Clause 34: lambda is preferred over std:: bind

call std::bind The returned function object is the binding object . Consider that somewhere in the program we want to set the alarm to go off after an hour and continue 30 second , Consider the following code ：

using Time = std::chrono::steady_clock::time_point;

enum class Sound {
    Beep, Siren, Whistle};

using Duration = std::chrono::steady_clock::duration;
//  At the moment t, Make a sound s, Constantly d
void setAlarm(Time t, Sound s, Duration d); 
//  Accept a specified sound , The sound will be 1 Hours later , And continue 30 second  
auto setSoundL = 
	[] (Sound s)
	{
    
		using namespace std::chrono;
		setAlarm(steady_clock::now() + hours(1), s, seconds(30));
	;

Then we write the corresponding std::bind Version of

auto setSoundB =
	std::bind(setAlarm,
			  steady_clock::now() + 1h, //  error ！ See below 
			  _1,
			  30s);

steady_clock::now() + 1h Passed as an argument to std::bind, Instead of setAlarm. It means that the expression is calling when evaluating std::bind moment . The final result is , The start time of the alarm setting is to call std::bind An hour after the moment of , Instead of setAlarm An hour when . The solution is in the original std::bind Nested inside 2 layer std::bind call :

auto setSoundB = 
	std::bind(setAlarm,
			  std::bind(std::plus<>(), steady_clock::now(), 1h),
			  _1,
			  30s);

Once you are right setAlarm Overloading is implemented , There will be new problems ：

enum class Volum {
    Normal, Loud, LoudPlusPlus };
void setAlarm(Time t, Sound s, Duration d, Volume v);

Previous lambda The formula will work as usual , But for std::bind Call to , Unable to compile

auto setSoundB = 
	std::bind(setAlarm,
			  std::bind(std::plus<>(), steady_clock::now(), 1h),
			  _1,
			  30s);

Because the compiler cannot determine which setAlarm Version passed to std::bind. All the information it gets is a function name , Only the function name itself is ambiguous . The solution is setAlarm Must be cast to the appropriate function pointer type ：

using SetAlarm3ParamType = void(*)(Time t,Sound s, Duration d);
auto setSoundB = 
	std::bind(static_cast<SetAlarm3ParamType>(setAlarm),
			  std::bind(std::plus<>(), steady_clock::now(), 1h),
			  _1,
			  30s);

stay C++11 in std::bind There are only two limited occasions where there is a reason to use ：

1. Mobile capture
2. Polymorphic function object