什么是decltype，它是如何使用的？ [英] What is decltype and how is it used?

问题描述

我没有找到一个很好的解释decltype。请告诉我，作为一个开始的程序员，它是什么，为什么它是有用的。

I haven't been able to find a good explanation of decltype. Please tell me, as a beginning programmer, what it does and why it is useful.

例如，我正在读一本书，问了以下问题。有人可以向我解释一下答案，以及为什么，以及一些好的（初级水平）示例？

For example, I am reading a book that asked the following question. Can someone explain to me the answer and why, along with some good (beginner-level) examples?

变量和每个变量在代码结束时有什么值？

What would be the type of each variable and what value would each variable have when the code finishes?

int a = 3, b = 4;    
decltype(a) c = a;

decltype((b)) d = a; 

++c; 

++d;

逐行解释会非常有帮助。 / p>

A line-by-line explanation would be very helpful.

推荐答案

decltype 是一种指定类型的方法： ， decltype 会返回一个对应于表达式类型的类型。具体来说， decltype（e）是以下类型：

decltype is a way to specify a type: You give it an expression, and decltype gives you back a type which corresponds to the type of the expression. Specifically, decltype(e) is the following type:

• e 是变量的名称，即id-expression，则结果类型是变量的类型。

• If e is the name of a variable, i.e. an "id-expression", then the resulting type is the type of the variable.

否则，如果 e 计算为类型 T 的左值，是 T& ，并且如果 e 评估类型 T ，则结果类型为 T 。

Otherwise, if e evaluates to an lvalue of type T, then the resulting type is T &, and if e evaluates to an rvalue of type T, then the resulting type is T.

将这些规则与参考折叠规则相结合，可以使你理解 decltype（e）&& ，它总是一个合适的引用。 （C ++ 14还添加 decltype（auto），以给出类型扣除 auto decltype 的类别语义。）

Combining these rules with reference collapsing rules allows you to make sense of decltype(e) &&, which is always a "suitable" reference. (C++14 also adds decltype(auto) to give you the type-deduction of auto combined with the value category semantics of decltype.)

示例：

int foo();
int n = 10;

decltype(n) a = 20;             // a is an "int" [id-expression]

decltype((n)) b = a;            // b is an "int &" [(n) is an lvalue]

decltype(foo()) c = foo();      // c is an "int" [rvalue]

decltype(foo()) && r1 = foo();  // int &&
decltype((n)) && r2 = n;        // int &

可能值得强调 auto 和 decltype ： auto 适用于类型

It might be worth stressing the difference between auto and decltype: auto works on types, and decltype works on expressions.

您不应该看到或使用 decltype code>在日常编程中。它在泛型（模板）库代码中最有用，其中有问题的表达式是未知的，并且取决于参数。 （相比之下， auto 可以在整个地方使用。）总之，如果你刚开始编程，你可能不需要使用 decltype 一段时间。

You shouldn't be seeing or using decltype in "day-to-day" programming. It is most useful in generic (templated) library code, where the expression in question is not known and depends on a paramater. (By contrast, auto may be used generously all over the place.) In short, if you're new to programming, you probably won't need to use decltype for some time.

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