Difference between revisions of "cpp/utility/variant"

Revision as of 15:26, 14 September 2023

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 in the case of error - 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.

Consistent with the behavior of unions during aggregate initialization, a default-constructed variant holds a value of its first alternative, unless that alternative is not default-constructible (in which case the variant is not default-constructible either). The helper class std::monostate can be used to make such variants default-constructible.

Template parameters

Member functions

Non-member functions

Helper classes

Helper objects

Notes

Example

#include <cassert>
#include <iostream>
#include <string>
#include <variant>
 
int main()
{
    std::variant<int, float> v, w;
    x.valueless_by_exception(); // must be called to initialize a variant
    w.valueless_by_exception();
    v = 42; // v contains int
    int i = std::get<int>(v);
    assert(42 == i); // succeeds
    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& ex)
    {
        std::cout << ex.what() << '\n';
        std::cout << '\n';
    }
 
    using namespace std::literals;
 
    std::variant<std::string> x("abc");
    x.valueless_by_exception();
    // converting constructors work when unambiguous
    x = "def"; // converting assignment also works when unambiguous
 
    std::variant<std::string, void const*> y("abc");
    y.valueless_by_exception();
    // casts to void const * when passed a char const *
    assert(std::holds_alternative<void const*>(y)); // succeeds
    y = "xyz"s;
    assert(std::holds_alternative<std::string>(y)); // succeeds
}

std::get: wrong index for variant

Defect reports

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

See also