如何使用模板和/或使用结构体？

huangapple
go评论86阅读模式
英文:

How to use templates and/or using`with structs?

问题

以下是您要翻译的部分：

  1. 我有以下定义：
  3. template<typename T>
  4. using refval= std::pair<int, T>;
  6. template<typename T>
  7. using indexedvals = std::vector<refval<T>>; 
  9. 为了使我的代码更易读，我想在“refval”的定义中使用“struct”而不是“pair”，这样在我的代码中可以使用成员名称“index”代替“first”，或者使用“value”代替“second”。
  11. 所以我尝试了：
  13. template<typename T>
  14. typedef struct_t refval {
  15.     refval(int iRefCell, T tValue):m_iRefCell(iRefCell),m_tValue(tValue){};
  16.     int m_iRefCell;
  17.     T   m_tValue;
  18. } refval;
  20. 但是这里编译器报错“error: template declaration of ‘typedef’”。
  22. 然后我尝试了：
  24. template<typename T>
  25. using refval = struct refval_t {
  26.     refval_t():m_iRefCell(-1){};
  27.     refval_t(int iRefCell, T tValue):m_iRefCell(iRefCell),m_tValue(tValue){};
  28.     int m_iRefCell;
  29.     T   m_tValue;
  30. };
  32. template<typename T>
  33. using indexedvals = std::vector<refval<T>>;
  36. 这里我得到了以下投诉：
  38. Sampling.h:12:32: error: types may not be defined in alias template declarations
  39.    12 | using refval = struct refval_t {
  40.       |                                ^
  41. Sampling.h:23:7: error: conflicting declaration of template ‘template<class T> using indexedvals = std::vector<refval_t>’
  42.    23 | using indexedvals = std::vector<refval<T>>;
  43.       |       ^~~~~~~~~~~
  45. 所以我想要做的是以一种方式定义我的模板结构，以便我可以像本帖开头的代码片段中那样在“using”语句中使用它。
  47. 我如何实现这一目标？
  49. （可能一个类似的问题是：如何实现“std::pair”以使其正常工作？）
英文:

I have the following definitions:

  1. template&lt;typename T&gt;
  2. using refval= std::pair&lt;int, T&gt;;
  4. template&lt;typename T&gt;
  5. using indexedvals = std::vector&lt;refval&lt;T&gt;&gt;;

In order to make my code more readable i wanted to use a struct instead of a pair in the definition of refval - so that in my code i could use a member name like index instead of first, or value instead of second.

So i tried

  1. template&lt;typename T&gt;
  2. typedef struct_t refval {
  3.     refval(int iRefCell, T tValue):m_iRefCell(iRefCell),m_tValue(tValue){};
  4.     int m_iRefCell;
  5.     T   m_tValue;
  6. } refval;

But here the compiler complained with "error: template declaration of ‘typedef’".

Then i tried

  1. template&lt;typename T&gt;
  2. using refval = struct refval_t {
  3.     refval_t():m_iRefCell(-1){};
  4.     refval_t(int iRefCell, T tValue):m_iRefCell(iRefCell),m_tValue(tValue){};
  5.     int m_iRefCell;
  6.     T   m_tValue;
  7. };
  9. template&lt;typename T&gt;
  10. using indexedvals = std::vector&lt;refval&lt;T&gt;&gt;;

Here i got the following complaints

  1. Sampling.h:12:32: error: types may not be defined in alias template declarations
  2.    12 | using refval = struct refval_t {
  3.       |                                ^
  4. Sampling.h:23:7: error: conflicting declaration of template ‘template&lt;class T&gt; using indexedvals = std::vector&lt;refval_t&gt;’
  5.    23 | using indexedvals = std::vector&lt;refval&lt;T&gt;&gt;; 
  6.       |       ^~~~~~~~~~~

So what i want to do is to define my templated struct in such a way that i can use it in a using statement as in the code snippets at the beginning of this post.

How can i achieve this?

(probably an analogous question would be: how is std::pair implemented so that this works?)

答案1

得分: 4

坦率地说，你正在猜测它是如何工作的，这永远不会有好结果。

你可以定义一个模板：

  1. template<typename T>
  2. struct refval_t {
  3.     refval_t():m_iRefCell(-1){};
  4.     refval_t(int iRefCell, T tValue):m_iRefCell(iRefCell),m_tValue(tValue){}
  5.     int m_iRefCell;
  6.     T   m_tValue;
  7. };

然后你可以定义一个别名模板：

  1. template<typename T>
  2. using refval = refval_t<T>;

然而，我认为实际上你并不需要这个别名，因为你可以直接使用模板。

请注意，在默认构造函数中也应初始化 m_tValue 成员。按照你当前的写法，很容易在它初始化之前就读取它，并引发未定义的行为。

英文:

Frankly, you are guessing how it works, and this is never going to end well.

You can define a template:

  1. template&lt;typename T&gt;
  2. struct refval_t {
  3.     refval_t():m_iRefCell(-1){};
  4.     refval_t(int iRefCell, T tValue):m_iRefCell(iRefCell),m_tValue(tValue){}
  5.     int m_iRefCell;
  6.     T   m_tValue;
  7. };

and then you can define an alias template:

  1. template&lt;typename T&gt;
  2. using refval = refval_t&lt;T&gt;;

However, I don't think you actually need the alias when you can just use the template directly.

Note that you should initialize the m_tValue member also in the default constructor. As you have it currently, it is too easy to read from it before it is initialized and invoke undefined behavior.

huangapple
  • 本文由 发表于 2023年4月17日 21:48:16
  • 转载请务必保留本文链接：https://go.coder-hub.com/76035867.html
  • c++
  • templates
  • using
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