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std::variant

From cppreference.com
< cpp‎ | utility
 
 
 
 
Defined in header <variant>
template <class... Types>
class variant;
(since C++17)

The class template std::variant represents a type-safe union. An instance of std::variant at any given time either holds a value of one of its alternative types, or it holds no value (this state is hard to achieve, see valueless_by_exception).

As with unions, if a variant holds a value of some object type T, the object representation of T is allocated directly within the object representation of the variant itself. Variant is not allowed to allocate additional (dynamic) memory.

A variant is not permitted to hold references, arrays, or the type void. Empty variants are also ill-formed (std::variant<std::monostate> can be used instead).

A variant is permitted to hold the same type more than once, and to hold differently cv-qualified versions of the same type.

As with unions, the default-initialized variant holds a value of its first alternative, unless that alternative is not default-constructible (in which case default constructor won't compile: the helper class std::monostate can be used to make such variants default-constructible)

Contents

[edit] Template parameters

Types - the types that may be stored in this variant. All types must be (possibly cv-qualified) non-array object types.

[edit] Member functions

constructs the variant object
(public member function) [edit]
destroys the variant, along with its contained value
(public member function) [edit]
assigns a variant
(public member function) [edit]
Observers
returns the zero-based index of the alternative held by the variant
(public member function) [edit]
checks if the variant is in the invalid state
(public member function) [edit]
Modifiers
constructs a value in the variant, in place
(public member function) [edit]
swaps with another variant
(public member function) [edit]

[edit] Non-member functions

(C++17)
calls the provided functor with the arguments held by one or more variants
(function template) [edit]
checks if a variant currently holds a given type
(function template) [edit]
reads the value of the variant given the index or the type (if the type is unique), throws on error
(function template) [edit]
(C++17)
obtains a pointer to the value of a pointed-to variant given the index or the type (if unique), returns null on error
(function template) [edit]
compares variant objects as their contained values
(function template) [edit]
specializes the std::swap algorithm
(function) [edit]

[edit] Helper classes

(C++17)
placeholder type for use as the first alternative in a variant of non-default-contructible types
(class) [edit]
exception thrown on invalid accesses to the value of a variant
(class) [edit]
obtains the size of the variant's list of alternatives at compile time
(class template) (variable template) [edit]
obtains the type of the alternative specified by its index, at compile time
(class template) (alias template) [edit]
specializes the std::hash algorithm
(class template specialization) [edit]

[edit] Helper objects

index of the variant in the invalid state
(constant) [edit]

[edit] Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR Applied to Behavior as published Correct behavior
LWG 2901 C++17 specialization of std::uses_allocator provided, but variant can't properly support allocators specialization removed

[edit] Example

#include <variant>
#include <string>
#include <cassert>
 
using namespace std::literals;
 
int main()
{
    std::variant<int, float> v, w;
    v = 12; // v contains int
    int i = std::get<int>(v);
    w = std::get<int>(v);
    w = std::get<0>(v); // same effect as the previous line
    w = v; // same effect as the previous line
 
//  std::get<double>(v); // error: no double in [int, float]
//  std::get<3>(v);      // error: valid index values are 0 and 1
 
    try {
      std::get<float>(w); // w contains int, not float: will throw
    }
    catch (const std::bad_variant_access&) {}
 
    std::variant<std::string> x("abc"); // converting constructors work when unambiguous
    x = "def"; // converting assignment also works when unambiguous
 
    std::variant<std::string, bool> y("abc"); // casts to bool when passed a char const *
    assert(std::holds_alternative<bool>(y)); // succeeds
    y = "xyz"s;
    assert(std::holds_alternative<std::string>(y)); //succeeds
}


[edit] See also

in-place construction tag
(class template) [edit]
(since C++17)
a wrapper that may or may not hold an object
(class template) [edit]
(since C++17)
Objects that hold instances of any CopyConstructible type.
(class) [edit]