Template member std::function of none templated class - c++

Question

I have a class that it has a templated function
class DialogManager
{
public:
template<class T>
using TimeoutCallback = std::function<void (T)>;
DialogManager();
virtual ~DialogManager();
template<class T>
void SetTimeoutCallback(TimeoutCallback<T> callback);
int GetDialoge(T obj);
private:
TimeoutCallback mTimeoutCallback;
}
I get below error:
error: invalid use of template-name ‘DialogManager::TimeoutCallback’
without an argument list
I've changed code to
template<class T>
TimeoutCallback<T> mTimeoutCallback;
but I a got another compiler's error
error: data member ‘mTimeoutCallback’ cannot be a member template
example usage is
#include <DialogManager.h>
class foo()
{
public:
foo()
{
DialogManager* dialog = new DialogManager();
}
DialogManager* mDialogManager;
};
struct data
{
data() {}
};
int main()
{
Foo* f1 = new Foo();
auto a1 = f1->mDialogManager->GetDialoge(1);
auto b1 = f1->mDialogManager->GetDialoge("test");
Foo* f2 = new Foo();
data d;
auto a2 = f2->mDialogManager->GetDialoge(d);
auto b2 = f2->mDialogManager->GetDialoge(45.231);
return 0;
}
`

Answer 1

If you want to share the same template type in different parts of class, make it template class instead of template function.
#include <functional>
template<class T>
class DialogManager
{
public:
using TimeoutCallback = std::function<void (T)>;
DialogManager();
virtual ~DialogManager();
void SetTimeoutCallback(TimeoutCallback callback);
private:
TimeoutCallback mTimeoutCallback;
};

Answer 2

If you are writing a class header file, then you generally want to template the class and use the template type's name as a variable type within that file. Then, inside the coder file, you can define the methods similar to how you were in the code your provided. It looks like you fixed the first compilation error, and the second error seems to be because you are templating individual methods within the header file. Here is a basic example for templating a header file:
`template<typename T>
class Node{
private:
T m_entry;
Node<T>* m_nextNodePtr;
public:
T getEntry() const;
void setEntry(T newEntry);
//more
}`
And here is the header file's accompanying coder file:
`template<typename T>
T Node<T>::getEntry() const{
return m_entry;
}
template<typename T>
void Node<T>::setEntry(T newEntry){
m_entry = newEntry;
return;
//more
}`
Hope this helps!

Answer 3

Your mTimeoutCallback is a template member variable; resolving the using, it's a
template<class T>
std::function<void (T)> mTimeoutCallback;
Unfortunately (a) template variables are available only starting from C++14 (and you tagged C++11) and (b) a template variable can be member of a class/struct only if static.
So your errors.