I am fairly new to C++ and I have a question regarding practices of std::bind. The following snippet is copied from this tutorial on the ROS2 website. The code creates a class where the timer_ field hosts a timer that is created using create_wall_timer(). creates_wall_timer() accepts a callback object of type CallbackT &&. In the constructor of the class, why does the author pass the result of std::bind(...) as the callback to create_timer() instead of a direct pointer or reference to the timer_callback method?
Apologies for the long questions. I am not really good at asking these questions. Hopefully, I didn't miss too much information that you need.
#include <chrono>
#include <functional>
#include <memory>
#include <string>
#include "rclcpp/rclcpp.hpp"
#include "std_msgs/msg/string.hpp"
using namespace std::chrono_literals;
/* This example creates a subclass of Node and uses std::bind() to register a
* member function as a callback from the timer. */
class MinimalPublisher : public rclcpp::Node
{
public:
MinimalPublisher()
: Node("minimal_publisher"), count_(0)
{
publisher_ = this->create_publisher<std_msgs::msg::String>("topic", 10);
timer_ = this->create_wall_timer(
500ms, std::bind(&MinimalPublisher::timer_callback, this));
}
private:
void timer_callback()
{
auto message = std_msgs::msg::String();
message.data = "Hello, world! " + std::to_string(count_++);
RCLCPP_INFO(this->get_logger(), "Publishing: '%s'", message.data.c_str());
publisher_->publish(message);
}
rclcpp::TimerBase::SharedPtr timer_;
rclcpp::Publisher<std_msgs::msg::String>::SharedPtr publisher_;
size_t count_;
};
You can't pass a pointer to a member function in isolation (unless that function is declared static), because it needs an instance of the [right kind of] object to be called on.
std::bind binds a pointer to an object (this, in this example) to the member function pointer (&MinimalPublisher::timer_callback) so that when the time comes to call the function, there is an instance of the required / desired object available.
To look at this from another (simpler) angle, consider what happens if you write:
MinimalPublisher::timer_callback ();
If MinimalPublisher::timer_callback is not a static function the compiler will complain, because a non-static function can only be called through a [pointer to a] MinimalPublisher object, so something like:
my_minimal_publisher_object.MinimalPublisher::timer_callback ();
or:
my_minimal_publisher_object_pointer->MinimalPublisher::timer_callback ();
You might like to experiment with this in your favourite online compiler.
Incidentally, std::bind has been largely superseded by capturing lambdas these days. So, in order to capture the required object instance (and taking my original example over at Wandbox as a starting point), you might do:
#include <functional>
struct MyStruct
{
void foo ();
};
int main()
{
MyStruct s;
std::function <void ()> f = [&s] { s.foo (); };
// Do things with f
}
Let's assume some third-party developer writes a function
int GetErrorCode(const object * p);
This function returns only certain int values, thus I am tempted to write my own ErrorCode enum class, that contains all the possible return values. Then write a bit updated function:
enum class ErrorCode : int {};
ErrorCode GetErrorCode2(const object * p){
return (ErrorCode)GetErrorCode(p);
}
The problem is I want my function to be named GetErrorCode and not that less-intuitive GetErrorCode2.
How can I possibly achieve that? Maybe there is a way to swap function names or something?
Use namespaces:
namespace MyLibrary {
ErrorCode GetErrorCode(const object *p) {
int origResult = ::ErrorCode(p);
// use :: to explicitly call outer function to avoid recursion
...
}
}
Then you can call the function as:
MyLibrary::GetErrorCode(obj);
Put your function in a namespace that you normally use. You might have a using namespace foo; - er better yet, using foo::GetErrorCode; - for it where you use it, preferably in a function's body.
I am making a Queue for running functions. I put the functions that require being called into a std::deque<bool(*)()> Then I later on cycle through the deque calling each function and letting it run, sometimes even doing things based on the return.
The problem I am having is actually with regards to placing these functions inside of the deque.
I have this deque inside a class called A2_Game. I also have a class called Button.
