<分区>
使用 curiously recurring template pattern 时,如果我试图从基类中引用属于派生类的 typedef,则仅无法引用它们; gcc 提示 no type named 'myType' in class Derived<...>
.这似乎与使用 typedef、模板和奇怪的重复关系的其他方式不一致。
考虑:
/* crtp.cpp */
#include <iostream>
using namespace std;
// case 1. simple.
class Base {
public:
typedef int a_t;
a_t foo;
};
class Derived : public Base {
a_t bar;
};
// case 2. template.
template<typename T>
class tBase {
public:
typedef T b_t;
T foo;
};
template <typename T>
class tDerived : public tBase<T> {
typename tBase<T>::b_t bar;
};
// case 3. curiously recurring.
template <typename T, typename D>
class tCuriousBase {
public:
typedef T c_t;
c_t foo;
};
template <typename T>
class tCuriousDerived : public tCuriousBase<T,tCuriousDerived<T> > {
typename tCuriousBase<T,tCuriousDerived<T> >::c_t bar;
};
// case 4. curiously recurring with member reference.
template <typename T, typename D>
class tCuriousMemberBase {
public:
T foo;
T get() {
return static_cast<D*>(this)->bar;
}
};
template <typename T>
class tCuriousMemberDerived : public tCuriousMemberBase<T, tCuriousMemberDerived<T> > {
public:
T bar;
tCuriousMemberDerived(T val) : bar(val) {}
};
// case 5. curiously recurring with typedef reference.
template <typename T, typename D>
class tCuriousTypeBase {
public:
typedef T d_t;
d_t foo;
typename D::c_t baz;
};
template <typename T>
class tCuriousTypeDerived : public tCuriousTypeBase<T, tCuriousTypeDerived<T> > {
public:
typedef T c_t;
typename tCuriousTypeBase<T,tCuriousTypeDerived<T> >::d_t bar;
};
// entry point
int main(int argc, char **argv) {
Derived::a_t one = 1;
tDerived<double>::b_t two = 2;
tCuriousDerived<double>::c_t three = 3;
double four = tCuriousMemberDerived<double>(4).get();
tCuriousTypeBase<double, tCuriousDerived<double> >::d_t five = 5;
// tCuriousTypeDerived<double>::d_t six = 6; /* FAILS */
cout << one << endl;
cout << two << endl;
cout << three << endl;
cout << four << endl;
cout << five << endl;
// cout << six << endl;
}
从(1)可以看出typedef确实是从base继承到derived的;可以通过派生类访问在基类中声明的 typedef。
从 (2) 可以看出,如果两个类都是模板,这仍然成立。
从 (3) 中,我们看到这种 typedef 继承在存在奇怪的重复模板关系的情况下仍然可以存在;我们在 three
的声明中通过派生类引用基类的 typedef .
从 (4) 中,我们看到派生类的成员变量 可以很容易地从基类访问。
从 (5) 中,我们看到我们可以访问在模板参数上定义的 typedef(这在我们声明 five
使用不通过继承相关的类型时有效)。
然而,一旦我们在 (6) 中使模板参数成为派生类,typedef 突然变得不可访问,即使它看起来与派生类中的任何成员变量一样定义良好。
这是为什么?