特定基类的C++类模板

Question

假设我有课程:

class Base{};

class A: public Base{
    int i;
};

class B:public Base{
    bool b;
};

现在我想定义一个模板化的类:

template < typename T1, typename T2 >
class BasePair{
    T1 first;
    T2 second;
};

但我想定义它,只有类Base的后代可以用作模板参数.

我怎样才能做到这一点？

Answer 1

C++ 11介绍 <type_traits>

template <typename T1, typename T2>
class BasePair{
static_assert(std::is_base_of<Base, T1>::value, "T1 must derive from Base");
static_assert(std::is_base_of<Base, T2>::value, "T2 must derive from Base");

    T1 first;
    T2 second;
};

Answer 2

更确切地说:

class B {};
class D1 : public B {};
class D2 : public B {};
class U {};

template <class X, class Y> class P {
    X x;
    Y y;
public:
    P() {
        (void)static_cast<B*>((X*)0);
        (void)static_cast<B*>((Y*)0);
    }
};

int main() {
    P<D1, D2> ok;
    P<U, U> nok; //error
}

Answer 3

C++还没有直接允许这个.您可以通过在类中使用STATIC_ASSERT和类型检查来间接实现它:

template < typename T1, typename T2 >
class BasePair{
    BOOST_STATIC_ASSERT(boost::is_base_of<Base, T1>);
    BOOST_STATIC_ASSERT(boost::is_base_of<Base, T2>);
    T1 first;
    T2 second;
};