std::variant
性病：变体
| Defined in header | | |
|---|---|---|
| template <class... Types> class variant; | | (since C++17) |
类模板std::variant表示类型安全。联合.一个实例std::variant在任何给定的时间内，要么持有其替代类型之一的值，要么不保存值%28，此状态很难实现，请参见valueless_by_exception29%。
与联合一样，如果变量持有某种对象类型的值。T的对象表示T直接在变体本身的对象表示中分配。变量不允许分配额外的%28动态%29内存。
不允许变体保存引用、数组或类型。void空变体也是格式错误的%28。std::variant<std::monostate>可以代替%29使用。
允许变体持有同一类型多次，并持有不同的cv-限定版本的同一类型。
和联合一样，默认初始化变量保存其第一个选项的值，除非该选项不是默认的可构造%28，在这种情况下，默认构造函数获得%27T编译：助手类。std::monostate可用于使此类变体默认为%29。
模板参数
| Types | - | the types that may be stored in this variant. All types must be (possibly cv-qualified) non-array object types. |
|---|
成员函数
| (constructor) | constructs the variant object (public member function) |
|---|---|
| (destructor) | destroys the variant, along with its contained value (public member function) |
| operator= | assigns a variant (public member function) |
观察员
索引返回变体%28公共成员函数%29所持有的备选方案的基于零的索引。
无价值[医]通过[医]异常检查变量是否处于无效状态%28公共成员函数%29
修饰符
在变体中构造一个值，替换%28公共成员函数%29
与另一个变体%28公共成员函数%29的交换掉期
非会员职能
| visit (C++17) | calls the provided functor with the arguments held by one or more variants (function template) |
|---|---|
| holds_alternative (C++17) | checks if a given type appears exactly once in a variant (function template) |
| std::get(std::variant) (C++17) | reads the value of the variant given the index or the type (if the type is unique), throws on error (function template) |
| get_if (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) |
| operator==operator!=operator<operator<=operator>operator>= (C++17) | compares variant objects as their contained values (function template) |
| std::swap(std::variant) (C++17) | specializes the std::swap algorithm (function) |
帮助者类
| monostate (C++17) | placeholder type for use as the first alternative in a variant of non-default-contructible types (class) |
|---|---|
| bad_variant_access (C++17) | exception thrown on invalid accesses to the value of a variant (class) |
| variant_sizevariant_size_v (C++17) | obtains the size of the variant's list of alternatives at compile time (class template) (variable template) |
| variant_alternativevariant_alternative_t (C++17) | obtains the type of the alternative specified by its index, at compile time (class template) (alias template) |
| std::hash<std::variant> (C++17) | specializes the std::hash algorithm (class template specialization) |
| std::uses_allocator<std::variant> (C++17) | specializes the std::uses_allocator type trait (class template specialization) |
辅助对象
| variant_npos (C++17) | index of the variant in the invalid state (constant) |
|---|
例
二次
#include <variant>
#include <string>
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 (std::bad_variant_access&) {}
std::variant<std::string> v("abc" // converting constructors work when unambiguous
v = "def"; // converting assignment also works when unambiguous
}
二次
另见
| in_place in_place_type in_place_index in_place_t in_place_type_t in_place_index_t (C++17) | in-place construction tag (class template) |
|---|---|
| optional (since C++17) | a wrapper that may or may not hold an object (class template) |
| any (since C++17) | Objects that hold instances of any CopyConstructible type. (class) |
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