#include <iostream>
#include <functional>
 
//using namespace std;
 
template <typename ...Args>
std::function<void(Args...)> wrapLog(std::function<void(Args...)> f) {
 
    return [f](Args... args) {                                            //捕获函数对象f
        std::cout << "start" << std::endl;
        f(args...);
        std::cout << "finish" << std::endl;
    };
}
 
template <typename> struct function_traits_base;
 
template <typename Function>
struct function_traits : public function_traits_base<decltype(&Function::operator())> {
 
};
 
//lambda 转换为 std::function<void(Args...)> 需要知道 传入参数类型和返回值类型 ，这里进行萃取 
template <typename ClassType, typename ReturnType, typename... Args>
struct function_traits_base<ReturnType(ClassType::*)(Args...) const>         //Args...可变模板参数 ，因为不知道lambda将传入几个参数
{
    //typedef ReturnType (*pointer)(Args...);
    typedef std::function<ReturnType(Args...)> function;            
};
 
template <typename Function>
typename function_traits<Function>::function    //返回值类型 function_traits<Function>::function
to_function(Function& lambda)                    //函数名to_function，传入参数lambda
{
    return typename function_traits<Function>::function(lambda);    //将lambda传入function_traits<Function>::function 
}
 
//functional里面有定义了个std::plus。如果把plus定义为global的，访问时在global查找定义就会和std::plus冲突，产生二义。
//auto plus = [](int a, int b) { std::cout << a + b << std::endl; };            //error，Plus不明确
std::function<void(int, int)> plus = [](int a, int b) { std::cout << a + b << std::endl; };
 
int main(int argc, char *argv[]) {
    //这是简单的lambda，其中包含琐碎的计算：
    //auto plus = [](int a, int b) { cout << a + b << endl; };
 
    //lambda和 std :: function 无法完成转换
    //auto loggedPlus = wrapLog(plus);                                                //test1
 
    //强制转换，OK，该行可以编译，但是很难看
    auto loggedPlus  = wrapLog(static_cast<std::function<void(int, int)>>(plus));   //test2
    loggedPlus(2, 3);
 
    //暂时最佳解决方案
    auto loggedPluss = wrapLog(to_function(plus));            //通过调用to_function函数 将 lambda表达式plus 转换为std::function<void(Args...)> 形式
    loggedPluss(2, 3);
 
