成员函数特化中的循环依赖...... "instantiation before specialization"

Question

下面的代码无法编译

#include <iostream>
using namespace std;
template<class T> class A;
template<class T> class B;
//@@@@@@@@@@@@@y
template<class T>
class B{
  int x=1;
public:
  void write(A<T> x);
  void add(A<T> x);
};
template<class T>
class A{
  int x=1;
public:
  void write(B<T> x);
  void add(B<T> x);
};

template<class T> void A<T>::write(B<T> x){cout<<"write generic A"<<endl;x.add(*this);};
template<class T> void A<T>::add(B<T> x){cout<<"add generic A"<<endl;};

template<> void A<int>::write(B<int> x){cout<<"write special A"<<endl;x.add(*this);};
template<> void A<int>::add(B<int> x){cout<<"add special A int"<<endl;};


template<class T> void B<T>::write(A<T> x){cout<<"write generic B"<<endl;x.add(*this);};
template<class T> void B<T>::add(A<T> x){cout<<"add generic B"<<endl;};

template<> void B<int>::write(A<int> x){cout<<"write special B"<<endl;x.add(*this);};
template<> void B<int>::add(A<int> x){cout<<"add special B"<<endl;};





int main(){
  B<int> b;
  A<int> a;
  b.write(a);
}

作为A和B的专门“write”方法，实例化A或B的“add”模板。编译编译时抱怨

error: specialization of ‘void B<T>::add(A<T>) [with T = int]’ after instantiation
   32 | template<> void B<int>::add(A<int> x){cout<<"add special B"<<endl;};

目前我看不出如何对函数进行排序来避免这个问题，除非我放弃专业化。

非常感谢任何建议。

Answer 1

您可以在每个 class 声明后声明模板方法的特化：

#include <iostream>
using namespace std;
template<class T> class A;
template<class T> class B;
//@@@@@@@@@@@@@y
template<class T>
class B{
  int x=1;
public:
  void write(A<T> x);
  void add(A<T> x);
};
// declare specializations:
template<> void B<int>::write(A<int> x);
template<> void B<int>::add(A<int> x);

template<class T>
class A{
  int x=1;
public:
  void write(B<T> x);
  void add(B<T> x);
};
// declare specializations:
template<> void A<int>::write(B<int> x);
template<> void A<int>::add(B<int> x);

template<class T> void A<T>::write(B<T> x){cout<<"write generic A"<<endl;x.add(*this);};
template<class T> void A<T>::add(B<T> x){cout<<"add generic A"<<endl;};

template<> void A<int>::write(B<int> x){cout<<"write special A"<<endl;x.add(*this);}; // method "B<int>::add(A<int> x)" is called but was not declared or defined before
template<> void A<int>::add(B<int> x){cout<<"add special A int"<<endl;};


template<class T> void B<T>::write(A<T> x){cout<<"write generic B"<<endl;x.add(*this);};
template<class T> void B<T>::add(A<T> x){cout<<"add generic B"<<endl;};

template<> void B<int>::write(A<int> x){cout<<"write special B"<<endl;x.add(*this);};
template<> void B<int>::add(A<int> x){cout<<"add special B"<<endl;};


int main(){
  B<int> b;
  A<int> a;
  b.write(a);
}

代码中有关于方法 B<int>::add(A<int> x) 在 A<int>::write(B<int> x) 中调用但之前未声明或定义的注释。

成员函数特化中的循环依赖...... "instantiation before specialization"

circular dependencies in member function specializations ....... "instantiation before specialization"

c++

templates

function

member

template-specialization