模板化派生类时访问基本成员数据错误 [英] Accessing base member data error when derived class is templated

问题描述

奇怪的重复性模板存在以下问题，当我尝试访问CRTP基类的数据成员时遇到问题.

I have the following problem with the curiously recurring template, with a problem when I try to access the data member of CRTP base class.

template<typename T>
struct Base {
  int protectedData=10;
};

struct Derived : public Base<Derived> {
public:
  void method() {
    std::cout<<protectedData<<std::endl;
  };
};

int main ()
{
  Derived a;
  a.method();
}

上面的代码可以编译并正常运行，可以打印出"10"，但是如果我将派生类进行模板化，例如:

The above code compiles and runs fine and I can get "10" printed, but if I have the derived class templated, like:

template<typename T>
struct Base {
  int protectedData=10;
};

template<typename T>
struct Derived : public Base<Derived<T> > {
public:
  void method() {
    std::cout<<protectedData<<std::endl;
  };
};

class A{};

int main ()
{
  Derived<A> a;
  a.method();
}

类A只是用作模板参数的虚拟类.但是编译器抱怨找不到"protectedData".错误信息如下:

class A is just a dummy class served as the template parameter. But the compiler complains cannot find the "protectedData". The error information is as following:

g++-4.9 test.cc -Wall -std=c++1y -Wconversion -Wextra
test.cc: In member function ‘void Derived<T>::method()’:
test.cc:26:11: error: ‘protectedData’ was not declared in this scope
    cout<<protectedData<<endl;

推荐答案

它实际上与CRTP无关，而事实上，对于访问依赖库的派生代码，您需要限定条件.

It doesn't really have to do with CRTP, but rather with the fact that for dependent-base-accessing derived code, you need to qualify things.

将行更改为

std::cout<<this->protectedData<<std::endl;

解决了.

请参见访问派生类中的基类成员

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