问题描述
我对类模板中的内部类有疑问。我有一个模板类（例如： Matrix< T> ）和一个子类（例如： Matrix< T> :: Row ）。现在，我想编写一个对子类实例进行操作的函数（例如： negate（Matrix< T> :: Row&））。我试图用 template< class T>声明函数。 negate（typename Matrix< T> :: Row&），但是当我尝试使用它时，编译器告诉我找不到匹配项。
I have a problem with inner classes in class templates. I have a template class (say: Matrix<T>), and a subclass (say: Matrix<T>::Row). Now I want to to write a function which operates on instances of the subclass (say: negate(Matrix<T>::Row &)). I tried to declare the function with template<class T> negate(typename Matrix<T>::Row &), but when I try to use it, the compiler tells me that it cannot find a match.
这是一个抽象示例：
template<class T>
class A
{
public:
class B
{
};
};
template<class T>
void x(typename A<T>::B &)
{
}
int main()
{
A<int>::B b;
x(b); // doesn't work: Error: Could not find a match
// for x<T>(A<int>::B) needed in main().
x<int>(b); // works fine
}
为什么编译器无法找到 x 在第一种情况下？有没有办法修改它的工作原理（无需显式指定类型 int ）？
Why does the compiler does not manage to find x in the first case? Is there a way to modify this that it works (without explicitly specifying the type int)?
（我也遇到类似的问题，其中 x 的形式为 template< class T ，类S> void x（typename A< T> :: B& ;, const S&）; ，我真的不想在执行调用时被迫显式命名所有类型。）
(I also have similar problems where x is of the form template<class T, class S> void x(typename A<T>::B &, const S &);, whence I would really like not to be forced to explicitly name all types while doing the call.)
我已经使用g ++ 4.4.3，g ++ 4.5.2和Sun Studio 5.9尝试了全部相同的结果。
I have tried this with g++ 4.4.3, g++ 4.5.2, and Sun Studio 5.9, all give the same result. Thanks a lot in advance for anything helpful!
推荐答案
编译器应如何推断呢？想象以下设置：
How should the compiler be able to deduce this? Imagine the following setup:
struct A { typedef int T; };
struct B { typedef int T; };
template <typename S> void foo(typename S::T);
现在，当您说 int x; foo（x）; ，没有明确的匹配方法。
Now when you say int x; foo(x);, there's no way to match this unambiguously.
要点是，您不是从给定值中推导出模板参数类模板，而是一个任意的独立类型。该类型在另一个类中定义的事实与此无关。
The point is that you are not deducing a template parameter from a given class template, but rather just an arbitrary, free-standing type. The fact that that type was defined inside another class is not relevant for that.
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