回调就是一段以参数传给其他代码的可执行代码。
Note: Most of the answers cover function pointers which is one possibility to achieve "callback" logic in C++, but as of today not the most favourable one I think.
Callback 是什么(?) 以及为什么要使用Callback (!)
A callback is a callable (see further down) accepted by a class or function, used to customize the current logic depending on that callback.
One reason to use callbacks is to write generic code which is independant from the logic in the called function and can be reused with different callbacks.
Many functions of the standard algorithms library <algorithm> use callbacks. For example the for_each algorithm applies an unary callback to every item in a range of iterators:
template<class InputIt, class UnaryFunction>
UnaryFunction for_each(InputIt first, InputIt last, UnaryFunction f)
{
for (; first != last; ++first) {
f(*first);
}
return f;
}
which can be used to first increment and then print a vector by passing appropriate callables， for example:
std::vector<double> v{ 1.0, 2.2, 4.0, 5.5, 7.2 };
double r = 4.0;
std::for_each(v.begin(), v.end(), [&](double & v) { v += r; });
std::for_each(v.begin(), v.end(), [](double v) { std::cout << v << " "; });
which prints：
5 6.2 8 9.5 11.2
Another application of callbacks is the notification of callers of certain events which enables a certain amount of static / compile time flexibility.
Personally, I use a local optimization library that uses two different callbacks:
- The first callback is called if a function value and the gradient based on a vector of input values is required (logic callback: function value determination / gradient derivation).
- The second callback is called once for each algorithm step and receives certain information about the convergence of the algorithm (notification callback).
Thus, the library designer is not in charge of deciding what happens with the information that is given to the programmer via the notification callback and he needn't worry about how to acutally determine function values becasue they're provided by the logic callback. Getting those things right is a task due to the library user and keeps the library slim and more generic.
Furthermore, callbacks can enable dynamic runtime behaviour.
Imagine some kind of game engine class which has a function that is fired, each time the users presses a button on his keyboard and a set of functions that control your game behaviour.With callbacks you can (re)decide at runtime which action will be taken.
void player_jump();
void player_crouch();
class game_core
{
std::array<void(*)(), total_num_keys> actions;
//
void key_pressed(unsigned key_id)
{
if(actions[key_id]) actions[key_id]();
}
// update keybind from menu
void update_keybind(unsigned key_id, void(*new_action)())
{
actions[key_id] = new_action;
}
};
Here the function key_pressed uses the callbacks stored in actions to obtain the desired behaviour when a certain key is pressed.If the player chooses to change the button for jumping, the engine can call
game_core_instance.update_keybind(newly_selected_key, &player_jump);
and thus change the behaviour of a call to key_pressed (which the calls player_jump) once this button is pressed the next time ingame.
在 C++(11) 中什么是callable ?
See C++ concepts: Callable on cppreference for a more formal description.
Callback functionality can be realized in several ways in C++(11) since several different things turn out to be callable*:
- Function pointers (including pointers to member functions)
std::functionobjects- Lambda expressions
- Bind expressions
- Function objects (classes with overloaded function call operator
operator())
* Note: Pointer to data members are callable as well but no function is called at all.
详细介绍几种写 callback 的重要方式
- X.1 "Writing" a callback in this post means the syntax to declare and name the callback type.
- X.2 "Calling" a callback refers to the syntax to call those objects.
- X.3 "Using" a callback means the syntax when passing arguments to a function using a callback.
Note: As of C++17, a call like f(...) can be written as std::invoke(f, ...) which also handles the pointer to member case.
1. 函数指针
A function pointer is the 'simplest' (in terms of generality; in terms of readability arguably the worst) type a callback can have.
