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doc: write about precedence

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ToruNiina
2021-05-15 00:24:51 +09:00
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README.md
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@@ -1255,8 +1255,16 @@ struct from<ext::foo>
In this way, since the conversion function is defined outside of the class,
you can add conversion between `toml::value` and classes defined in another library.
Note that you cannot implement both of the functions because the overload
resolution of `toml::get` will be ambiguous.
In some cases, a class has a templatized constructor that takes a template, `T`.
It confuses `toml::get/find<T>` because it makes the class "constructible" from
`toml::value`. To avoid this problem, `toml::from` and `from_toml` always
precede constructor. It makes easier to implement conversion between
`toml::value` and types defined in other libraries because it skips constructor.
But, importantly, you cannot define `toml::from<T>` and `T.from_toml` at the same
time because it causes ambiguity in the overload resolution of `toml::get<T>` and `toml::find<T>`.
So the precedence is `toml::from<T>` == `T.from_toml()` > `T(toml::value)`.
If you want to convert any versions of `toml::basic_value`,
you need to templatize the conversion function as follows.