[英]accessing class member regardless of it being a function or a data member
I want to write a generic accessor for a class member regardless whether it is a function or or a data member:我想为 class 成员编写通用访问器,无论它是 function 还是数据成员:
#include <type_traits>
namespace traits
{
template <typename T, typename = void>
struct _name;
template <typename T>
struct _name<T, std::void_t<decltype(std::declval<T>().name)>>
{
constexpr decltype(auto) operator()(const T &t) const
{
return t.name;
}
};
template <typename T>
struct _name<T, std::void_t<decltype(std::declval<T>().name())>>
{
constexpr decltype(auto) operator()(const T &t) const
{
return t.name();
}
};
template <typename T>
decltype(auto) name(const T &t)
{
return _name<T>{}(t);
}
}
#include <string>
struct beb
{
std::string name = "beb";
};
struct bob
{
std::string name() const
{
return "bob";
}
};
#include <iostream>
int main()
{
std::cout << traits::name(bob());
std::cout << traits::name(beb());
}
I am using SFINAE with void_t
specialization, but it works with only single specialization.我正在使用具有
void_t
专业化的 SFINAE,但它仅适用于单一专业化。 Otherwise it gives an error saying error C2953: 'traits::_name<T,void>': class template has already been defined
.否则会给出错误
error C2953: 'traits::_name<T,void>': class template has already been defined
。
MSVC latest: https://godbolt.org/z/h9WT58z8P - does not compile MSVC 最新: https://godbolt.org/z/h9WT58z8P - 不编译
GCC 12.2: https://godbolt.org/z/vc3K1M7x5 - compiles GCC 12.2: https://godbolt.org/z/vc3K1M7x5 - 编译
Clang 15.0.0: https://godbolt.org/z/dqGEMfYWK - does not compile Clang 15.0.0: https://godbolt.org/z/dqGEMfYWK - 无法编译
_name
is a customization point within a traits
namespace. _name
是traits
命名空间中的自定义点。 By default it accesses name
using name
or name()
.默认情况下,它使用
name
或name()
访问name
。 But one can provide a specialization to use, for example getName()
.但是可以提供一种专门的使用方法,例如
getName()
。
You seem to be reinventing std::invoke
.您似乎正在重新发明
std::invoke
。 This function embodies the definition of Callable concept, and that definition has two special cases:这个function体现了Callable概念的定义,该定义有两种特殊情况:
std::invoke(&C::dataMember, obj)
is equivalent to obj.*dataMember
std::invoke(&C::dataMember, obj)
等效于obj.*dataMember
std::invoke(&C::memFun, obj, a, b, c)
is equivalent to (obj.*memFun)(a, b, c)
std::invoke(&C::memFun, obj, a, b, c)
is equivalent to (obj.*memFun)(a, b, c)
Putting this together, your name
can be implemented simply as综上所述,您的
name
可以简单地实现为
template <typename T>
decltype(auto) name(const T &t)
{
return std::invoke(&T::name, t);
}
It will do the right thing whether name
is a data member or a member function.无论
name
是数据成员还是成员 function,它都会做正确的事情。 Demo演示
If you want _name
as a customization point, just add an extra indirection: make name
call _name
, and the default implementation of _name
call std::invoke
.如果您希望
_name
作为自定义点,只需添加一个额外的间接:使name
调用_name
,而_name
的默认实现调用std::invoke
。
声明:本站的技术帖子网页,遵循CC BY-SA 4.0协议,如果您需要转载,请注明本站网址或者原文地址。任何问题请咨询:yoyou2525@163.com.