My code resembles the following:
class Button
{
bool DoInput();
}
class A2_Game
{
std::deque<bool(*)()> Input_Functions;
bool EnterName()
}
A2_Game::OtherMethod()
{
Button* btn1 = new Button();
Input_Functions.push_back(&A2_Game::EnterName); //The compiler told me to do this and it still won't compile the line..
Input_Functions.push_back(btn1->DoInput);
//Loop
}
I cannot determine how to fix my compile errors. I suspect some of you may be able to just tell me outright what needs to be changed/done in order to get this to compile, by looking at what I've shown here. In case that is !true then here are the compile errors.
error C2664: 'std::deque<_Ty>::push_back' : cannot convert parameter 1 from 'bool (__thiscall A2_Game::* )(void)' to 'bool (__cdecl *const &)(void)'
error C3867: 'Button::Doinput': function call missing argument list; use '&Button::Doinput' to create a pointer to member
if you want to push back functions you can use std::function (or boost if your compiler doesn't support c++11)
std::deque<std::function<bool()> > function_list;
Button* btn1 = new Button();
function_list.push_back([this](){return EnterName();});
function_list.push_back([btn1](){return btn1->DoInput();});
make sure everything in the lambda is still going to be valid when you call it from the function_list.
EDIT:
boost equivalent
std::deque<boost::function<bool()> > function_list;
Button* btn1 = new Button();
function_list.push_back(boost::bind(&A2_Game::EnterName,this));
function_list.push_back(boost::bind(&Button::DoInput,btn1));
The problem is that the signature of the class methods don't match with the function signature bool (*)(). The signatures of the two methods are bool (Button::*)(); or bool (A2_Game::*)(); respectively. (The actual class to which the method belongs is part of its signature!)
The solution here is to use functors/function objects. Functors are wrapper objects around "callable elements" that are useful if you want to treat functions like objects (in a OOP sense). If you have boost at hand your code could look similar to this (code compiles):
#include <boost/function.hpp>
#include <deque>
class Button
{
public:
bool DoInput() { return true; }
};
class A2_Game
{
public:
typedef boost::function<bool()> Functor;
std::deque<Functor> Input_Functions;
bool EnterName() { return true; }
void OtherMethod();
};
void A2_Game::OtherMethod()
{
Button* btn1 = new Button();
Input_Functions.push_back(boost::bind(&A2_Game::EnterName, this));
Input_Functions.push_back(boost::bind(&Button::DoInput, btn1));
}
boost::bind combines a function pointer with the reference to an actual class instance and returns an function object of the same type as A2_Game::Functor.
Note that boost::function has been integrated into the C++11 standard (see here), so if your project supports C++11 simply use #include <functional> and std instead of boost namespaces.
I am looking for a way to call different functions by a string input.
I have a map that ties each unique string to a function pointer and a lookup function to search the map and return a pointer if found.
Now the trick is, I need a way to store and return pointers to functions with at least different return types, if possible, also with different signatures.
The usage would be:
Get a string input from a network socket ->
find and execute the found function -> shove the result straight back into the socket to be serialized and sent, not caring what actually happened.
Is this doable? If not, how would one approach this task?
That can be done with a bit of boilerplate code in different ways. If the number of signatures is small enough you can hold multiple vectors of function pointers (one per function type) and then a map that maps the function name with a type identifier (used to select the vector) and the position within the vector.
The second option would be to store a boost::variant (again, if the set of signatures is small). You would need to provide a visitor object that evaluates the function (for each function type stored) and yields the result. The type is managed by the boost::variant type so there would be no need for the type tag to be stored in the map.
You can also use full type erasure and store in the map a tag determining the type of function to be called and a boost::any object storing the function pointer. You can use the type information to retrieve the pointer and execute the function, but you will have to manually handle the switch based on function type.
The simplest approach, on the other hand, is to write adapters that have a fixed interface. Then just store the pointers to the adapters in the map.
While you can't store different function pointers, you can store objects which contain those functions.
