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# decltype那些事
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# Things about decltype
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## 关于作者：
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## About Author：
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个人公众号：
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## 1.基本使用
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decltype的语法是:
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## 1.Basic
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The syntax of decltype is:
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```
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decltype (expression)
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```
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这里的括号是必不可少的,decltype的作用是“查询表达式的类型”，因此，上面语句的效果是，返回 expression 表达式的类型。注意，decltype 仅仅“查询”表达式的类型，并不会对表达式进行“求值”。
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Brackets are essential here.The function of decltype is "the type of query expression".Therefore, the effect of the above statement is to return the type of the expression expression.Note that decltype only "queries" the type of the expression and does not "evaluate" the expression.
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### 1.1 推导出表达式类型
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### 1.1 Deduce expression type
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```
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int i = 4;
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decltype(i) a; //推导结果为int。a的类型为int。
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decltype(i) a; //The result is int. The type of a is int
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```
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### 1.2 与using/typedef合用，用于定义类型。
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### 1.2 Used with using / typedef to define types.
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```c++
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using size_t = decltype(sizeof(0));//sizeof(a)的返回值为size_t类型
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@ -37,11 +36,12 @@ for (vectype i = vec.begin; i != vec.end(); i++)
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}
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```
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这样和auto一样，也提高了代码的可读性。
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like auto, improves the readability of the code.
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### 1.3 重用匿名类型
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在C++中，我们有时候会遇上一些匿名类型，如:
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### 1.3 Reuse anonymous types
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In C + +, we sometimes encounter some anonymous types, such as:
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```c++
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struct
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@ -51,15 +51,13 @@ struct
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}anon_s;
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```
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而借助decltype，我们可以重新使用这个匿名的结构体：
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With decltype, we can reuse this anonymous structure:
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```c++
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decltype(anon_s) as ;//定义了一个上面匿名的结构体
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decltype(anon_s) as ;//Defines an anonymous structure above
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```
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### 1.4 泛型编程中结合auto，用于追踪函数的返回值类型
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### 1.4 In generic programming, auto is used to trace the return value type of function
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这也是decltype最大的用途了。
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```c++
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template <typename T>
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@ -69,24 +67,26 @@ auto multiply(T x, T y)->decltype(x*y)
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}
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```
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完整代码见：[decltype.cpp](decltype.cpp)
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Code：[decltype.cpp](decltype.cpp)
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## 2.判别规则
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## 2.Discriminant rules
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对于decltype(e)而言，其判别结果受以下条件的影响：
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如果e是一个没有带括号的标记符表达式或者类成员访问表达式，那么的decltype（e）就是e所命名的实体的类型。此外，如果e是一个被重载的函数，则会导致编译错误。
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否则 ，假设e的类型是T，如果e是一个将亡值，那么decltype（e）为T&&
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否则，假设e的类型是T，如果e是一个左值，那么decltype（e）为T&。
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否则，假设e的类型是T，则decltype（e）为T。
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标记符指的是除去关键字、字面量等编译器需要使用的标记之外的程序员自己定义的标记，而单个标记符对应的表达式即为标记符表达式。例如：
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For decltype (E), the results are affected by the following conditions:
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If e is a marker expression or class member access expression without parentheses, decltype (E) of is the type of entity named by E.In addition, if e is an overloaded function, it will result in compilation errors.
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Otherwise, assume that the type of E is t, and if e is a dying value, then decltype (E) is t&&
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Otherwise, assume that the type of E is t, and if e is a dying value, then decltype (E) is t&&
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Otherwise, assuming that the type of E is t, decltype (E) is t.
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Markers are defined by programmers except for keywords, literals and other tags that the compiler needs to use. The expression corresponding to a single marker is a marker expression. For example:
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```c++
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int arr[4]
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```
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则arr为一个标记符表达式，而arr[3]+0不是。
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Then arr is a marker expression，ranther than arr[3]+0
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举例如下：
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Example：
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```c++
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int i = 4;
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@ -94,11 +94,11 @@ int arr[5] = { 0 };
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int *ptr = arr;
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struct S{ double d; }s ;
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void Overloaded(int);
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void Overloaded(char);//重载的函数
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void Overloaded(char);// reload function
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int && RvalRef();
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const bool Func(int);
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//规则一：推导为其类型
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//Rule 1: derivation is its type
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decltype (arr) var1; //int 标记符表达式
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decltype (ptr) var2;//int * 标记符表达式
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@ -111,11 +111,11 @@ decltype(s.d) var3;//doubel 成员访问表达式
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decltype (RvalRef()) var5 = 1;
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//规则三：左值，推导为类型的引用。
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//规则三：Lvalue, derived as a reference to the type
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decltype ((i))var6 = i; //int&
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decltype (true ? i : i) var7 = i; //int& 条件表达式返回左值。
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decltype (true ? i : i) var7 = i; //int& A conditional expression returns an lvalue
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decltype (++i) var8 = i; //int& ++i返回i的左值。
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@ -126,7 +126,7 @@ decltype(*ptr)var10 = i;//int& *操作返回左值
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decltype("hello")var11 = "hello"; //const char(&)[9] 字符串字面常量为左值，且为const左值。
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//规则四：以上都不是，则推导为本类型
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//Rule 4: if none of the above is true, this type is derived
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decltype(1) var12;//const int
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@ -135,4 +135,4 @@ decltype(Func(1)) var13=true;//const bool
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decltype(i++) var14 = i;//int i++返回右值
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```
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学习参考：https://www.cnblogs.com/QG-whz/p/4952980.html
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From：https://www.cnblogs.com/QG-whz/p/4952980.html
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