[英]“using” keyword in C++
I am learning C++. 我正在学习C ++。 And my professor uses some code which is something like
我的教授使用了一些类似的代码
using filePath = std::string;
using setOfPaths = std::set<filePath>;
using iterOfSet = setOfPaths::iterator;
using listOfIter = std::list<iterOfSet>;
using iterList = listOfIter::iterator;
using fileName = std::string;
using mapOfFileName = std::map<fileName, listOfIter>;
using iterOfMap = mapOfFileName::iterator;
setOfPaths _setOfPaths;
mapOfFileName _mapOfFileName;
iterOfSet setIter;
I want to know why we are using the using
keyword. 我想知道为什么我们使用
using
关键字。 Why can't we simply write 为什么我们不能简单地写
std::string filepath;
std::set<filepath> setOfPaths;
...
...
What is the benefit of having using
keyword? using
关键字有什么好处?
The using
keyword is used to define type aliases. using
关键字用于定义类型别名。 The reasons your professor is using it are: 你的教授使用它的原因是:
typename
typename
You can use type aliases to semantically (and only that) restrict a specific type, making the name more descriptive for the specific use. 您可以使用类型别名来语义(并且仅限于此)限制特定类型,使名称对特定用途更具描述性。
An example is: 一个例子是:
using fileName = std::string;
The fileName
alias is used to describe a file name string, not just any string. fileName
别名用于描述文件名字符串,而不仅仅是任何字符串。 This makes for readable function signatures too. 这也使得可读功能签名成为可能。
I feel like I have to iterate this again: it's simply an alias. 我觉得我必须再次迭代这个:它只是一个别名。 Any function taking
fileName
as an argument will work just fine with any std::string
argument. 将
fileName
作为参数的任何函数都可以与任何std::string
参数一起使用。
typename
s typename
Some may seem unnecessary, like: 有些似乎是不必要的,如:
using setOfPaths = std::set<filePath>;
but in some cases they can be actually used to avoid having to specify typename
in situations like: 但在某些情况下,它们实际上可用于避免在以下情况下指定
typename
:
template<typename Type>
struct something {
using something_iter = typename std::set<Type>::iterator;
};
with: 有:
template<typename Container>
using itertype = typename Container::iterator;
template<typename Type>
struct something {
using something_iter = itertype<std::set<Type>>;
};
By moving typename
in a specific alias we can reuse itertype
in multiple other occasions effectively avoiding typename
. 通过在特定别名中移动
typename
,我们可以在多个其他场合重用itertype
,从而有效地避免使用typename
。
typedef
typedef
There's another way to define type aliases: typedef
. 还有另一种定义类型别名的方法:
typedef
。 That keyword is inherited from C and does not allow for templated aliases, like: 该关键字继承自C,不允许模板化别名,例如:
template<typename Type>
using vec = std::vector<Type>;
This is not actually any more type safe than not using aliases at all. 这实际上不比不使用别名更安全。 Again,
fileName
and std::string
are exactly the same type. 同样,
fileName
和std::string
的类型完全相同。 You can use both interchangeably. 您可以互换使用。
A possible next step would be to define a specific fileName
class/struct type with its own specific invariants. 可能的下一步是定义具有其自己的特定不变量的特定
fileName
类/结构类型。
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