问题描述
如何使用模板参数覆盖虚函数?
virtual T accept(Visitor<T>);
#include <iostream>
template<typename T>
class Visitor{
public:
template<typename K>
T visit(K);
};
template<typename T>
class Expr{
public:
virtual T accept(Visitor<T>);
};
template<typename T>
class Lit: public Expr<T>{
public:
int value;
Lit(int a){
value = a;
}
T accept(Visitor<T> v) override{
v.visit(*this);
}
};
template<typename T>
class Add: public Expr<T>{
public:
Expr<T> left,right;
Add(Expr<T> a,Expr<T> b){
left = a; right = b;
}
T accept(Visitor<T> v) override{
v.visit(*this);
}
};
class tv: public Visitor<int>{
public:
int visit(Lit<int> a){
return a.value;
}
int visit(Add<int> a){
return a.left.accept(*this) + a.right.accept(*this);
}
};
int main() {
Add<int> a(Lit<int>(1),Lit<int>(2));
tv testvisitor;
std::cout << a.accept(testvisitor);
return 0;
}
解决方法
就我而言,失败如下:
/tmp/ccysNlSn.o: In function `Add<int>::accept(Visitor<int>)':
main.cpp:(.text._ZN3AddIiE6acceptE7VisitorIiE[_ZN3AddIiE6acceptE7VisitorIiE]+0x2b): undefined reference to `int Visitor<int>::visit<Add<int> >(Add<int>)'
/tmp/ccysNlSn.o:(.rodata._ZTV4ExprIiE[_ZTV4ExprIiE]+0x10): undefined reference to `Expr<int>::accept(Visitor<int>)'
/tmp/ccysNlSn.o: In function `Lit<int>::accept(Visitor<int>)':
main.cpp:(.text._ZN3LitIiE6acceptE7VisitorIiE[_ZN3LitIiE6acceptE7VisitorIiE]+0x2b): undefined reference to `int Visitor<int>::visit<Lit<int> >(Lit<int>)'
collect2: error: ld returned 1 exit status
这里的主要错误如下:
undefined reference to `int Visitor<int>::visit<Add<int> >(Add<int>)'
undefined reference to `Expr<int>::accept(Visitor<int>)'
这些错误意味着您尚未提供 int Visitor<int>::visit<Add<int> >(Add<int>)
和 Expr<int>::accept(Visitor<int>)
的实现。所以只要提供一个空的实现就可以解决它们:
template<typename T>
class Visitor{
public:
template<typename K>
T visit(K) {}; // Added an empty body here
};
template<typename T>
class Expr{
public:
virtual T accept(Visitor<T>) {}; // Added an empty body here
};
这将可以很好地编译和链接,但当然您需要将实际代码提供给实现。