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让boost.variant支持lambda表达式访问

让boost.variant支持lambda表达式访问

之前写个过一篇博客叫 《浅谈boost.variant的几种访问方式》 ,里面讲到了可以通过访问者方式来获取variant的值,但是在重载函数 operator() 里面只能够获取variant的值,如果要捕获外部变量或调用外部函数比较麻烦,那么有没有一种方法来简化variant的访问呢?当然有,下面我们让variant支持lambda表达式访问(个人博客也发表了 《让boost.variant支持lambda表达式访问》 )。

代码

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

template<typename Function, typename... Args>
struct make_overload_impl : make_overload_impl<Function>::type, 
 make_overload_impl<Args...>::type
{
 using type = make_overload_impl;
 using make_overload_impl<Function>::type::operator();
 using make_overload_impl<Args...>::type::operator();
 constexpr explicit make_overload_impl(Function&& func, Args&&... args)
 : make_overload_impl<Function>::type(std::forward<Function>(func)),
 make_overload_impl<Args...>::type(std::forward<Args>(args)...)
 {}
};

template<typename Function>
struct make_overload_impl<Function>
{
 using type = Function;
};

template<typename Return, typename... Args>
struct make_overload_impl<Return(*)(Args...)>
{
 using type = make_overload_impl;
 using Function = Return(*)(Args...);
 constexpr explicit make_overload_impl(const Function&& func) : _func(func) {}
 constexpr Return operator()(Args&&... args) const
 {
 return _func(std::forward<Args>(args)...);
 }

private:
 Function _func;
};

struct make_overload
{
 template<typename... Function, typename Overload = 
 typename make_overload_impl<typename std::decay<Function>::type...>::type>
 constexpr Overload operator()(Function&&... func) const
 {
 return Overload(std::forward<Function>(func)...);
 }
};

template<typename... Args>
auto make_visitor(Args&&... args)
{
 return make_overload()(std::forward<Args>(args)...);
}

int main()
{
 auto visitor = make_visitor
 (
 [](int& i) { std::cout << i << std::endl; },
 [](std::string& i) { std::cout << i << std::endl; }
 );
 boost::variant<int, std::string> v;
 v = "Hello world";
 boost::apply_visitor(visitor, v);
 v = 100;
 boost::apply_visitor(visitor, v);

 return 0;
}

该代码也上传到了我的 github

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