Let's have a simple function foo:
int foo (int x) { return 2+x; }
1.1 写一个函数指针/类型notation
A function pointer type has the notation
return_type (*)(paramter_type_1, paramter_type_2, paramter_type_3)
// i.e. a pointer to foo has the type:
int (*)(int)
where a named function pointer type will look like
return_type (* name) (paramter_type_1, paramter_type_2, paramter_type_3)
// i.e. f_int_t is a type: function pointer taking one int argument, returning int
typedef int (*f_int_t) (int);
// foo_p is a pointer to function taking int returning int
// initialized by pointer to function foo taking int returning int
int (* foo_p)(int) = &foo;
// can alternatively be written as
f_int_t foo_p = &foo;
The using declaration gives us the option to make things a little bit more readable, since the typedef for f_int_t can also be written as:
using f_int_t = int(*)(int);
Where (at least for me) it is clearer that f_int_t is the new type alias and recognition of the function pointer type is also easier
And a declaration of a function using a callback of function pointer type will be:
// foobar having a callback argument named moo of type
// pointer to function returning int taking int as its argument
int foobar (int x, int (*moo)(int));
// if f_int is the function pointer typedef from above we can also write foobar as:
int foobar (int x, f_int_t moo);
1.2 callback调用notation
The call notation follows the simple function call syntax:
int foobar (int x, int (*moo)(int))
{
return x + moo(x); // function pointer moo called using argument x
}
// analog
int foobar (int x, f_int_t moo)
{
return x + moo(x); // function pointer moo called using argument x
}
1.3 Callback使用notation及兼容类型
A callback function taking a function pointer can be called using function pointers.
Using a function that takes a function pointer callback is rather simple:
int a = 5;
int b = foobar(a, foo); // call foobar with pointer to foo as callback
// can also be
int b = foobar(a, &foo); // call foobar with pointer to foo as callback
1.4 例子
A function ca be written that doesn't rely on how the callback works:
void tranform_every_int(int * v, unsigned n, int (*fp)(int))
{
for (unsigned i = 0; i < n; ++i)
{
v[i] = fp(v[i]);
}
}
where possible callbacks could be
int double_int(int x) { return 2*x; }
int square_int(int x) { return x*x; }
used like
int a[5] = {1, 2, 3, 4, 5};
tranform_every_int(&a[0], 5, double_int);
// now a == {2, 4, 6, 8, 10};
tranform_every_int(&a[0], 5, square_int);
// now a == {4, 16, 36, 64, 100};
2. 指向成员函数的指针
A pointer to member function (of some class C) is a special type of (and even more complex) function pointer which requires an object of type C to operate on.
struct C
{
int y;
int foo(int x) const { return x+y; }
};
2.1 写一个指向成员函数/类型的指针notation
A pointer to member function type for some class T has the notation
// can have more or less parameters
return_type (T::*)(paramter_type_1, paramter_type_2, paramter_type_3)
// i.e. a pointer to C::foo has the type
int (C::*) (int)
where a named pointer to member function will -in analogy to the function pointer- look like this:
return_type (T::* name) (paramter_type_1, paramter_type_2, paramter_type_3)
// i.e. a type `f_C_int` representing a pointer to member function of `C`
// taking int returning int is:
typedef int (C::* f_C_int_t) (int x);
// The type of C_foo_p is a pointer to member function of C taking int returning int
// Its value is initialized by a pointer to foo of C
int (C::* C_foo_p)(int) = &C::foo;
// which can also be written using the typedef:
f_C_int_t C_foo_p = &C::foo;
Example: Declaring a function taking a pointer to member function callback as one of its arguments:
// C_foobar having an argument named moo of type pointer to member function of C
// where the callback returns int taking int as its argument
// also needs an object of type c
int C_foobar (int x, C const &c, int (C::*moo)(int));
// can equivalently declared using the typedef above:
int C_foobar (int x, C const &c, f_C_int_t moo);
2.2 callback调用notation
The pointer to member function of C can be invoked, with respect to an object of type C by using member access operations on the dereferenced pointer.Note: Parenthesis required!
int C_foobar (int x, C const &c, int (C::*moo)(int))
{
return x + (c.*moo)(x); // function pointer moo called for object c using argument x
}
// analog
int C_foobar (int x, C const &c, f_C_int_t moo)
{
return x + (c.*moo)(x); // function pointer moo called for object c using argument x
}
Note: If a pointer to C is available the syntax is equivalent (where the pointer to C must be dereferenced as well):
int C_foobar_2 (int x, C const * c, int (C::*meow)(int))
{
if (!c) return x;
// function pointer meow called for object *c using argument x
return x + ((*c).*meow)(x);
}
// or equivalent:
int C_foobar_2 (int x, C const * c, int (C::*meow)(int))
{
if (!c) return x;
// function pointer meow called for object *c using argument x
return x + (c->*meow)(x);
}
2.3 callback使用notation及兼容类型
A callback function taking a member function pointer of class T can be called using a member function pointer of class T.