#include <iostream>
#include <cmath>
#include <map>
#include <string>
using namespace std;
class Functor{
public:
template<class T>
void operator()(T data){}
};
template<class T>
class BaseFunctor : public Functor{
public:
virtual void CallFunction(T data){ }
};
class FunctionPointer1 : public BaseFunctor<void *>{
public:
void doFunction1(){
cout << "Do Function 1"<<endl;
}
template<class T>
void CallFunction(T data){ doFunction1(); }
template<class T>
void operator()(T data){ this->CallFunction(data); }
};
class FunctionPointer2 : public BaseFunctor<int>{
public:
void doFunction2(int variable){ cout << "Do function 2 with integer variable" << variable <<endl; }
template<class T>
void CallFunction(T data) { doFunction2(data);}
template<class T>
void operator()(T data){ this->CallFunction(data); }
};
class FunctionPerformer{
private:
map<string,Functor> functions;
public:
FunctionPerformer(){
//init your map.
FunctionPointer1 function1;
FunctionPointer2 function2;
//-- follows
functions["Function1"] = function1;
functions["Functions2"] = function2;
//-- follows
}
Functor getFunctionFromString(string str){
return functions[str]
}
};
int main(int argc, char *argv[])
{
map<string,Functor> functions;
FunctionPerformer performer;
Functor func1, func2; // to hold return values from perfomer()
FunctionPointer1 *fn1; // to casting and execute the functions
FunctionPointer2 *fn2; // to casting and execute the functions
func1 = performer.getFunctionFromString("Function1");//get data
func2 = performer.getFunctionFromString("Function2");
//following two lines to cast the object and run the methods
fn1 = reinterpret_cast<FunctionPointer1 *>(&func1);
(*fn1)(NULL);
//following two lines to cast the object and run the methods
fn2 = reinterpret_cast<FunctionPointer2 *>(&func2);
(*fn2)(10);
system("Pause");
return 0;
}
I think the edited part makes it clearer?
This code can be optimized a little. Play around with it.
This is doable in C++11 with Variadic Templates. Check my answer at https://stackoverflow.com/a/33837343/1496826
No, it's really not doable, you need a real interpreted language if you want to do something like this. As soon as the signature is not constant then you need something a lot more involved.
How about making all those functions have the same signature? You could make all return types implement an interface, or use a collection, class, union or struct. Same for the arguments.
Can't you use specialization and templates to work around the issue?
template <class T>
T FooBar(void * params);
template<> int FooBar<int>( void * params );
template<> char FooBar<char>( void * params );
Instead of storing the function pointers themselves, which are too different from one another to be accommodated into the same data structure, you can store adaptors that take care of bridging the mismatch. This is a form of type-erasure. An example:
// Imaginary important resources
blaz_type get_blaz();
qux_type get_qux();
// The functions we'd like to put in our map
int foo(blaz_type);
std::string bar(qux_type);
using context_type = std::tuple<blaz_type, qux_type>;
using callback_type = std::function<void(context_type, socket_type&)>;
using std::get;
std::map<std::string, callback_type> callbacks = {
{
"foo"
, [](context_type context, socket_type& out)
{ marshall(out, foo(get<0>(std::move(context)))); }
}
, {
"bar"
, [](context_type context, socket_type& out)
{ marshall(out, bar(get<1>(std::move(context)))); }
}
};
In this example the adaptors are not stateful so you can actually use void (*)(context_type, socket_type&) as the callback_type.
Do note that this kind of design is a bit brittle in that the context_type needs to know about every kind of parameter a stored callback might ever need. If at some later point you need to store a callback which needs a new kind of parameter, you need to modify context_type -- if you improve the above design not to use magic numbers like 0 and 1 as parameters to std::get you could save yourself some pains (especially in the reverse situation of removing types from context_type). This is not an issue if all callbacks take the same parameters, in which case you can dispense yourself with the context_type altogether and pass those parameters to the callbacks directly.
Demonstration on LWS.
I've read through this article, and what I take from it is that when you want to call a pointer to a member function, you need an instance (either a pointer to one or a stack-reference) and call it so:
(instance.*mem_func_ptr)(..)
or
(instance->*mem_func_ptr)(..)
My question is based on this: since you have the instance, why not call the member function directly, like so:
instance.mem_func(..) //or: instance->mem_func(..)
What is the rational/practical use of pointers to member functions?
