I'm used to seeing syntax like this for function pointers
int (*pointer_name) (float, char *);
void call_function (void (*)(int), int);
In some C++03 functional libraries I see types used this way:
abc::function<void(*)(int,float)> f;
In C++11's std::function
I see the type given this way
std::function<void(int,float)> f;
There is a missing (*)
. Why?
The C++03 function<T>
has T
being an identical type to the corresponding function pointer. It's easy to imagine the implementation.
std::function
in C++11 is supported by core language enhancements. Have template argument types been extended to accomodate callability?
std::function
(and its inspiration, boost::function
) does not only store function pointers. It can also store function objects. In that sense, passing a function signature as a template parameter is similar to how a smart pointer usually take the type of the pointee as a template parameter, not a pointer type!
Contrast:
int* p; // indirection to an object of type int
std::unique_ptr<int> q; // indirection to an object of type int
with
typedef void signature_type(); // a function type
// indirection to something callable with signature_type as a signature
// i.e. f() has type void
// only work for freestanding functions however
signature_type* f;
// indirection to something callable with signature_type as a signature
// i.e. g() has type void
// not restricted to function pointers!
std::function<signature_type> g;
This is a useful convention.
There is nothing magic here, the type
void(int,float)
is the type of a function without the names. It matches a function like void g(int x, float y)
.
With templates you don't have to use function pointers, you can use function types as well.
As with other elements, functions have a type, and you can use either the type or the pointer to the type in different contexts. The missing (*)
you are expecting is just the pointer-to syntax.
int (*pointer_name) (float, char *);
typedef int my_function_type(float,char*);
my_function_type * pointer_name2;
The types of pointer_name
and pointer_name2
are the same: pointer to a function that returns int
and takes two arguments of types float
and char*
. Note that this is exactly equivalent to other types like int
, with the difference that you cannot declare a variable to be of type function, only pointer to function.
The interface of std::function
(or boost::function
) just takes the signature of the function. The type argument is not a pointer to function but rather the type of a function (like my_function_type
in the code above)
Function types aren't new in C++11 (see 8.3.5 in C++98). IIRC, the improvement over what TR1 and boost provide for function
are quite minor.