, because the idea is to encourage the convention that whenever a Better Enum exists, it has a valid value. This is borrowed from typed functional programming.
• Better Enums is still under development, and this option is the most future-proof.

So, if you uncomment this code, the file won't compile:

    Channel      channel;

This may be too strict, and I may relax it in the future. In the meantime, the solution sketched in the special values demo can replace default constructors for some purposes, and in a more flexible way. I may eventually have the default constructor calling a template function like the one in that demo.

---

    return 0;
}

%% description = Type safety features and limitations.