問題描述

在下面的例子中，A 有一個(gè)成員 typedef Instantiate，它導(dǎo)致 B 的實(shí)例化.

In the following example, A has a member typedef Instantiate which causes the instantiation of B<A>.

template<typename T>
struct B
{
    typedef typename T::Before Before; // ok
    typedef typename T::After After; // error: no type named 'After' in 'A<int>'
};

template<typename T>
struct A
{
    typedef int Before;
    typedef typename B<A>::After Instantiate;
    typedef int After;
};

template struct A<int>; // instantiate A<int>

我嘗試過的所有編譯器都報(bào)告說，雖然 A::Before 可見，但 A::After 不可見.這種行為是否符合標(biāo)準(zhǔn)?如果是這樣，標(biāo)準(zhǔn)在哪里指定在 B<A> 的實(shí)例化過程中 A 中的哪些名稱應(yīng)該是可見的?

All the compilers I've tried report that, while A::Before is visible, A::After is not. Is this behaviour compliant with the standard? If so, where does the standard specify which names in A should be visible during instantiation of B<A>?

如果依賴名稱是在模板實(shí)例化點(diǎn)查找"，那么在名稱由模板參數(shù)(例如 T::After)限定的情況下，這意味著什么?

If dependent names are "looked up at the point of the template instantiation", what does this mean in the scenario of a name qualified by a template parameter such as T::After?

請(qǐng)注意，當(dāng) A 不是模板時(shí)會(huì)發(fā)生相同的行為:

Note that the same behaviour occurs when A is not a template:

template<typename T>
struct B
{
    typedef typename T::Before Before; // ok
    typedef typename T::After After; // error: no type named 'After' in 'A'
};

struct A
{
    typedef int Before;
    typedef B<A>::After Instantiate;
    typedef int After;
};

.. 和 G++ 接受以下內(nèi)容，但 Clang 不接受:

.. and G++ accepts the following, but Clang does not:

template<typename T>
struct B
{
    static const int value = 0;
    static const int i = T::value; // clang error: not a constant expression
};

struct A
{
    static const int value = B<A>::value;
};

在閱讀了 C++03 標(biāo)準(zhǔn)之后:

After some reading of the C++03 standard:

[temp.dep.type] 一個(gè)類型是依賴的，如果它是一個(gè)模板參數(shù)

[temp.dep.type] A type is dependent if it is a template parameter

因此 T 是依賴的.

[temp.res] 在查找模板定義中使用的名稱的聲明時(shí)，通常的查找規(guī)則用于非依賴名稱.依賴模板參數(shù)的名稱查找被推遲到實(shí)際模板參數(shù)已知.

[temp.res] When looking for the declaration of a name used in a template definition, the usual lookup rules are used for nondependent names. The lookup of names dependent on the template parameters is postponed until the actual template argument is known.

T::After 的查找因此被推遲，直到 T 的參數(shù)已知.

The lookup of T::After is therefore postponed until the argument for T is known.

[temp.inst] 除非已顯式實(shí)例化類模板特化……當(dāng)在需要完全定義的對(duì)象類型的上下文中引用特化時(shí)，類模板特化會(huì)被隱式實(shí)例化.

[temp.inst] Unless a class template specialization has been explicitly instantiated ... the class template specialization is implicitly instantiated when the specialization is referenced in a context that requires a completely-defined object type.

因此 A<int>::Instantiate 的聲明需要 B<A> 的實(shí)例化(因?yàn)樗糜谇短酌Q說明符.)

Therefore the declaration of A<int>::Instantiate requires the instantiation of B<A> (because it is used in a nested-name-specifier.)

A<int>::After 在 A<int>::Instantiate 的聲明點(diǎn)不可見，所以編譯器的行為是有道理的 -但我在 C++03 中沒有看到任何明確描述這種行為的內(nèi)容.最接近的是這個(gè)有點(diǎn)模糊的段落:

A<int>::After is not visible at the point of declaration of A<int>::Instantiate, so the behaviour of the compiler makes sense - but I haven't seen anything in C++03 that explicitly describes this behaviour. The closest thing was this somewhat vague paragraph:

[temp.dep.res]在解析依賴名稱時(shí)，會(huì)考慮來自以下來源的名稱:

[temp.dep.res] In resolving dependent names, names from the following sources are considered:

——在模板定義點(diǎn)可見的聲明.

— Declarations that are visible at the point of definition of the template.

推薦答案

typename T::Before 是否有效在規(guī)范中沒有明確說明.它是缺陷報(bào)告的主題(因?yàn)榭梢苑浅：侠淼亻喿x標(biāo)準(zhǔn)來禁止它):http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_active.html#287.

Whether typename T::Before is valid is not explicitly said by the spec. It is subject of a defect report (because the Standard can very reasonably be read to forbid it): http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_active.html#287 .

typename T::After 是否無效也可以很合理地被規(guī)范讀取為真的，實(shí)際上它有相當(dāng)多的意義(并且前面提到的 DR 仍然保持格式錯(cuò)誤).因?yàn)槟幸粋€(gè)類 A 的實(shí)例化，它在成員 Baz 具有的期間引用了另一個(gè)類 A尚未聲明，這會(huì)引用回 A<Foo>::Bar.在非模板的情況下，這也是格式錯(cuò)誤的(嘗試暫時(shí)忘記"您正在處理模板:B<A>::After 的查找肯定已完成A 模板被完全解析，但不是在它的具體實(shí)例化完全創(chuàng)建之后.實(shí)際上是它的實(shí)例化做參考！)

Whether typename T::After is invalid can also very reasonably be read to be true by the spec, and actually it makes quite a bit of sense (and aforementioned DR still keeps it ill-formed). Because you have an instantiation of a class A<Foo>, which references another class A<Bar> during a period where a member Baz has not yet been declared, and that makes a reference back to A<Foo>::Bar. That is ill-formed in the case of non-templates aswell (try to "forget" for a moment that you are dealing with templates: surely the lookup of B<A>::After is done after the A template was completely parsed, but not after the specific instantiation of it was completely created. And it is the instantiation of it that actually will do the reference!).

struct A {
   typedef int Foo;
   typedef A::Foo Bar; // valid
   typedef A::Baz Lulz; // *not* valid
   typedef int Baz; 
};

這篇關(guān)于實(shí)例化模板時(shí)，其不完整參數(shù)類型的成員是否應(yīng)該可見?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)！