[edit]
I'm playing with X-development & reached the stage where I am implementing widgets; the event-loop-thread for translating the X-events to my classes & widgets needs to start threads for each widget/window when an event for them arrives; to do this properly I thought I needed function-pointers to the event-handlers in my classes.
Not so: what I did discover was that I could do the same thing in a much clearer & neater way by simply using a virtual base class. No need whatsoever for pointers to member-functions. It was while developing the above that the doubt about the practical usability/meaning of pointers to member-functions arose.
The simple fact that you need a reference to an instance in order to use the member-function-pointer, obsoletes the need for one.
[edit - #sbi & others]
Here is a sample program to illustrate my point:
(Note specifically 'Handle_THREE()')
#include <iostream>
#include <string>
#include <map>
//-----------------------------------------------------------------------------
class Base
{
public:
~Base() {}
virtual void Handler(std::string sItem) = 0;
};
//-----------------------------------------------------------------------------
typedef void (Base::*memfunc)(std::string);
//-----------------------------------------------------------------------------
class Paper : public Base
{
public:
Paper() {}
~Paper() {}
virtual void Handler(std::string sItem) { std::cout << "Handling paper\n"; }
};
//-----------------------------------------------------------------------------
class Wood : public Base
{
public:
Wood() {}
~Wood() {}
virtual void Handler(std::string sItem) { std::cout << "Handling wood\n"; }
};
//-----------------------------------------------------------------------------
class Glass : public Base
{
public:
Glass() {}
~Glass() {}
virtual void Handler(std::string sItem) { std::cout << "Handling glass\n"; }
};
//-----------------------------------------------------------------------------
std::map< std::string, memfunc > handlers;
void AddHandler(std::string sItem, memfunc f) { handlers[sItem] = f; }
//-----------------------------------------------------------------------------
std::map< Base*, memfunc > available_ONE;
void AddAvailable_ONE(Base *p, memfunc f) { available_ONE[p] = f; }
//-----------------------------------------------------------------------------
std::map< std::string, Base* > available_TWO;
void AddAvailable_TWO(std::string sItem, Base *p) { available_TWO[sItem] = p; }
//-----------------------------------------------------------------------------
void Handle_ONE(std::string sItem)
{
memfunc f = handlers[sItem];
if (f)
{
std::map< Base*, memfunc >::iterator it;
Base *inst = NULL;
for (it=available_ONE.begin(); ((it != available_ONE.end()) && (inst==NULL)); it++)
{
if (it->second == f) inst = it->first;
}
if (inst) (inst->*f)(sItem);
else std::cout << "No instance of handler for: " << sItem << "\n";
}
else std::cout << "No handler for: " << sItem << "\n";
}
//-----------------------------------------------------------------------------
void Handle_TWO(std::string sItem)
{
memfunc f = handlers[sItem];
if (f)
{
Base *inst = available_TWO[sItem];
if (inst) (inst->*f)(sItem);
else std::cout << "No instance of handler for: " << sItem << "\n";
}
else std::cout << "No handler for: " << sItem << "\n";
}
//-----------------------------------------------------------------------------
void Handle_THREE(std::string sItem)
{
Base *inst = available_TWO[sItem];
if (inst) inst->Handler(sItem);
else std::cout << "No handler for: " << sItem << "\n";
}
//-----------------------------------------------------------------------------
int main()
{
Paper p;
Wood w;
Glass g;
AddHandler("Paper", (memfunc)(&Paper::Handler));
AddHandler("Wood", (memfunc)(&Wood::Handler));
AddHandler("Glass", (memfunc)(&Glass::Handler));
AddAvailable_ONE(&p, (memfunc)(&Paper::Handler));
AddAvailable_ONE(&g, (memfunc)(&Glass::Handler));
AddAvailable_TWO("Paper", &p);
AddAvailable_TWO("Glass", &g);
std::cout << "\nONE: (bug due to member-function address being relative to instance address)\n";
Handle_ONE("Paper");
Handle_ONE("Wood");
Handle_ONE("Glass");
Handle_ONE("Iron");
std::cout << "\nTWO:\n";
Handle_TWO("Paper");
Handle_TWO("Wood");
Handle_TWO("Glass");
Handle_TWO("Iron");
std::cout << "\nTHREE:\n";
Handle_THREE("Paper");
Handle_THREE("Wood");
Handle_THREE("Glass");
Handle_THREE("Iron");
}
{edit] Potential problem with direct-call in above example:
In Handler_THREE() the name of the method must be hard-coded, forcing changes to be made anywhere that it is used, to apply any change to the method. Using a pointer to member-function the only additional change to be made is where the pointer is created.
