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std::variant 类似于 union,可以存储范围内的一种类型的值。
在使用时,需要判断 variant 存了哪种类型的值,可以使用
index
、holds_alternative
更简单的方法是使用
std::visit
,visit 会自动用相应的类型调用 lambdavisit 中的 lambda 可以有多个 variant 的参数,而且 lambda 还可以有返回值,可以进一步简化代码