模板基础构造函数调用成员初始化列表错误 [英] Template base constructor call in member initialization list error

问题描述

我有一个基类，如下所示。

I have a base class that looks like the following.

template<typename T>
class Base
{
   public:
      Base(int someValue);

      virtual T someFunc() =0;
};

template<typename T>
Base<T>::Base(int someValue)
{}

然后是以下。

#include "base.hpp"

class Foo
   : public Base<Foo>
{
   public:
      Foo(int someValue);

      virtual Foo someFunc();
};

Foo::Foo(int someValue)
   : Base(someValue)
{}

我从gcc 4.2.1得到以下错误。

I get the following error from gcc 4.2.1.

error: class ‘Foo’ does not have any field named ‘Base’

我应该提到这个编译在我的Fedora框正在运行gcc 4.6.2。在我的os x Lion机器上编译时出现此错误。

I should mention this compiles fine on my Fedora box wich is running gcc 4.6.2. This error occurs when compiling on my os x Lion machine.

感谢您提供帮助。

问题似乎在调用构造函数时，我没有在Foo类中指示模板类型。以下修复了os x中的错误。

Problem seems that I am not indicating type of template in the Foo class when calling the constructor. The following fixes the error in os x.

: Base<Foo>(someValue, parent)

EDIT

是的，这看起来像一个错误。我之前提到的修复os x下的错误和代码编译在fedora与该修复。将在os x中查看gcc是否有更新。

Yes this does look like a bug. What I mentioned before fixes the error under os x and code compiles fine in fedora with that fix. Will go and see if there is an update to gcc in os x.

推荐答案

第一：

[C ++ 11：12.6.2 / 3]： A mem-initializer-list 可以使用表示基类类型的任何 class-or-decltype 初始化一个基类。

[C++11: 12.6.2/3]: A mem-initializer-list can initialize a base class using any class-or-decltype that denotes that base class type.

struct A { A(); };
typedef A global_A;
struct B { };
struct C: public A, public B { C(); };
C::C(): global_A() { } // mem-initializer for base A

-end example ]

和基本应该是这里的基础的有效的 inject-class-name （也就是说，您可以使用它代替 Base< T> ）：

And Base should be a valid injected-class-name for the base here (that is, you can use it in place of Base<T>):

[C ++ 11：14.6.1 / 1]： 与普通（非模板）类类似，类模板具有 inject-class-name （第9条）。 inject-class-name 可用作模板名称或类型名称。 模板参数列表作为模板的模板参数，或作为详细说明了一个朋友类模板声明的
，它引用了类模板本身。否则，它等同于<> 中包含的类模板的 template-name ，后跟 template-parameters / code> 。

[C++11: 14.6.1/1]: Like normal (non-template) classes, class templates have an injected-class-name (Clause 9). The injected-class-name can be used as a template-name or a type-name. When it is used with a template-argument-list, as a template-argument for a template template-parameter, or as the final identifier in the elaborated-type-specifier of a friend class template declaration, it refers to the class template itself. Otherwise, it is equivalent to the template-name followed by the template-parameters of the class template enclosed in <>.

[C ++ 11：14.6.1 / 3]：类模板的 injected-class-name 类模板专门化可以作为 template-name 或 type-name 使用，无论它在范围内。 [示例：

[C++11: 14.6.1/3]: The injected-class-name of a class template or class template specialization can be used either as a template-name or a type-name wherever it is in scope. [ Example:

template <class T> struct Base {
   Base* p;
};

template <class T> struct Derived: public Base<T> {
   typename Derived::Base* p; // meaning Derived::Base<T>
};

template<class T, template<class> class U = T::template Base> struct Third { };
Third<Base<int> > t; // OK: default argument uses injected-class-name as a template

]

我没有发现任何内容表示这不适用于 ctor-

I haven't found anything to indicate that this doesn't apply in the ctor-initializer, so I'd say that this is a compiler bug.

我已删除的测试用例在GCC 4.1.2 和 GCC 4.3.4 中失败，但在GCC 4.5.1（C ++ 11模式）中成功。它似乎由 GCC错误189 解决;在 GCC 4.5发行说明中：

My stripped-down testcase fails in GCC 4.1.2 and GCC 4.3.4 but succeeds in GCC 4.5.1 (C++11 mode). It seems to be resolved by GCC bug 189; in the GCC 4.5 release notes:

G ++现在实现了 DR 176 。以前，G ++不支持将模板基类的
inject-class-name用作类型名称，而
查找名称在
封闭范围中找到模板的声明。现在查找名称找到inject-class-name，
，它可以用作类型或模板，取决于
，无论名称是否后跟模板参数列表。作为此更改的
结果，以前接受的一些代码可能是
错误，因为

G++ now implements DR 176. Previously G++ did not support using the injected-class-name of a template base class as a type name, and lookup of the name found the declaration of the template in the enclosing scope. Now lookup of the name finds the injected-class-name, which can be used either as a type or as a template, depending on whether or not the name is followed by a template argument list. As a result of this change, some code that was previously accepted may be ill-formed because

• inject-class-name不可访问，因为它来自私有基础，或


• inject-class-name不能用作模板模板参数的参数。

在这两种情况下，代码都可以通过添加
nested-name-specifier来明确命名模板。第一个可以
使用-fno-access-control;第二个仅被拒绝
with -pedantic。

In either of these cases, the code can be fixed by adding a nested-name-specifier to explicitly name the template. The first can be worked around with -fno-access-control; the second is only rejected with -pedantic.

---

具有Qt的下拉测试用例抽象出来：

---

My stripped-down testcase with Qt abstracted out:

template <typename T>
struct Base { };

struct Derived : Base<Derived> { // I love the smell of CRTP in the morning
   Derived();
};

Derived::Derived() : Base() {};

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