[edit] Practical uses gleaned from the answers:
From answer by Chubsdad:
What: A dedicated 'Caller'-function is used to invoke the mem-func-ptr;Benefit: To protect code using function(s) provided by other objectsHow: If the particular function(s) are used in many places and the name and/or parameters change, then you only need to change the name where it is allocated as pointer, and adapt the call in the 'Caller'-function. (If the function is used as instance.function() then it must be changed everywhere.)
From answer by Matthew Flaschen:
What: Local specialization in a classBenefit: Makes the code much clearer,simpler and easier to use and maintainHow: Replaces code that would conventionally be implement using complex logic with (potentially) large switch()/if-then statements with direct pointers to the specialization; fairly similar to the 'Caller'-function above.
The same reason you use any function pointer: You can use arbitrary program logic to set the function pointer variable before calling it. You could use a switch, an if/else, pass it into a function, whatever.
EDIT:
The example in the question does show that you can sometimes use virtual functions as an alternative to pointers to member functions. This shouldn't be surprising, because there are usually multiple approaches in programming.
Here's an example of a case where virtual functions probably don't make sense. Like the code in the OP, this is meant to illustrate, not to be particularly realistic. It shows a class with public test functions. These use internal, private, functions. The internal functions can only be called after a setup, and a teardown must be called afterwards.
#include <iostream>
class MemberDemo;
typedef void (MemberDemo::*MemberDemoPtr)();
class MemberDemo
{
public:
void test1();
void test2();
private:
void test1_internal();
void test2_internal();
void do_with_setup_teardown(MemberDemoPtr p);
};
void MemberDemo::test1()
{
do_with_setup_teardown(&MemberDemo::test1_internal);
}
void MemberDemo::test2()
{
do_with_setup_teardown(&MemberDemo::test2_internal);
}
void MemberDemo::test1_internal()
{
std::cout << "Test1" << std::endl;
}
void MemberDemo::test2_internal()
{
std::cout << "Test2" << std::endl;
}
void MemberDemo::do_with_setup_teardown(MemberDemoPtr mem_ptr)
{
std::cout << "Setup" << std::endl;
(this->*mem_ptr)();
std::cout << "Teardown" << std::endl;
}
int main()
{
MemberDemo m;
m.test1();
m.test2();
}
My question is based on this: since you have the instance, why not call the member function directly[?]
Upfront: In more than 15 years of C++ programming, I have used members pointers maybe twice or thrice. With virtual functions being around, there's not all that much use for it.
You would use them if you want to call a certain member functions on an object (or many objects) and you have to decide which member function to call before you can find out for which object(s) to call it on. Here is an example of someone wanting to do this.
I find the real usefulness of pointers to member functions comes when you look at a higher level construct such as boost::bind(). This will let you wrap a function call as an object that can be bound to a specific object instance later on and then passed around as a copyable object. This is a really powerful idiom that allows for deferred callbacks, delegates and sophisticated predicate operations. See my previous post for some examples:
https://stackoverflow.com/questions/1596139/hidden-features-and-dark-corners-of-stl/1596626#1596626
Member functions, like many function pointers, act as callbacks. You could manage without them by creating some abstract class that calls your method, but this can be a lot of extra work.
One common use is algorithms. In std::for_each, we may want to call a member function of the class of each member of our collection. We also may want to call the member function of our own class on each member of the collection - the latter requires boost::bind to achieve, the former can be done with the STL mem_fun family of classes (if we don't have a collection of shared_ptr, in which case we need to boost::bind in this case too). We could also use a member function as a predicate in certain lookup or sort algorithms. (This removes our need to write a custom class that overloads operator() to call a member of our class, we just pass it in directly to boost::bind).
