問(wèn)題描述

我有一個(gè)如下所示的基類(lèi):

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(cuò)誤.

I get the following error from gcc 4.2.1.

錯(cuò)誤:Foo"類(lèi)沒(méi)有任何名為Base"的字段

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

我應(yīng)該提到這在我運(yùn)行 gcc 4.6.2 的 Fedora 機(jī)器上編譯得很好.在我的 os x Lion 機(jī)器上編譯時(shí)出現(xiàn)此錯(cuò)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(wèn)題似乎是我在調(diào)用構(gòu)造函數(shù)時(shí)沒(méi)有在 Foo 類(lèi)中指明模板的類(lèi)型.以下修復(fù)了 os x 中的錯(cuò)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(shí)看起來(lái)像一個(gè)錯(cuò)誤.我之前提到的修復(fù)了 os x 下的錯(cuò)誤，并且代碼在 Fedora 中通過(guò)該修復(fù)可以正常編譯.去看看os x中g(shù)cc有沒(méi)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(lèi)decltype 表示該基類(lèi)類(lèi)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(jié)束示例 ]

而且 Base 應(yīng)該是一個(gè)有效的 injected-class-name 在這里的基礎(chǔ)(也就是說(shuō)，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(lèi)一樣，類(lèi)模板有一個(gè)注入類(lèi)名(第 9 條).injected-class-name 可以用作 template-name 或 type-name. 當(dāng)它與模板參數(shù)列表，作為模板模板參數(shù)的模板參數(shù)，或者作為中的最終標(biāo)識(shí)符詳細(xì)類(lèi)型說(shuō)明符在友元類(lèi)模板聲明中，它指的是類(lèi)模板本身.否則，它等價(jià)于 template-name 后跟 template-parameters 包含在 <> 中的類(lèi)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(lèi)模板或類(lèi)模板特化的 injected-class-name 可以用作template-name 或 type-name 范圍內(nèi)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(jié)束示例 ]

我沒(méi)有發(fā)現(xiàn)任何內(nèi)容表明這不適用于 ctor-initializer，所以我認(rèn)為這是一個(gè)編譯器錯(cuò)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(jiǎn)測(cè)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(fā)行說(shuō)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(xiàn)在實(shí)施 DR 176.以前 G++ 不支持使用模板基類(lèi)的注入類(lèi)名稱(chēng)作為類(lèi)型名稱(chēng)，以及名稱(chēng)的查找發(fā)現(xiàn)模板的聲明在封閉范圍.現(xiàn)在查找名稱(chēng)找到注入的類(lèi)名稱(chēng)，它可以用作類(lèi)型或模板，具體取決于名稱(chēng)后面是否跟有模板參數(shù)列表.作為一個(gè)此更改的結(jié)果，以前接受的某些代碼可能是格式錯(cuò)誤，因?yàn)?em class="showen">

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(lèi)名不可訪問(wèn)，因?yàn)樗鼇?lái)自私有庫(kù)，或者
• 注入的類(lèi)名不能用作模板模板參數(shù)的參數(shù).

在這兩種情況中的任何一種情況下，都可以通過(guò)添加嵌套名稱(chēng)說(shuō)明符顯式命名模板.第一個(gè)可以使用 -fno-access-control 解決；第二個(gè)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(shí)>

我用 Qt 抽象出來(lái)的精簡(jiǎn)測(cè)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() {};

這篇關(guān)于成員初始化列表錯(cuò)誤中的模板基構(gòu)造函數(shù)調(diào)用的文章就介紹到這了，希望我們推薦的答案對(duì)大家有所幫助，也希望大家多多支持html5模板網(wǎng)！