Using a function that takes a pointer to member function callback is -in analogy to function pointers- quite simple as well:
C my_c{2}; // aggregate initialization
int a = 5;
int b = C_foobar(a, my_c, &C::foo); // call C_foobar with pointer to foo as its callback
3. std::function 对象( 头文件 <functional>)
The std::function class is a polymorphic function wrapper to store, copy or invoke callables.
3.1 写std::function 对象/类型notation
The type of a std::function object storing a callable looks like:
std::function<return_type(paramter_type_1, paramter_type_2, paramter_type_3)>
// i.e. using the above function declaration of foo:
std::function<int(int)> stdf_foo = &foo;
// or C::foo:
std::function<int(const C&, int)> stdf_C_foo = &C::foo;
3.2 callback调用notation
The class std::function has operator() defined which can be used to invoke its target.
int stdf_foobar (int x, std::function<int(int)> moo)
{
return x + moo(x); // std::function moo called
}
// or
int stdf_C_foobar (int x, C const &c, std::function<int(C const &, int)> moo)
{
return x + moo(c, x); // std::function moo called using c and x
}
3.3 callback使用notation及兼容类型
The std::function callback is more generic than function pointers or pointer to member function since different types can be passed and implicitly converted into a std::function object.
3.3.1 函数指针和指向成员函数的指针
A function pointer
int a = 2;
int b = stdf_foobar(a, &foo);
// b == 6 ( 2 + (2+2) )
or a pointer to member function
int a = 2;
C my_c{7}; // aggregate initialization
int b = stdf_C_foobar(a, c, &C::foo);
// b == 11 == ( 2 + (7+2) )
can be used.
3.3.2 Lambda 表达式
An unnamed closure from a lambda expression can be stored in a std::function object:
int a = 2;
int c = 3;
int b = stdf_foobar(a, [c](int x) -> int { return 7+c*x; });
// b == 15 == a + (7*c*a) == 2 + (7+3*2)
3.3.3 std::bind 表达式
The result of a std::bind expression can be passed. For example by binding parameters to a function pointer call:
int foo_2 (int x, int y) { return 9*x + y; }
using std::placeholders::_1;
int a = 2;
int b = stdf_foobar(a, std::bind(foo_2, _1, 3));
// b == 23 == 2 + ( 9*2 + 3 )
int c = stdf_foobar(a, std::bind(foo_2, 5, _1));
// c == 49 == 2 + ( 9*5 + 2 )
Where also objects can be bound as the object for the invokation of pointer to member functions:
int a = 2;
C const my_c{7}; // aggregate initialization
int b = stdf_foobar(a, std::bind(&C::foo, my_c, _1));
// b == 1 == 2 + ( 2 + 7 )
3.3.4 函数对象
Objects of classes having a proper operator() overload can be stored inside a std::function object, as well.
struct Meow
{
int y = 0;
Meow(int y_) : y(y_) {}
int operator()(int x) { return y * x; }
};
int a = 11;
int b = stdf_foobar(a, Meow{8});
// b == 99 == 11 + ( 8 * 11 )
3.4 例子
Changing the function pointer example to use std::function
void stdf_tranform_every_int(int * v, unsigned n, std::function<int(int)> fp)
{
for (unsigned i = 0; i < n; ++i)
{
v[i] = fp(v[i]);
}
}
gives a whole lot more utility to that function because (see 3.3) we have more possibilities to use it:
// using function pointer still possible
int a[5] = {1, 2, 3, 4, 5};
stdf_tranform_every_int(&a[0], 5, double_int);
// now a == {2, 4, 6, 8, 10};
// use it without having to write another function by using a lambda
stdf_tranform_every_int(&a[0], 5, [](int x) -> int { return x/2; });
// now a == {1, 2, 3, 4, 5}; again
// use std::bind :
int nine_x_and_y (int x, int y) { return 9*x + y; }
using std::placeholders::_1;
// calls nine_x_and_y for every int in a with y being 4 every time
stdf_tranform_every_int(&a[0], 5, std::bind(nine_x_and_y, _1, 4));
// now a == {13, 22, 31, 40, 49};
4. 模板化的callback类型
Using templates, the code calling the callback can be even more general than using std::function objects.
