I have a custom Dialog class (derived from wxDialog). I want to add a setter so you can set a call back function MyDialog::m_f with some function of type std::function<bool(wxString&)>. Later, the dialog will call this function. I want to be able to pass in a lambda function (otherwise I could solve my problem with ordinary function pointers). It must also be possible to call this setter multiple times, the last set value for m_f should be valid, thus erasing any previous set values (the value being a new function). But, if I declare the member m_f, does it have to be initialized in the initializer list in the constructor? For my use case that would not really work. I have to be able to set MyDialog::m_f after the dialog has been constructed (but before ShowModal is called).
Should my member m_f be a (unique) pointer to a std::function<bool(wxString&)>, so it can be set to nullptr in MyDialog::MyDialog?
My basic problem is I do not fully understand std::function. How can you make a variable of some std::function<...> type and assign a value (concrete function) to it later? What does it mean to have a variable like described which is uninitialized? How can I test whether it is (un)assigned? Is it at all possible to delay this assignment, that is: have a separate declaration and later initialization, or should a variable of std::function<...> be initialized immediately (like a const or reference)?
Thanks for any help.
BTW, the language is C++11 and we can't upgrade due to restrictions at work.
How can you make a variable of some std::function<...> type and assign a value (concrete function) to it later?
std::function has a default constructor that results in an "empty" instance. In your case, just declare the data member std::function<...> m_f and don't explicitly initialize it.
What does it mean to have a variable like described which is uninitialized?
That depends on the type of the variable. In the std::function, it's simply unusable. If you try to inoke an empty std::function, and exception will be thrown.
How can I test whether it is (un)assigned?
std::function has an explicit conversion operator to bool. It evaluates to true when the object is non-empty, i.e.
if (m_f)
m_f(/* parameters... */);
else
; // do nothing, not set yet
How can you make a variable of some std::function<...> type and assign a value (concrete function) to it later?
If you want to later assign lambda expression to the function object through a setter, you can turn this member function into a template, e.g.
template<class Fct>
void setCallback(Fct&& f)
{
m_f = std::forward<Fct>(f);
}
Another option is to pass a std::function with the desired signature to a non-template member function.