Boost variant visitor with an extra parameter

问题

I have code that resembles below.

typedef uint32_t IntType;
typedef IntType IntValue;
typedef boost::variant<IntValue, std::string>  MsgValue;

MsgValue v;

Instead of saying this,

IntValue value = boost::apply_visitor(d_string_int_visitor(), v);

I would like to pass an extra parameter like this: But operator() gives a compile error.

//This gives an error since the overload below doesn't work.
IntValue value = boost::apply_visitor(d_string_int_visitor(), v, anotherStr);

class d_string_int_visitor : public boost::static_visitor<IntType>
{
public:
    inline IntType operator()(IntType i) const
    {
        return i;
    }

    inline IntValue operator()(const std::string& str) const noexcept
    {
        // code in here
    }

    //I want this, but compiler error.
    inline IntValue operator()(const std::string& str, const std::string s) const noexcept
    {
        // code in here
    }
};

回答1:

You can bind the extra string argument to the visitor using std::bind. First, add the std::string parameter to all of the visitor's operator() overloads.

class d_string_int_visitor : public boost::static_visitor<IntType>
{
public:
    inline IntType operator()(IntType i, const std::string& s) const
    {
        return i;
    }

    inline IntValue operator()(const std::string& str, const std::string& s) const noexcept
    {
        // code in here
        return 0;
    }
};

Now create a visitor to which you have bound the second string argument.

auto bound_visitor = std::bind(d_string_int_visitor(), std::placeholders::_1, "Hello World!");
boost::apply_visitor(bound_visitor, v);

Live demo

However, a better solution would be to pass the string as the visitor's constructor argument.

回答2:

typedef uint32_t IntType;
typedef IntType IntValue;
typedef boost::variant<IntValue, std::string>  MsgValue;

MsgValue v;

IntValue value = boost::apply_visitor([&](auto&& one){
  return d_string_int_visitor{}(decltype(one)(one), anotherStr);
}, v);

assuming every overload of d_string_int_visitor can handle the extra parameter.

As a bonus, you can even do away with the wrapping class if you want:

IntValue to_int_value(IntValue v, std::string const& format) { return v; }
IntValue to_int_value(std::string const& str, std::string const& format);

IntValue value = boost::apply_visitor([&](auto&& one){
  return to_int_value(decltype(one)(one), anotherStr);
}, v);

where we create an anonymous lambda that forwards to a traditional set of function overloads.

The auto&& one and decltype(one)(one) is a technique to do perfect forwarding from a lambda (C++14). You could replace the second with std::forward<decltype(one)>(one), but I find the short version readable. Unlike std::forward, it does the "wrong" thing with value-types, but we know that one is an l or r value reference.

来源：https://stackoverflow.com/questions/29618636/boost-variant-visitor-with-an-extra-parameter

标签

c++

c++11

boost

c++14

boost-variant