如何将boost访问者概念与包含状态变量的类一起使用？

Question

我正在尝试使用boost :: static_visitor在boost :: variant类型上实现影响某些变量状态的操作.我的方法是在命令访问者类中包含所有状态变量,但似乎这是不可能的.

这是我的代码示例:

#include <string>
#include <sstream>
#include <vector>
#include <boost/variant.hpp>
#include <boost/foreach.hpp>


struct TypeA
{
    int varA;
    int varB;
};

struct TypeB
{
   std::string varA;
   std::string varB;
};

typedef boost::variant<TypeA, TypeB> MyVariantType;

class MyCommandVisitor : public boost::static_visitor<>
{
public:
//These are just examples, the actions only need to be able to touch
// internal variables.
void operator()(TypeA & t) const
{
   m_runningSum += t.varA;
   m_outStream << "TYPEA ACTION: " << t.varB << std::endl;
}

void operator(TypeB & t) const
{
   m_charCount += t.varA.size();
   m_outStream << t.varB <<  " ACTION " << t.varA << std::endl;
}

std::string emitWork(std::vector<MyVariantType> listOfVariants)
{
    m_outStream.clear();
    m_runningSum = 0;
    m_charCount = 0;
    BOOST_FOREACH(MyVariantType & v, listOfVariants)
    {
        boost::apply_visitor(*this, v);
    }
    return m_outStream.str();
}

protected:
int m_runningSum;
int m_charCount;
std::stringstream outStream;
}; //End class MyCommandVisitor


int main(int argc, char **argv)
{
    TypeA ta;
    ta.varA = 1;
    ta.varB = 2; 

    TypeB tb;
    tb.varA = "String1";
    tb.varB = "String2";
    std::vector<MyVariantType> listOfWork;
    listOfWork.push_back(ta);
    listOfWork.push_back(tb);
    MyCommandVisitor myCV;

    std::string result = myCV.emitWork(listOfWork);

    std::cout << "Result:\n" << result << std::endl << std::endl;
    return 0;
}

我希望这个片段能够理解我想要实现的目标.但是,它不会编译,给出[释义]错误:

error: no operator "<<" matches these operands
   operand types are: const std::stringstream << const char [N]
m_outStream << "TYPE A ACTION: " << t.varB << std::endl;
             ^

我假设这个错误是由于const修饰符必须放在operator()函数原型的末尾,这使得编译器认为成员变量不能被函数修改.

我的问题是:

使用必须在访问之间保持状态的变量来完成访问者模式(使用boost :: variant)的正确方法是什么？

Answer 1

有几个错别字,但我做了一些mods,它现在有效.基本上你的static_visitor类在每次访问时都会发生变化,因此operator()方法不能是const.

#include <string>
#include <sstream>
#include <vector>
#include <boost/variant.hpp>
#include <boost/foreach.hpp>
#include <iostream>


struct TypeA
{
    int varA;
    int varB;
};

struct TypeB
{
   std::string varA;
   std::string varB;
};

typedef boost::variant<TypeA, TypeB> MyVariantType;

class MyCommandVisitor : public boost::static_visitor<>
{
public:
//These are just examples, the actions only need to be able to touch
// internal variables.
void operator()(TypeA & t)
{
   m_runningSum += t.varA;
   m_outStream << "TYPEA ACTION: " << t.varB << std::endl;
}

void operator()(TypeB & t)
{
   m_charCount += t.varA.size();
   m_outStream << t.varB <<  " ACTION " << t.varA << std::endl;
}

std::string emitWork(std::vector<MyVariantType> listOfVariants)
{
    m_outStream.clear();
    m_runningSum = 0;
    m_charCount = 0;
    BOOST_FOREACH(MyVariantType & v, listOfVariants)
    {
        boost::apply_visitor(*this, v);
    }
    return m_outStream.str();
}

protected:
int m_runningSum;
int m_charCount;
std::stringstream m_outStream;
}; //End class MyCommandVisitor


int main(int argc, char **argv)
{
    TypeA ta;
    ta.varA = 1;
    ta.varB = 2; 

    TypeB tb;
    tb.varA = "String1";
    tb.varB = "String2";
    std::vector<MyVariantType> listOfWork;
    listOfWork.push_back(ta);
    listOfWork.push_back(tb);
    MyCommandVisitor myCV;

    std::string result = myCV.emitWork(listOfWork);

    std::cout << "Result:\n" << result << std::endl << std::endl;
    return 0;
}

在http://www.compileonline.com/compile_cpp11_online.php上运行给出:

Compiling the source code....
$g++ -std=c++11 main.cpp -o demo -lm -pthread -lgmpxx -lgmp -lreadline 2>&1

Executing the program....
$demo 
Result:
TYPEA ACTION: 2
String2 ACTION String1