The callback class can now accept generalized function-objects:
class Thing {
public:
int value;
void operator()() {
printf("hi! %d\n", value);
}
};
Callback<void()> cb(Thing(2));
However, with the intention of avoiding implicit dynamic-memory
allocations, the Callback class is limited to a single word of storage.
Exceeding this size will eliminate the function-object type from the
overload set and fail to compile.
Effort was invested to make this situation very clear to the user. Here
is an example error message with noise removed:
[ERROR] ./main.cpp: In function 'int main()':
./mbed-os/hal/api/Ticker.h:101:10: note:
no known conversion for argument 1 from 'BigFunc' to 'mbed::Callback<void()>'
The real benefit of this change is the ability for users to hook into
the attributes of the Callback class. This mostly allows lifetime
management of the function-objects from third-party libraries (such as
the Event class from mbed-events).
Note: The convenient `callback` function may become ambiguous if
provided with a type that defines multiple incompatible `operator()`
member functions.
This allows additional attributes to be attached to the internally
generated type such as move and destructor operations with no increase
in RAM footprint.
The current overloads can't take advantage of this, but it does open
the possibility for more powerful overloads that can provide these
additional attributes.
Changes to mbed-os memory consumption:
.text .data .bss
before 57887 2292 7692
after 57842 2292 7691
Before:
Callback<void()> a = callback(obj, member)
Callback<void()> b = callback(context, function)
After:
Callback<void()> a = callback(obj, member)
Callback<void()> b = callback(function, context)
This ordering is more intuitive based on feedback from users. This order
was initially considered but proved problematic when integrated with
other variable arguments in attach functions.
With `callback` as a separate convenience function, this style
no longer presents a problem.
One limitation of C++ is that implicit casts do not occur when
matching template overloads, as a consequence the callback's
argument type requires a strict match.
Unfortunately, the prevents the previously common pattern of using
void pointers as function arguments, causing unnecessary problems
for users porting code.
Thing *t;
void doit(void *p) { blablabla }
Callback<void()> cb(t, doit);
To avoid this, explicit overloads on void pointers were added. This
avoids a template expansion, and allows the implicit cast to occur
as the user would expect.
- Marked `call` and `operator()` functions as const
- Moved to static_cast for internal function pointer to avoid losing
compiler checked const-safety
- Added test for `operator=` with non-callback types
- Moved from zero-cast to value-initializer when callback is null
- Added `operator==` and `operator!=`
- Removed special handling of null callback
- Replicated doxygen to all overloads
- Added correct nops where uninitialized callbacks are called
- Added assertion for null callback
- Removed copy-constructor from callback constructor
Additionally, the following changes were don to avoid combinatorial
explosion in function overloads as a result of adding cv-qualifiers:
- Added convenience function for inferred type
- Deprecated callback overloads qhere cv-qualifiers are not scalable
Supported overloads:
callback(void (*f)(A...));
callback(const Callback<R(A...)> &);
callback(T *t, void (*f)(T*, A...));
callback(const T *t, void (*f)(const T*, A...));
callback(volatile T *t, void (*f)(volatile T*, A...));
callback(const volatile T *t, void (*f)(const volatile T*, A...));
callback(T *t, void (T::*f)(A...));
callback(const T *t, void (T::*f)(A...) const);
callback(volatile T *t, void (T::*f)(A...) volatile);
callback(const volatile T *t, void (T::*f)(A...) const volatile);
Before, the following results in a compilation error:
const struct Object *obj;
void obj_doit(const Object *obj);
Callback<void()> cb(obj, obj_doit);
This is especially noticable when migrating from the old Thread
constructor, which previously _required_ const.
Short term fix for all cv qualifiers through a C cast:
void *_obj = (void*)obj;
per @c1728p9
Update the Callback class to handle a NULL thunk by returning 0
rather than trying to call the thunk. This fixes a crash that occurs
on some targets when the TX uart handler is not attached.
Background:
The K64F HAL uart implementation calls the TX interrupt handler
every time a uart interrupt occurs while the TX register is empty.
It does not check to see if the TX interrupt has been enabled.
This means that the TX interrupt can and typically does get
run on RX events. This causes a crash with the newer callback
code which did not (prior to this patch) support a NULL thunk.
- Adopt C++11 style template arguments, requires rename to Callback
- Add constructor for C style callback functions
- Add constructor for Callbacks
- Add static function for passing to C style callbacks
Sam Grove
Christopher Haster
Vincent Coubard
Russ Butler
Jimmy Brisson