    return 0;
}

I2luY2x1ZGUgPGlvc3RyZWFtPgojaW5jbHVkZSA8ZnVuY3Rpb25hbD4KCi8vdXNpbmcgbmFtZXNwYWNlIHN0ZDsKCnRlbXBsYXRlIDx0eXBlbmFtZSAuLi5BcmdzPgpzdGQ6OmZ1bmN0aW9uPHZvaWQoQXJncy4uLik+IHdyYXBMb2coc3RkOjpmdW5jdGlvbjx2b2lkKEFyZ3MuLi4pPiBmKSB7CgogICAgcmV0dXJuIFtmXShBcmdzLi4uIGFyZ3MpIHsgICAgICAgICAgICAgICAgICAgICAgICAgICAgICAgICAgICAgICAgICAgIC8v5o2V6I635Ye95pWw5a+56LGhZgogICAgICAgIHN0ZDo6Y291dCA8PCAic3RhcnQiIDw8IHN0ZDo6ZW5kbDsKICAgICAgICBmKGFyZ3MuLi4pOwogICAgICAgIHN0ZDo6Y291dCA8PCAiZmluaXNoIiA8PCBzdGQ6OmVuZGw7CiAgICB9Owp9Cgp0ZW1wbGF0ZSA8dHlwZW5hbWU+IHN0cnVjdCBmdW5jdGlvbl90cmFpdHNfYmFzZTsKCnRlbXBsYXRlIDx0eXBlbmFtZSBGdW5jdGlvbj4Kc3RydWN0IGZ1bmN0aW9uX3RyYWl0cyA6IHB1YmxpYyBmdW5jdGlvbl90cmFpdHNfYmFzZTxkZWNsdHlwZSgmRnVuY3Rpb246Om9wZXJhdG9yKCkpPiB7Cgp9OwoKLy9sYW1iZGEg6L2s5o2i5Li6IHN0ZDo6ZnVuY3Rpb248dm9pZChBcmdzLi4uKT4g6ZyA6KaB55+l6YGTIOS8oOWFpeWPguaVsOexu+Wei+WSjOi/lOWbnuWAvOexu+WeiyDvvIzov5nph4zov5vooYzokIPlj5YgCnRlbXBsYXRlIDx0eXBlbmFtZSBDbGFzc1R5cGUsIHR5cGVuYW1lIFJldHVyblR5cGUsIHR5cGVuYW1lLi4uIEFyZ3M+CnN0cnVjdCBmdW5jdGlvbl90cmFpdHNfYmFzZTxSZXR1cm5UeXBlKENsYXNzVHlwZTo6KikoQXJncy4uLikgY29uc3Q+ICAgICAgICAgLy9BcmdzLi4u5Y+v5Y+Y5qih5p2/5Y+C5pWwIO+8jOWboOS4uuS4jeefpemBk2xhbWJkYeWwhuS8oOWFpeWHoOS4quWPguaVsAp7CiAgICAvL3R5cGVkZWYgUmV0dXJuVHlwZSAoKnBvaW50ZXIpKEFyZ3MuLi4pOwogICAgdHlwZWRlZiBzdGQ6OmZ1bmN0aW9uPFJldHVyblR5cGUoQXJncy4uLik+IGZ1bmN0aW9uOyAgICAgICAgICAgIAp9OwoKdGVtcGxhdGUgPHR5cGVuYW1lIEZ1bmN0aW9uPgp0eXBlbmFtZSBmdW5jdGlvbl90cmFpdHM8RnVuY3Rpb24+OjpmdW5jdGlvbiAgICAvL+i/lOWbnuWAvOexu+WeiyBmdW5jdGlvbl90cmFpdHM8RnVuY3Rpb24+OjpmdW5jdGlvbgp0b19mdW5jdGlvbihGdW5jdGlvbiYgbGFtYmRhKSAgICAgICAgICAgICAgICAgICAgLy/lh73mlbDlkI10b19mdW5jdGlvbu+8jOS8oOWFpeWPguaVsGxhbWJkYQp7CiAgICByZXR1cm4gdHlwZW5hbWUgZnVuY3Rpb25fdHJhaXRzPEZ1bmN0aW9uPjo6ZnVuY3Rpb24obGFtYmRhKTsgICAgLy/lsIZsYW1iZGHkvKDlhaVmdW5jdGlvbl90cmFpdHM8RnVuY3Rpb24+OjpmdW5jdGlvbiAKfQoKLy9mdW5jdGlvbmFs6YeM6Z2i5pyJ5a6a5LmJ5LqG5Liqc3RkOjpwbHVz44CC5aaC5p6c5oqKcGx1c+WumuS5ieS4umdsb2JhbOeahO+8jOiuv+mXruaXtuWcqGdsb2JhbOafpeaJvuWumuS5ieWwseS8muWSjHN0ZDo6cGx1c+WGsueqge+8jOS6p+eUn+S6jOS5ieOAggovL2F1dG8gcGx1cyA9IFtdKGludCBhLCBpbnQgYikgeyBzdGQ6OmNvdXQgPDwgYSArIGIgPDwgc3RkOjplbmRsOyB9OyAgICAgICAgICAgIC8vZXJyb3LvvIxQbHVz5LiN5piO56GuCnN0ZDo6ZnVuY3Rpb248dm9pZChpbnQsIGludCk+IHBsdXMgPSBbXShpbnQgYSwgaW50IGIpIHsgc3RkOjpjb3V0IDw8IGEgKyBiIDw8IHN0ZDo6ZW5kbDsgfTsKCmludCBtYWluKGludCBhcmdjLCBjaGFyICphcmd2W10pIHsKICAgIC8v6L+Z5piv566A5Y2V55qEbGFtYmRh77yM5YW25Lit5YyF5ZCr55CQ56KO55qE6K6h566X77yaCiAgICAvL2F1dG8gcGx1cyA9IFtdKGludCBhLCBpbnQgYikgeyBjb3V0IDw8IGEgKyBiIDw8IGVuZGw7IH07CgogICAgLy9sYW1iZGHlkowgc3RkIDo6IGZ1bmN0aW9uIOaXoOazleWujOaIkOi9rOaNogogICAgLy9hdXRvIGxvZ2dlZFBsdXMgPSB3cmFwTG9nKHBsdXMpOyAgICAgICAgICAgICAgICAgICAgICAgICAgICAgICAgICAgICAgICAgICAgICAgIC8vdGVzdDEKCiAgICAvL+W8uuWItui9rOaNou+8jE9L77yM6K+l6KGM5Y+v5Lul57yW6K+R77yM5L2G5piv5b6I6Zq+55yLCiAgICBhdXRvIGxvZ2dlZFBsdXMgID0gd3JhcExvZyhzdGF0aWNfY2FzdDxzdGQ6OmZ1bmN0aW9uPHZvaWQoaW50LCBpbnQpPj4ocGx1cykpOyAgIC8vdGVzdDIKICAgIGxvZ2dlZFBsdXMoMiwgMyk7CgogICAgLy/mmoLml7bmnIDkvbPop6PlhrPmlrnmoYgKICAgIGF1dG8gbG9nZ2VkUGx1c3MgPSB3cmFwTG9nKHRvX2Z1bmN0aW9uKHBsdXMpKTsgICAgICAgICAgICAvL+mAmui/h+iwg+eUqHRvX2Z1bmN0aW9u5Ye95pWwIOWwhiBsYW1iZGHooajovr7lvI9wbHVzIOi9rOaNouS4unN0ZDo6ZnVuY3Rpb248dm9pZChBcmdzLi4uKT4g5b2i5byPCiAgICBsb2dnZWRQbHVzcygyLCAzKTsKCiAgICByZXR1cm4gMDsKfQ==