Default argument with template in C++

Question

I'm designing an interface, by which users can define a class that tells what they want to do.
The code is something like the following,

#include <stdio.h>

class Dummy{
public:
    void do(){ printf("do nothing\n"); }
};

class Task{
public:
    void do(){ printf("do something\n"); }
};

template <class TASK>
void func(TASK &task = Dummy()){
    task.do();
}

int main(){
    func(); // do nothing
    Task task;
    func(task); // do something
}

How to make it work?

Answer 1

The main issue is this func argument:

TASK &task = Dummy()

It will not work unless it is const. This happens because non-const lvalue reference to type cannot bind to a temporary.

But if you can use const there, you can easily solve your problem:

class Dummy{
public:
    void doit() const { printf("do nothing\n"); }
};

class Task{
public:
    void doit() const { printf("do something\n"); }
};

template <class TASK = Dummy>
void func(const TASK &task = TASK()){
    task.doit();
}

int main(){
    func(); // do nothing
    Task task;
    func(task); // do something
}