对基类模板成员函数的模棱两可的访问 [英] Ambiguous access to base class template member function

问题描述

在Visual Studio 2008中，编译器无法将对下面 _tmain 中的 SetCustomer 的调用解析为明确的内容:

In Visual Studio 2008, the compiler cannot resolve the call to SetCustomer in _tmain below and make it unambiguous:

template <typename TConsumer>
struct Producer
{
    void SetConsumer(TConsumer* consumer) { consumer_ = consumer; }

    TConsumer* consumer_;
};

struct AppleConsumer
{
};

struct MeatConsumer
{
};

struct ShillyShallyProducer : public Producer<AppleConsumer>,
                              public Producer<MeatConsumer>
{
};

int _tmain(int argc, _TCHAR* argv[])
{
    ShillyShallyProducer producer;
    AppleConsumer consumer;
    producer.SetConsumer(&consumer);   //  <--- Ambiguous call!!

    return 0;
}

这是编译错误:

// error C2385: ambiguous access of 'SetConsumer'
//    could be the 'SetConsumer' in base 'Producer<AppleConsumer>'
//    or could be the 'SetConsumer' in base 'Producer<MeatConsumer>'

我认为模板参数查找机制足够聪明，可以推断出正确的基础 Producer .为什么不呢?

I thought the template argument lookup mechanism would be smart enough to deduce the correct base Producer. Why isn't it?

我可以通过将 Producer 更改为

template <typename TConsumer>
struct Producer
{
    template <typename TConsumer2>
    void SetConsumer(TConsumer2* consumer) { consumer_ = consumer; }

    TConsumer* consumer_;
};

并以

调用 SetConsumer

    producer.SetConsumer<AppleConsumer>(&consumer);   // Unambiguous call!!

但是如果我不必...那就更好了.

but it would be nicer if I didn't have to...

推荐答案

我认为模板参数查找机制足够聪明，可以推断出正确的基本Producer.

I thought the template argument lookup mechanism would be smart enough to deduce the correct base Producer.

这与模板无关，它来自使用多个基类-名称查找已经模棱两可，并且仅在此之后才进行重载解析.

This hasn't to do with templates, it comes from using multiple base classes - the name lookup is already ambiguous and overload resolution only takes place after that.

一个简化的示例如下:

struct A { void f()    {} };
struct B { void f(int) {} };
struct C : A, B {};

C c;
c.f(1); // ambiguous

变通办法是明确限定调用或将功能引入派生类范围:

Workarounds are explicitly qualifying the call or to introduce the functions into the derived classes scope:

 struct ShillyShallyProducer : public Producer<AppleConsumer>,
                               public Producer<MeatConsumer>
 {
     using Producer<AppleConsumer>::SetConsumer;
     using Producer<MeatConsumer >::SetConsumer;
 };

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