我好奇地重复使用模板时遇到以下问题,当我尝试访问CRTP基类的数据成员时遇到了问题.

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模板化,例如：

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：

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;

解决方法:

它实际上与CRTP无关,而是与以下事实有关：对于依赖库访问派生代码,您需要限定条件.

将行更改为

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

解决了.

参见accessing a base class member in derived class.