为什么不能在D中实现结构的默认构造函数？ [英] Why can I not implement default constructors for structs in D?

问题描述

编写代码

  struct S 
 {
 this（）//编译时错误
 {
} 
} 
  

给我一​​个错误消息说

仅允许使用@disable且没有主体的结构的默认构造函数。

为什么？

解决方案

这种情况之一比最初预期的要复杂得多。

D与C ++相比，重要且有用的功能之一是，每个单个类型（包括所有用户类型）都有一些初始的非垃圾值，可以在编译时对其进行评估。它用作 T.init ，并具有两个重要的用例：

1. 模板约束可以使用T.init值来检查是否可以对给定类型执行某些操作（引用Kenji Hara的代码段）：

     template isSomething（T）{
   枚举isSomething = is（typeof（{
    // T t1; //如果T是嵌套结构或@disable this（）
   则不好// T t2 = void; auto x = t2; //如果T是不可更改类型
   则不好T T = T.init; //避免默认构造检查
    ...使用t。 .. 
   }））； 
   } 
     




2. 您的变量始终正确初始化，除非您明确使用 int i = void 语法。

鉴于此，出现了一个难题。我们应该保证T（）和T.init是相同的（就像许多C ++程序员所期望的那样）还是允许默认构造轻松地破坏该保证。据我所知，尽管令人惊讶，但还是做出了第一种方法更安全的决定。

但是，讨论不断涌现，提出了各种改进建议（例如，允许CTFE-能够使用默认构造函数）。 这样的线程最近才出现。 / p>

Writing code like

struct S
{
    this() // compile-time error
    {
    }
}

gives me an error message saying

default constructor for structs only allowed with @disable and no body.

Why??

解决方案

This is one of cases much more tricky than one can initially expect.

One of important and useful features D has over C++ is that every single type (including all user types) has some initial non-garbage value that can be evaluated at compile-time. It is used as T.init and has two important use cases:

1. Template constraints can use T.init value to check if certain operations can be done on given type (quoting Kenji Hara's snippet):

   template isSomething(T) {
      enum isSomething = is(typeof({
          //T t1;                      // not good if T is nested struct, or has @disable this()
          //T t2 = void; auto x = t2;  // not good if T is non-mutable type
          T t = T.init;                // avoid default construct check
          ...use t...
      }));
   }
   

2. Your variables are always initialized properly unless you explicitly use int i = void syntax. No garbage possible.

Given that, difficult question arises. Should we guarantee that T() and T.init are the same (as many programmers coming from C++ will expect) or allow default construction that may easily destroy that guarantee. As far as I know, decision was made that first approach is safer, despite being surprising.

However, discussions keep popping with various improvements proposed (for example, allowing CTFE-able default constructor). One such thread has appeared very recently.

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