Today I ran into a C++ scenario where I found std::function to be useful. I wanted to create a std::queue of member function pointers to functions that are children of the same parent class. For example:<\/p>\n
\n\nclass Parent\n{\npublic:\n Parent() {};\n virtual ~Parent() {};\n};\nclass ChildA : public Parent\n{\npublic:\n ChildA() : Parent() {};\n ~ChildA() {};\n void Execute01() {printf("A");};\n void Execute02() {printf("B");};\n void Execute03() {printf("C");};\n};\nclass ChildB : public Parent\n{\npublic:\n void Execute01() {printf("D");};\n void Execute02() {printf("E");};\n void Execute03() {printf("F");};\n};\n<\/pre>\n<\/div>\nTo keep it simple, this examples shows two child classes (ChildA, ChildB), but I wanted it to work for lots more child classes. I wanted to push() member function pointers to ChildA::Execute01() etc into a std::queue… But I ran into an issue. The declaration for a member function pointer requires the class to be specified, and the execution requires an instance of the class the be specified. So the following snippet is valid:<\/p>\n
\n\n\ntypedef void (ChildA::*funcPtr)(); \nstd::queue<funcPtr> theQueue; \nChildA childA; \ntheQueue.push(&ChildA::Execute01); \nfuncPtr fn = theQueue.front(); \n(childA.*fn)();\n<\/pre>\n<\/div>\nHowever, I can’t push() a pointer to a ChildB member function into this same std::queue. If I try referencing the parent class as follows:<\/p>\n
\n\n\ntypedef void (Parent::*funcPtr)();\nstd::queue<funcPtr> theQueue;\nChildA childA;\ntheQueue.push(&ChildA::Execute01);\nfuncPtr fn = theQueue.front();\n(childA.*fn)();\n<\/pre>\n<\/div>\nThen Visual Studio 2013 gives me an error:<\/p>\n
Error 9 error C2664: ‘void std::queue<funcPtr,std::deque<_Ty,std::allocator<_Ty>>>::push(void (__cdecl Parent::* const &)(void))’ : cannot convert argument 1 from ‘void (__cdecl ChildB::* )(void)’ to ‘void (__cdecl Parent::* &&)(void)’ <\/p>\n
Ignoring C++14’s variable templates, there are two types of templates – function templates and class templates… So we can’t just use a template on the function pointer’s typedef. Instead, my next solution was to make a queue of Delegate’s and use a template to define the delegate class’s children:<\/p>\n