Note that templates are a compile-time feature and are a design tool for compile-time polymorphism. If runtime dynamic behaviour is to be achieved through callbacks, templates will help but they won't induce runtime dynamics.
4.1 写(类型notation) 及调用模板化的callback
Generalizing i.e. the std_ftransform_every_int code from above even further can be achieved by using templates:
template<class R, class T>
void stdf_transform_every_int_templ(int * v,
unsigned const n, std::function<R(T)> fp)
{
for (unsigned i = 0; i < n; ++i)
{
v[i] = fp(v[i]);
}
}
with an even more general (as well as easiest) syntax for a callback type being a plain, to-be-deduced templated argument:
template<class F>
void transform_every_int_templ(int * v,
unsigned const n, F f)
{
std::cout << "transform_every_int_templ<"
<< type_name<F>() << ">\n";
for (unsigned i = 0; i < n; ++i)
{
v[i] = f(v[i]);
}
}
Note: The included output prints the type name deduced for templated type F. The implementation of type_name is given at the end of this post.
The most general implementation for the unary transformation of a range is part of the standard library, namely std::transform, which is also templated with respect to the iterated types.
template<class InputIt, class OutputIt, class UnaryOperation>
OutputIt transform(InputIt first1, InputIt last1, OutputIt d_first,
UnaryOperation unary_op)
{
while (first1 != last1) {
*d_first++ = unary_op(*first1++);
}
return d_first;
}
4.2 使用模板化的callback和兼容类型的例子
The compatible types for the templated std::function callback method stdf_transform_every_int_templ are identical to the above mentioned types (see 3.4).
Using the templated version however, the signature of the used callback may change a little:
// Let
int foo (int x) { return 2+x; }
int muh (int const &x) { return 3+x; }
int & woof (int &x) { x *= 4; return x; }
int a[5] = {1, 2, 3, 4, 5};
stdf_transform_every_int_templ<int,int>(&a[0], 5, &foo);
// a == {3, 4, 5, 6, 7}
stdf_transform_every_int_templ<int, int const &>(&a[0], 5, &muh);
// a == {6, 7, 8, 9, 10}
stdf_transform_every_int_templ<int, int &>(&a[0], 5, &woof);
Note: std_ftransform_every_int (non templated version; see above) does work with foo but not using muh.
// Let
void print_int(int * p, unsigned const n)
{
bool f{ true };
for (unsigned i = 0; i < n; ++i)
{
std::cout << (f ? "" : " ") << p[i];
f = false;
}
std::cout << "\n";
}
The plain templated paramter of transform_every_int_templ can be every possible callable type.
