Question

typedef boost::variant<int, double> Type;
class Append: public boost::static_visitor<>
{
public:
    void operator()(int)
    {}

    void operator()(double)
    {}

};

Type type(1.2);
Visitor visitor;
boost::apply_visitor(visitor, type);

是否可以更改访问者，使其收到如下额外数据：

class Append: public boost::static_visitor<>
{
public:
    void operator()(int, const std::string&)
    {}

    void operator()(double, const std::string&)
    {}
};

此字符串值在Append对象的生命周期内发生更改。在这种情况下，通过构造函数传递字符串不是一个选项。

Answer 1

为每个调用赋予的“附加参数”是this指针。使用它来传递您需要的任何其他信息：

#include <boost/variant.hpp>
typedef boost::variant<int, double> Type;
class Append: public boost::static_visitor<>
{
public:
    void operator()(int)
    {}

    void operator()(double)
    {}
    std::string argument;
};

int main() {
    Type type(1.2);
    Append visitor;
    visitor.argument = "first value";
    boost::apply_visitor(visitor, type);
    visitor.argument = "new value";
    boost::apply_visitor(visitor, type);
}

Answer 2

另一个选择是绑定额外的参数。您的访客班级可能看起来像这样：

class Append: public boost::static_visitor<>
{
public:
    void operator()(const std::string&, int)
    {}

    void operator()(const std::string&, double)
    {}
};

这样称呼：

std::string myString = "foo";
double value = 1.2;
auto visitor = std::bind( Append(), myString, std::placeholders::_1 );
boost::apply_visitor( visitor, value );

Answer 3

这个解决了您的问题：

#include <iostream>
#include <string>
#include <boost/variant.hpp>

typedef boost::variant<int, double> Type;
typedef boost::variant<const std::string> Extra;
class Append: public boost::static_visitor<>
{
public:
    void operator()(const int& a1, const std::string& a2) const {
        std::cout << "arg 1 = "<< a1 << "\n";
        std::cout << "arg 2 = "<< a2 << "\n";
    }

    void operator()(const double& a1, const std::string& a2) const {
        std::cout << "arg 1 = "<< a1 << "\n";
        std::cout << "arg 2 = "<< a2 << "\n";
    }
};

int main()
{
    Type type(1.2);
    Extra str("extra argument");
    boost::apply_visitor(Append(), type, str);
}

这里是有效的 Demo 。您可以发送额外的参数-任意数量。限制是必须将它们包装在boost :: variant中。但是，编译器会在内部使用单一类型来优化变体。如果您需要两个以上的参数，请#include <boost/variant/multivisitors.hpp>，请参见https://www.boost.org/doc/libs/1_70_0/doc/html/boost/apply_visitor.html

具有多个参数的boost :: static_visitor

3 个答案: