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If a pointer-to-member is passed to the Signal::connect() function with
arguments that don't match the Signal type, the pointer-to-member
version of connect() will not match during template argument resolution,
but the functor version will. This results in a compilation error in the
BoundMethodFunctor class, due to the pointer-to-member not being a
functor and thus not being callable directly. The error messages are
quite cryptic. With the following error applied,
diff --git a/test/signal.cpp b/test/signal.cpp
index 5c6b304dac0b..6dd11ac45313 100644
--- a/test/signal.cpp
+++ b/test/signal.cpp
@@ -107,6 +107,7 @@ protected:
/* Test signal emission and reception. */
called_ = false;
signalVoid_.connect(this, &SignalTest::slotVoid);
+ signalVoid_.connect(this, &SignalTest::slotInteger1);
signalVoid_.emit();
if (!called_) {
gcc outputs
../../include/libcamera/base/bound_method.h: In instantiation of ‘R libcamera::BoundMethodFunctor<T, R, Func, Args>::activate(Args ..., bool) [with T = SignalTest; R = void; Func = void (SignalTest::*)(int); Args = {}]’:
../../include/libcamera/base/bound_method.h:143:4: required from here
../../include/libcamera/base/bound_method.h:146:37: error: must use ‘.*’ or ‘->*’ to call pointer-to-member function in ‘((libcamera::BoundMethodFunctor<SignalTest, void, void (SignalTest::*)(int)>*)this)->libcamera::BoundMethodFunctor<SignalTest, void, void (SignalTest::*)(int)>::func_ (...)’, e.g. ‘(... ->* ((libcamera::BoundMethodFunctor<SignalTest, void, void (SignalTes
t::*)(int)>*)this)->libcamera::BoundMethodFunctor<SignalTest, void, void (SignalTest::*)(int)>::func_) (...)’
146 | return func_(args...);
| ~~~~~^~~~~~~~~
and clang isn't much better:
../../include/libcamera/base/bound_method.h:146:11: error: called object type 'void (SignalTest::*)(int)' is not a function or function pointer
return func_(args...);
^~~~~
../../include/libcamera/base/bound_method.h:137:2: note: in instantiation of member function 'libcamera::BoundMethodFunctor<SignalTest, void, void (SignalTest::*)(int)>::activate' requested here
BoundMethodFunctor(T *obj, Object *object, Func func,
^
../../include/libcamera/base/signal.h:80:27: note: in instantiation of member function 'libcamera::BoundMethodFunctor<SignalTest, void, void (SignalTest::*)(int)>::BoundMethodFunctor' requested here
SignalBase::connect(new BoundMethodFunctor<T, void, Func, Args...>(obj, nullptr, func));
^
../../test/signal.cpp:110:15: note: in instantiation of function template specialization 'libcamera::Signal<>::connect<SignalTest, void (SignalTest::*)(int), nullptr>' requested here
signalVoid_.connect(this, &SignalTest::slotInteger1);
^
Improve error reporting by disabling the functor version of connect()
when the Func argument isn't invocable with the Signal arguments. gcc
will then complain with
../../test/signal.cpp:110:36: error: no matching function for call to ‘libcamera::Signal<>::connect(SignalTest*, void (SignalTest::*)(int))’
110 | signalVoid_.connect(this, &SignalTest::slotInteger1);
| ~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
and clang with
../../test/signal.cpp:110:15: error: no matching member function for call to 'connect'
signalVoid_.connect(this, &SignalTest::slotInteger1);
~~~~~~~~~~~~^~~~~~~
which are more readable.
This change requires usage of std::is_invocable<>, which is only
available starting in C++17. This is fine for usage of the Signal class
within libcamera, as the project is compiled with C++17, but we try to
keep the public API compatible C++14. Condition the additional checks
based on the C++ version.
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Paul Elder <paul.elder@ideasonboard.com>
Acked-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
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It can be useful to connect a signal to a functor, and in particular a
lambda function, while still operating in the context of a receiver
object (to support both object-based disconnection and queued
connections to Object instances).
Add a BoundMethodFunctor class to bind a functor, and a corresponding
Signal::connect() function. There is no corresponding disconnect()
function, as a lambda passed to connect() can't be later passed to
disconnect(). Disconnection typically uses disconnect(T *object), which
will cover the vast majority of use cases.
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Umang Jain <umang.jain@ideasonboard.com>
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