The other use, as I mentioned, are callbacks, often in event-driven code. When an operation has completed we want a method of our class called to handle the completion. This can often be wrapped into a boost::bind functor. In this case we have to be very careful to manage the lifetime of these objects correctly and their thread-safety (especially as it can be very hard to debug if something goes wrong). Still, it once again can save us from writing large amounts of "wrapper" code.
There are many practical uses. One that comes to my mind is as follows:
Assume a core function such as below (suitably defined myfoo and MFN)
void dosomething(myfoo &m, MFN f){ // m could also be passed by reference to
// const
m.*f();
}
Such a function in the presence of pointer to member functions, becomes open for extension and closed for modification (OCP)
Also refer to Safe bool idiom which smartly uses pointer to members.
The best use of pointers to member functions is to break dependencies.
Good example where pointer to member function is needed is Subscriber/Publisher pattern :
http://en.wikipedia.org/wiki/Publish/subscribe
In my opinion, member function pointers do are not terribly useful to the average programmer in their raw form. OTOH, constructs like ::std::tr1::function that wrap member function pointers together with a pointer to the object they're supposed to operate on are extremely useful.
Of course ::std::tr1::function is very complex. So I will give you a simple example that you wouldn't actually use in practice if you had ::std::tr1::function available:
// Button.hpp
#include <memory>
class Button {
public:
Button(/* stuff */) : hdlr_(0), myhandler_(false) { }
~Button() {
// stuff
if (myhandler_) {
delete hdlr_;
}
}
class PressedHandler {
public:
virtual ~PressedHandler() = 0;
virtual void buttonPushed(Button *button) = 0;
};
// ... lots of stuff
// This stores a pointer to the handler, but will not manage the
// storage. You are responsible for making sure the handler stays
// around as long as the Button object.
void setHandler(const PressedHandler &hdlr) {
hdlr_ = &hdlr;
myhandler_ = false;
}
// This stores a pointer to an object that Button does not manage. You
// are responsible for making sure this object stays around until Button
// goes away.
template <class T>
inline void setHandlerFunc(T &dest, void (T::*pushed)(Button *));
private:
const PressedHandler *hdlr_;
bool myhandler_;
template <class T>
class PressedHandlerT : public Button::PressedHandler {
public:
typedef void (T::*hdlrfuncptr_t)(Button *);
PressedHandlerT(T *ob, hdlrfuncptr_t hdlr) : ob_(ob), func_(hdlr) { }
virtual ~PressedHandlerT() {}
virtual void buttonPushed(Button *button) { (ob_->*func_)(button); }
private:
T * const ob_;
const hdlrfuncptr_t func_;
};
};
template <class T>
inline void Button::setHandlerFunc(T &dest, void (T::*pushed)(Button *))
{
PressedHandler *newhandler = new PressedHandlerT<T>(&dest, pushed);
if (myhandler_) {
delete hdlr_;
}
hdlr_ = newhandler;
myhandler_ = true;
}
// UseButton.cpp
#include "Button.hpp"
#include <memory>
class NoiseMaker {
public:
NoiseMaker();
void squee(Button *b);
void hiss(Button *b);
void boo(Button *b);
private:
typedef ::std::auto_ptr<Button> buttonptr_t;
const buttonptr_t squeebutton_, hissbutton_, boobutton_;
};
NoiseMaker::NoiseMaker()
: squeebutton_(new Button), hissbutton_(new Button), boobutton_(new Button)
{
squeebutton_->setHandlerFunc(*this, &NoiseMaker::squee);
hissbutton_->setHandlerFunc(*this, &NoiseMaker::hiss);
boobutton_->setHandlerFunc(*this, &NoiseMaker::boo);
}
Assuming Button is in a library and not alterable by you, I would enjoy seeing you implement that cleanly using a virtual base class without resorting to a switch or if else if construct somewhere.
