检查的boost ::变体LT; T＆GT;对于空 [英] check boost::variant<T> for null

问题描述

我在我的节目一个boost ::变种，我要检查，如果变量本身初始化，还是否有包含在它的一个值的类型。

I have a boost::variant in my program and I want to check if the variant itself is initialized and also if there is a value contained in one of it's types.

我就试过变种空（），但似乎并没有工作。无论是不检查对NULL。

I've tried empty() on the variant, but that doesn't seem to work. Neither does checking against NULL.

有谁知道如何检查呢？

编辑：好吧，看来它永远不会是空的，但不会永远是它包含类型的值，所以我怎么检查一个没有价值的情况

Ok, It seems it will never be empty, but there will not always be a value in it's contained types, so how do I check for a no-value situation?

推荐答案

A 的boost ::变种总是被初始化。

如果你没有明确initalized它，第一个项目是使用它的默认构造函数构造：

If you did not initalized it explicitly, the first item was constructed using its default constructor:

struct Foo {};
struct Bar {};

struct Visitor: boost::static_visitor<>
{
  void operator()(Foo const& foo) const { std::cout << "Foo\n"; }
  void operator()(Bar const& bar) const { std::cout << "Bar\n"; }
};

int main(int argc, char* argv[])
{
  boost::variant<Foo,Bar> var;
  boost::apply_visitor(Visitor(), var); // prints Foo
  return 0;
}

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