“使用” C ++中的关键字 [英] "using" keyword in C++
问题描述
我正在学习C ++。我的教授使用了一些代码,例如使用filePath = std :: string;
I am learning C++. And my professor uses some code which is something like
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;
我想知道为什么我们使用 c>关键字。为什么我们不能简单地写
I want to know why we are using the using
keyword. Why can't we simply write
std::string filepath;
std::set<filepath> setOfPaths;
...
...
拥有 using
关键字?
推荐答案
使用
关键字用于定义类型别名。您的教授使用它的原因是:
The using
keyword is used to define type aliases. The reasons your professor is using it are:
- 可读性
- 更具描述性
- 避免不必要的
typename
- readability
- being more descriptive
- avoid unnecessary
typename
您可以使用类型别名在语义上(并且仅此)限制特定类型,从而使名称对特定用途更具描述性。
You can use type aliases to semantically (and only that) restrict a specific type, making the name more descriptive for the specific use.
一个例子是:
using fileName = std::string;
fileName
别名用于描述文件名字符串,而不仅仅是任何字符串。
The fileName
alias is used to describe a file name string, not just any string. This makes for readable function signatures too.
我觉得我必须再次迭代:它只是一个别名。任何以 fileName
作为参数的函数都可以与任何 std :: string
参数一起使用。
I feel like I have to iterate this again: it's simply an alias. Any function taking fileName
as an argument will work just fine with any std::string
argument.
有些似乎不必要,例如:
Some may seem unnecessary, like:
using setOfPaths = std::set<filePath>;
,但是在某些情况下,它们实际上可以用来避免指定 typename
在以下情况下:
but in some cases they can be actually used to avoid having to specify typename
in situations like:
template<typename Type>
struct something {
using something_iter = typename std::set<Type>::iterator;
};
具有:
template<typename Container>
using itertype = typename Container::iterator;
template<typename Type>
struct something {
using something_iter = itertype<std::set<Type>>;
};
通过在特定别名中移动 typename
可以在其他多种情况下重用 itertype
,从而有效避免 typename
。
By moving typename
in a specific alias we can reuse itertype
in multiple other occasions effectively avoiding typename
.
还有另一种定义类型别名的方法: typedef
。该关键字从C继承,不允许使用模板别名,例如:
There's another way to define type aliases: typedef
. That keyword is inherited from C and does not allow for templated aliases, like:
template<typename Type>
using vec = std::vector<Type>;
关于类型安全的说明
此实际上,与根本不使用别名相比,类型安全性更高。同样, fileName
和 std :: string
是完全相同的类型。可以互换使用。
A note on type safety
This is not actually any more type safe than not using aliases at all. Again, fileName
and std::string
are exactly the same type. You can use both interchangeably.
下一步可能是定义特定的 fileName
类/结构类型,它自己的特定不变量。
A possible next step would be to define a specific fileName
class/struct type with its own specific invariants.
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