The whole point of pointers of pointer-to-member function type is that they act as a run-time way to reference a specific method. When you use the "usual" syntax for method access
object.method();
pointer->method();
the method part is a fixed, compile-time specification of the method you want to call. It is hardcoded into your program. It can never change. But by using a pointer of pointer-to-member function type you can replace that fixed part with a variable, changeable at run-time specification of the method.
To better illustrate this, let me make the following simple analogy. Let's say you have an array
int a[100];
You can access its elements with fixed compile-time index
a[5]; a[8]; a[23];
In this case the specific indices are hardcoded into your program. But you can also access array's elements with a run-time index - an integer variable i
a[i];
the value of i is not fixed, it can change at run-time, thus allowing you to select different elements of the array at run-time. That is very similar to what pointers of pointer-to-member function type let you do.
The question you are asking ("since you have the instance, why not call the member function directly") can be translated into this array context. You are basically asking: "Why do we need a variable index access a[i], when we have direct compile-time constant access like a[1] and a[3]?" I hope you know the answer to this question and realize the value of run-time selection of specific array element.
The same applies to pointers of pointer-to-member function type: they, again, let you to perform run-time selection of a specific class method.
The use case is that you have several member methods with the same signature, and you want to build logic which one should be called under given circumstances. This can be helpful to implement state machine algorithms.
Not something you use everyday...
Imagine for a second you have a function that could call one of several different functions depending on parameters passed.
You could use a giant if/else if statement
You could use a switch statement
Or you could use a table of function pointers (a jump table)
If you have a lot of different options the jump table can be a much cleaner way of arranging your code ...
Its down to personal preference though. Switch statement and jump table correspond to more or less the same compiled code anyway :)
Member pointers + templates = pure win.
e.g. How to tell if class contains a certain member function in compile time
or
template<typename TContainer,
typename TProperty,
typename TElement = decltype(*Container().begin())>
TProperty grand_total(TContainer& items, TProperty (TElement::*property)() const)
{
TProperty accum = 0;
for( auto it = items.begin(), end = items.end(); it != end; ++it) {
accum += (it->*property)();
}
return accum;
}
auto ship_count = grand_total(invoice->lineItems, &LineItem::get_quantity);
auto sub_total = grand_total(invoice->lineItems, &LineItem::get_extended_total);
auto sales_tax = grand_total(invoice->lineItems, &LineItem::calculate_tax);
To invoke it, you need a reference to an instance, but then you can call the func direct & don't need a pointer to it.
This is completely missing the point. There are two indepedent concerns here:
what action to take at some later point in time
what object to perform that action on
Having a reference to an instance satisfies the second requirement. Pointers to member functions address the first: they are a very direct way to record - at one point in a program's execution - which action should be taken at some later stage of execution, possibly by another part of the program.
EXAMPLE
Say you have a monkey that can kiss people or tickle them. At 6pm, your program should set the monkey loose, and knows whom the monkey should visit, but around 3pm your user will type in which action should be taken.
A beginner's approach
So, at 3pm you could set a variable "enum Action { Kiss, Tickle } action;", then at 6pm you could do something like "if (action == Kiss) monkey->kiss(person); else monkey->tickle(person)".
Issues
But that introducing an extra level of encoding (the Action type's introduced to support this - built in types could be used but would be more error prone and less inherently meaningful). Then - after having worked out what action should be taken at 3pm, at 6pm you have to redundantly consult that encoded value to decide which action to take, which will require another if/else or switch upon the encoded value. It's all clumsy, verbose, slow and error prone.
Member function pointers
A better way is to use a more specialised varibale - a member function pointer - that directly records which action to perform at 6pm. That's what a member function pointer is. It's a kiss-or-tickle selector that's set earlier, creating a "state" for the monkey - is it a tickler or a kisser - which can be used later. The later code just invokes whatever function's been set without having to think about the possibilities or have any if/else-if or switch statements.
To invoke it, you need a reference to an instance, but then you can call the func direct & don't need a pointer to it.
Back to this. So, this is good if you make the decision about which action to take at compile time (i.e. a point X in your program, it'll definitely be a tickle). Function pointers are for when you're not sure, and want to decouple the setting of actions from the invocation of those actions.