boostorg/variant2
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Peter Dimov
2019-05-12 00:08:27 +03:00
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doc/variant2/overview.adoc
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@ -9,7 +9,9 @@ http://www.boost.org/LICENSE_1_0.txt
[#overview] [#overview]
# Overview # Overview
:idprefix: :idprefix: overview_
## Description
This library implements a type-safe discriminated/tagged union type, This library implements a type-safe discriminated/tagged union type,
`variant<T...>`, that is API-compatible with the {cpp}17 Standard's `variant<T...>`, that is API-compatible with the {cpp}17 Standard's
@ -39,6 +41,8 @@ struct V
}; };
``` ```
## Usage Examples
Variants can be used to represent dynamically-typed values. A configuration Variants can be used to represent dynamically-typed values. A configuration
file of the form file of the form
@ -167,6 +171,8 @@ int main()
} }
``` ```
## Construction and Assignment
If we look at the If we look at the
``` ```
@ -194,6 +200,8 @@ variant::variant(float x);
and the standard overload resolution rules are used to pick the one that will and the standard overload resolution rules are used to pick the one that will
be used. So `variant<int, float>((short)1)` will hold an `int`. be used. So `variant<int, float>((short)1)` will hold an `int`.
## Inspecting the Value
Putting values into a `variant` is easy, but taking them out is necessarily a Putting values into a `variant` is easy, but taking them out is necessarily a
bit more convoluted. It's not possible for `variant<int, float>` to define a bit more convoluted. It's not possible for `variant<int, float>` to define a
member function `get() const`, because such a function will need its return member function `get() const`, because such a function will need its return
@ -265,6 +273,8 @@ void f( variant<int, float> const& v )
} }
``` ```
## Visitation
Last but not least, there's `visit`. `visit(f, v)` calls the a function object Last but not least, there's `visit`. `visit(f, v)` calls the a function object
`f` with the value contained in the `variant` `v` and returns the result. When `f` with the value contained in the `variant` `v` and returns the result. When
`v` is `variant<int, float>`, it will call `f` with either an `int` or a `v` is `variant<int, float>`, it will call `f` with either an `int` or a
@ -311,6 +321,8 @@ void g( variant<int, float> const& v )
`f(x1, x2)`, where `x1` is the value contained in `v1` and `x2` is the value `f(x1, x2)`, where `x1` is the value contained in `v1` and `x2` is the value
in `v2`. in `v2`.
## Default Construction
The default constructor of `variant` value-initializes the first type in The default constructor of `variant` value-initializes the first type in
the list. `variant<int, float>{}` holds `0` (of type `int`), and the list. `variant<int, float>{}` holds `0` (of type `int`), and
`variant<float, int>{}` holds `0.0f`. `variant<float, int>{}` holds `0.0f`.