c++ - 为什么这个二进制 apply_visitor 不工作？

Question

#include <string>
#include "boost/variant/variant.hpp"
#include "boost/variant/apply_visitor.hpp"

using namespace std;

class Base
{
public:
    Base(){}
    ~Base(){}
    void AddField(int tag, int value){std::cout << "Base::AddField " << tag << ", " << value << std::endl;}
    void AddField(int tag, string value){std::cout << "Base::AddField " << tag << ", " << value << std::endl;}
};

class A : public Base
{
public:
    A(){}
    ~A(){}
};
class B : public Base
{
public:
    B(){}
    ~B(){}
};

class foo_visitor
    : public boost::static_visitor<>
{
public:
    foo_visitor(int tag){mTag = tag;}
    template <typename T>
    void operator()(T &a, int &v) const {
        a->AddField(mTag, v);
    }
private:
    int mTag;
};

int main(int argc, char **argv) 
{
    typedef boost::variant<A*,B*> AB;
    AB ab = new A();    
    int tag = 1;
    int v = 2;
    boost::apply_visitor(foo_visitor(tag), ab, v);
    return 0;
}

我遇到这个编译错误:

apply_visitor_unary.hpp:60:43: error: request for member ‘apply_visitor’ in ‘visitable’, which is of non-class type ‘int’

我的代码有什么问题？

Answer 1

int 实际上不是 variant。

Overloads accepting two operands invoke the binary function call operator of the given visitor on the content of the given variant operands.

http://www.boost.org/doc/libs/1_52_0/doc/html/boost/apply_visitor.html