[英]How can I override a virtual function with template argument in C++?
如何使用模板參數覆蓋虛擬 function? 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;
}
就我而言,失敗如下:
/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
這里的主要錯誤如下:
undefined reference to `int Visitor<int>::visit<Add<int> >(Add<int>)'
undefined reference to `Expr<int>::accept(Visitor<int>)'
這些錯誤意味着您沒有提供int Visitor<int>::visit<Add<int> >(Add<int>)
和Expr<int>::accept(Visitor<int>)
的實現。 因此,只需提供一個空實現即可修復它們:
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
};
這將很好地編譯和鏈接,但是您當然需要將實際代碼提供給實現。
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