我真的需要实现用户提供的构造函数的const对象吗？ [英] Do I really need to implement user-provided constructor for const objects?

问题描述

我有代码：

class A {
  public:
    A() = default;

  private:
    int i = 1;
};

int main() {
  const A a;
  return 0;
}

它在g ++上编译良好（参见ideone ），但在clang ++上失败，并显示错误：

It compiles fine on g++ (see ideone), but fails on clang++ with error:

默认初始化const类型'const A'需要用户提供的默认构造函数

default initialization of an object of const type 'const A' requires a user-provided default constructor

我在 LLVM bug-tracker 并获得了INVALID。

I reported this issue on LLVM bug-tracker and got it INVALID.

我看到它绝对无意义试图说服ang开发商。另一方面，我没有看到这种限制的原因。

I see it absolutly pointless to try to convince the clang developers. On the other side, I don't see the reason for such restriction.

任何人都可以建议，如果C + +11标准不知何故暗示这个代码无效？或者我应该向g ++报告一个错误？

Can anyone advise, if the C++11 Standard somehow implies this code to be invalid? Or should I just report a bug to g++? Or maybe there is enough freedom in language rules to handle this code in many ways?

推荐答案

N3797§8.5/ 7说：

N3797 §8.5/7 says:

如果程序调用一个const限定类型T的对象的默认初始化，T将是一个类型类型，提供默认构造函数。

If a program calls for the default initialization of an object of a const-qualified type T, T shall be a class type with a user-provided default constructor.

没有更多的示例或解释。我同意这似乎很奇怪。此外，当类类型需要用户声明的构造函数时，在C ++ 11中更新的规则比在C ++ 03中更加限制。 （您的构造函数是用户声明的。）

There's no further example or explanation of this. I agree it seems pretty bizarre. Furthermore the rule was updated in C++11 to be more restrictive than it was in C++03, when class types needed user-declared constructors. (Your constructor is user-declared.)

解决方法是使用 {} 或者使用Dietmar的聪明的 inline 定义。

The workaround is be to ask for value initialization using {}, or use Dietmar's clever out-of-class inline definition.

GCC确实提供了诊断

GCC does provide a diagnosis (and quite a nice one, referring to the newer C++11 requirements) if you add another member without an initializer.

  private:
    int i = 1;
    int j;

unmem.cpp:11:11: error: uninitialized const ‘a’ [-fpermissive]
   const A a;
           ^
unmem.cpp:1:7: note: ‘const class A’ has no user-provided default constructor
 class A {
       ^
unmem.cpp:3:5: note: constructor is not user-provided because it is explicitly defaulted in the class body
     A() = default;
     ^
unmem.cpp:7:9: note: and the implicitly-defined constructor does not initialize ‘int A::j’
     int j;

GCC 来源指代 DR 253 ，为什么必须初始化空或全初始化的const对象？标准中的一个开放性问题，最后更新于2011年8月（后C ++ 11）与此注释：

The GCC source refers to DR 253, Why must empty or fully-initialized const objects be initialized? This is an open issue in the standard, last updated in August 2011 (post-C++11) with this note:

构造函数初始化所有子对象，不需要初始化器。

If the implicit default constructor initializes all subobjects, no initializer should be required.

因此，Clang符合C ++ 11与C ++ 14），GCC正在实施标准化委员会的最新思想。

Therefore whereas Clang complies with C++11 (and will comply as-is with C++14), GCC is implementing the latest thinking of the standardization committee.

提交了 GCC错误。我预测，当（和如果）修复错误时，您需要 -pedantic 才能获得诊断。

Filed a GCC bug. I predict that you'll need -pedantic to get a diagnosis when (and if) the bug is fixed.

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