如何使用对象的引用来初始化 boost::any ？
我想在 boost::any 对象中存储对对象的引用。如何初始化 boost::any 对象？我尝试了 std::ref()，但是 boost::any 是用 std::reference_wrapper 初始化的。例如，以下
#include <boost/any.hpp>
#include <cxxabi.h>
#include <iostream>
int main(void)
{
int s;
int i = 0;
boost::any x(std::ref(i));
std::cout << abi::__cxa_demangle(x.type().name(), 0, 0, &s) << "\n";
return 0;
}
打印
std::reference_wrapper<int>
我希望 boost::any 包含 int& 。
I want to store a reference to an object in a boost::any object. How do I initialize the boost::any object? I tried std::ref(), but boost::any gets initialized with std::reference_wrapper<>. For example, the following
#include <boost/any.hpp>
#include <cxxabi.h>
#include <iostream>
int main(void)
{
int s;
int i = 0;
boost::any x(std::ref(i));
std::cout << abi::__cxa_demangle(x.type().name(), 0, 0, &s) << "\n";
return 0;
}
prints
std::reference_wrapper<int>
I want the boost::any to contain int& instead.
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boost::any类没有允许这样的接口：您需要使用构造函数指定引用的类型。我认为您不能显式指定模板化构造函数的类型，因为我看不到任何可以粘贴它的地方。即使您可以显式指定模板参数，它在 C++2003 中也不起作用，因为没有可用的引用折叠，并且该参数被声明为采用T const&：您会试图创建一个T& const&不会飞。我认为你最好的选择是如果你坚持要像远程引用一样使用
std::reference_wrapper，或者只是使用T*。也就是说，通常可以有一个类似于 boost::any 类型的模板化静态因子方法，用于显式指定模板参数。然而，由于
boost::any是故意设计来处理值类型的，所以并没有这样做。我有点怀疑是否也应该这样做：使用指针是非常好的选择。如果您确实需要引用类型，您可能必须自己实现它。The
boost::anyclass doesn't have an interface allowing something like this: you would need to specify the type of the reference with the constructor. I don't think that you can explicitly specify the type of templated constructor because I don't see any place you could stick it. Even if you can explicitly specify the template parameter, it wouldn't work in C++2003 bcause there is no reference collapsing available and the parameter is declared as taking aT const&: you'd be trying to create aT& const&which won't fly.I think your best option is to either use
std::reference_wrapper<T>if you insist on something looking remotely reference like or just to useT*.That said, it would be generally possible to have a templatized static factor method of a type similar to
boost::anywhich would be used to explicitly specify the template argument. However, sinceboost::anyis deliberately designed to deal with value types this isn't done. I'm a bit dubious whether it should be done as well: using a pointer is perfectly good alternative. If you really need a reference type you'll probably have to implement it yourself.该行为是正确的、预期的和适当的。
std::ref是一个辅助函数，用于创建std::reference_wrapper类型的对象，引用包装器是一个具有值语义的类，用于保存引用-- 如果您希望容器跟踪外部引用，那么这正是您想要放入容器中的东西。因此，只需采用您拥有的解决方案即可。
如果愿意的话，您不能拥有直接、裸引用的容器，就像不能拥有引用数组一样。包装纸的设计正是为了满足这种需求。
The behaviour is correct, expected and appropriate.
std::refis a helper function that creates an object of typestd::reference_wrapper<T>, and the reference wrapper is a class with value semantics that holds a reference -- that's exactly the sort of thing you want to put into a container if you want the container to track outside references.So just go with the solution you have.
If you will, you cannot have a container of direct, naked references, much like you cannot have an array of references. The wrapper is designed precisely to accommodate such needs.