int a[5] = { 1, 2, 3, 4, 5 };
print_int(a, 5);
transform_every_int_templ(&a[0], 5, foo);
print_int(a, 5);
transform_every_int_templ(&a[0], 5, muh);
print_int(a, 5);
transform_every_int_templ(&a[0], 5, woof);
print_int(a, 5);
transform_every_int_templ(&a[0], 5, [](int x) -> int { return x + x + x; });
print_int(a, 5);
transform_every_int_templ(&a[0], 5, Meow{ 4 });
print_int(a, 5);
using std::placeholders::_1;
transform_every_int_templ(&a[0], 5, std::bind(foo_2, _1, 3));
print_int(a, 5);
transform_every_int_templ(&a[0], 5, std::function<int(int)>{&foo});
print_int(a, 5);
The above code prints:
1 2 3 4 5
transform_every_int_templ <int(*)(int)>
3 4 5 6 7
transform_every_int_templ <int(*)(int&)>
6 8 10 12 14
transform_every_int_templ <int& (*)(int&)>
9 11 13 15 17
transform_every_int_templ <main::{lambda(int)#1} >
27 33 39 45 51
transform_every_int_templ <Meow>
108 132 156 180 204
transform_every_int_templ <std::_Bind<int(*(std::_Placeholder<1>, int))(int, int)>>
975 1191 1407 1623 1839
transform_every_int_templ <std::function<int(int)>>
977 1193 1409 1625 1841
上面使用的type_name实现
#include <type_traits>
#include <typeinfo>
#include <string>
#include <memory>
#include <cxxabi.h>
template <class T>
std::string type_name()
{
typedef typename std::remove_reference<T>::type TR;
std::unique_ptr<char, void(*)(void*)> own
(abi::__cxa_demangle(typeid(TR).name(), nullptr,
nullptr, nullptr), std::free);
std::string r = own != nullptr?own.get():typeid(TR).name();
if (std::is_const<TR>::value)
r += " const";
if (std::is_volatile<TR>::value)
r += " volatile";
if (std::is_lvalue_reference<T>::value)
r += " &";
else if (std::is_rvalue_reference<T>::value)
r += " &&";
return r;
}
一段使用了回调函数的 sample code，用到的回调函数为 META_R_cnf，回调函数的参数由library实现给出并处理，实际code里没有见到有参数。
CMETAHANDLE m_META_HANDLE_Obj;
void _stdcall META_NVR_W_cnf(const FT_NVRAM_WRITE_CNF *cnf, const short token, void *usrData);
void _stdcall META_NVR_R_cnf(const FT_NVRAM_READ_CNF *cnf, const short token,void *usrData );
FT_NVRAM_READ_CNF NVRAM_ReadCnf;
FT_NVRAM_WRITE_CNF NVRAM_WriteCnf;
int bfinished = 0;
bool ReadGPIOFromNV( GPIO_VALUE& gpio_value )
{
int buf_size = 0;
char* buffer;
FT_NVRAM_READ_REQ nvram_read_req;
FT_NVRAM_READ_CNF nvram_read_cnf;
char tmp_value = 0;
short rd_token;
int rd_data;
int iloop=0;
bfinished=0;
META_RESULT meta_result = META_NVRAM_GetRecLen( "NVRAM_EF_PCBA_VARIANT_LID", &buf_size );
if ( meta_result != META_SUCCESS)
return false;
buffer = (char*)malloc(sizeof(char)*buf_size);
nvram_read_req.LID="NVRAM_EF_PCBA_VARIANT_LID";
nvram_read_req.RID=1;
nvram_read_cnf.RID=1;
nvram_read_cnf.buf= (unsigned char*)buffer;
nvram_read_cnf.len=buf_size;
meta_result = META_NVRAM_Read_r( m_META_HANDLE_Obj.Get_MainHandle(), &nvram_read_req, &nvram_read_cnf,
META_NVR_R_cnf, &rd_token, &rd_data );
if(meta_result!=META_SUCCESS)
{
free( buffer );
return false;
}
while(!bfinished)
{
Sleep(100);
iloop++;
if(iloop==30)
{
free( buffer );
return false;
}
}
char str[30];
for( int i = 0; i < 3; i++ )
{
sprintf( str, "pcba_id%d", i );
meta_result=META_NVRAM_GetRecFieldValue( "NVRAM_EF_PCBA_VARIANT_LID", str,
buffer, buf_size, &tmp_value, sizeof(tmp_value) );
if( meta_result!=META_SUCCESS)
{
free( buffer );
return false;
}
else
gpio_value.value[i] = tmp_value;
}
free( buffer );
return true;
}
void _stdcall META_NVR_R_cnf(const FT_NVRAM_READ_CNF *cnf, const short token,void *usrData)
{
if(cnf->status==0)
NVRAM_ReadCnf=*cnf;
bfinished=1;
}
void _stdcall META_NVR_W_cnf( const FT_NVRAM_WRITE_CNF *cnf, const short token, void *usrData)
{
if(cnf->status==0)
NVRAM_WriteCnf=*cnf;
bfinished=1;
}