[英]C++ template and inheritance, function not replaced in derived class
Here I attached a minimal case, where in child class, cmp
will replace the one in parent class, but it doesn't work, it always calls the one from the parent 在这里我附加了一个最小的情况,在子类中,
cmp
将替换父类中的一个,但它不起作用,它总是调用父类中的一个
#include <iostream>
template <typename T>
class A
{
public:
void tell(T a, T b)
{
std::cout << (cmp (a, b) ? a : b) << " is better" << std::endl;
}
protected:
bool cmp (T & a, T & b)
{
return a > b;
}
};
template <typename T>
class B: public A<T>
{
protected:
bool cmp (T & a, T & b)
{
return a < b;
}
};
int main ( int argc , char **argv )
{
B<int> b;
// should be: 3 is better here
b.tell(5, 3);
return 0;
}
You must declare polymorphic functions virtual
. 您必须将多态函数声明为
virtual
。 Otherwise you will not get polymorphic behaviour 否则你不会得到多态行为
virtual bool cmp (T & a, T & b)
Your cmp
function must be declared virtual
in A
. 您的
cmp
函数必须在A
声明为virtual
。
protected:
virtual bool cmp (T & a, T & b)
{
return a > b;
}
It will implicitly remain virtual in derived classes, although I'd explicitly declare cmp
virtual in B
as well, for clarity's sake. 它将在派生类中隐式保持虚拟,尽管为了清楚起见我也明确地在
B
声明了cmp
virtual。
你应该让cmp
成为基类中的虚函数:
virtual bool cmp (T & a, T & b);
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