什么可以保留在“struct”和“{”除了结构名称？ [英] What can stay between "struct" and "{" except the structure name?

问题描述

这里给出了如何在C ++中使用数据结构的一个清楚的例子。 1 这是链接页面上给出的示例之一：

  struct product {
 int weight; 
 float price; 
}; 
 
产品苹果; 
 product banana，melon; 
  

但是，我有一个不遵循此模板的代码，我不明白。我有以下给定：

  struct result：mppp :: data :: table< 
 row< semantics :: user，int>，
 row< semantics :: exitdatum，spmm :: date>，
 row< userid，int& 
> {}; 
  

我不明白为什么而不是结构名称我们有这么复杂的结构， 。此外，我不明白为什么结构体的body是空的（{和}之间没有任何内容）。

任何人都可以解释一下

感谢您的答案。

现在更清楚了。上面示例中的：表示继承。但是这些结构的意思： aaa ？

解决方案

该代码使用继承。您可以在名称后面指定 struct 的基类，使用：字符分隔它们， public ， protected 或 private 限定符之一指定继承类型（ public 是默认值，如果没有指定（*））：

  struct A {}; // Fine 
 
 struct B：public A {}; //也很好
 
 struct C：B {}; //再次很好，`public`默认是假设
 
 struct D：A，B {}; //也可能（多重继承）
 
 struct E {}; 
 
 struct F：public E，private D {} //限定符可以不同
 
 struct：A，F {} obj; // structs可以是匿名的
  

在你的情况下，基类是模板的一个实例： / p>

  template< typename T> 
 struct X {}; 
 
 struct Y：X< A> {}; // Fine 
  

---

（*）虽然相同的限定符适用于 class 类型的继承，但在这种情况下默认为 private 。

A clear example of how data structures can be used in C++ is given [here].1 This is one of the examples given on the linked page:

struct product {
  int weight;
  float price;
} ;

product apple;
product banana, melon;

However, I have a code that does not follow this template and I cannot understand it. What I have is given bellow:

struct result : mppp::data::table <
    row<semantics::user,       int>,
    row<semantics::exitdatum, spmm::date>,
    row<userid,                int>
> {};

I do not understand why instead of struct name we have such a complex construction and how it should be understood. Moreover, I do not understand why the "body" of the struct is empty (there is nothing between "{" and "}").

Can anybody please explain me that?

ADDED

Thank you for the answers. Now it is more clear. The : in the above example means inheritance. But what all these structures mean: aaa<bbb>?

解决方案

That code uses inheritance. You can specify the base classes of a struct after their name, separating them with a : character and, possibly, using one of the public, protected, or private qualifiers for specifying the type of inheritance (public being the default if none is specified (*)):

struct A { }; // Fine

struct B : public A { }; // Also fine

struct C : B { }; // Fine again, `public` is assumed by default

struct D : A, B { }; // Also possible (multiple inheritance)

struct E { };

struct F : public E, private D { } // Qualifiers can differ

struct : A, F { } obj; // structs can be anonymous 

In your case, the base class is an instance of a template:

template<typename T>
struct X { };

struct Y : X<A> { }; // Fine

---

(*) It is also worth mentioning that while the same qualifiers apply to inheritance for class types, the default is assumed to be private in that case.

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