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本文介紹了成員初始化列表錯誤中的模板基構造函數調用的處理方法，對大家解決問題具有一定的參考價值，需要的朋友們下面隨著小編來一起學習吧！

問題描述

我有一個如下所示的基類:

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:

#include "base.hpp"

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

      virtual Foo someFunc();
};

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

我從 gcc 4.2.1 得到以下錯誤.

I get the following error from gcc 4.2.1.

錯誤:Foo"類沒有任何名為Base"的字段

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

我應該提到這在我運行 gcc 4.6.2 的 Fedora 機器上編譯得很好.在我的 os x Lion 機器上編譯時出現此錯誤.

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

編輯

問題似乎是我在調用構造函數時沒有在 Foo 類中指明模板的類型.以下修復了 os x 中的錯誤.

Problem seems to be 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)

編輯

是的，這確實看起來像一個錯誤.我之前提到的修復了 os x 下的錯誤，并且代碼在 Fedora 中通過該修復可以正常編譯.去看看os x中gcc有沒有更新.

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.

推薦答案

第一:

[C++11: 12.6.2/3]: mem-initializer-list 可以使用任何 class-or- 初始化基類decltype 表示該基類類型.

[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.

[ 示例:

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

——結束示例 ]

而且 Base 應該是一個有效的 injected-class-name 在這里的基礎(也就是說，你可以用它代替 Base;):

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]: 像普通(非模板)類一樣，類模板有一個注入類名(第 9 條).injected-class-name 可以用作 template-name 或 type-name. 當它與模板參數列表，作為模板模板參數的模板參數，或者作為中的最終標識符詳細類型說明符在友元類模板聲明中，它指的是類模板本身.否則，它等價于 template-name 后跟 template-parameters 包含在 <> 中的類模板的template-parameters.

[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]: 類模板或類模板特化的 injected-class-name 可以用作template-name 或 type-name 范圍內的任何位置.[ 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

——結束示例 ]

我沒有發現任何內容表明這不適用于 ctor-initializer，所以我認為這是一個編譯器錯誤.

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.

我的精簡測試用例在 GCC 4.1.2 中失敗和 GCC 4.3.4 但在 GCC 4.5.1 (C++11 模式).它似乎由 GCC bug 189 解決；在 GCC 4.5 發行說明中:

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++ 現在實施 DR 176.以前 G++ 不支持使用模板基類的注入類名稱作為類型名稱，以及名稱的查找發現模板的聲明在封閉范圍.現在查找名稱找到注入的類名稱，它可以用作類型或模板，具體取決于名稱后面是否跟有模板參數列表.作為一個此更改的結果，以前接受的某些代碼可能是格式錯誤，因為

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

注入的類名不可訪問，因為它來自私有庫，或者
注入的類名不能用作模板模板參數的參數.

在這兩種情況中的任何一種情況下，都可以通過添加嵌套名稱說明符顯式命名模板.第一個可以使用 -fno-access-control 解決；第二個只被拒絕與-迂腐.

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.

<小時>

我用 Qt 抽象出來的精簡測試用例:

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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成員初始化列表錯誤中的模板基構造函數調用

問題描述

推薦答案

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