can I pass an object to a class via template c++

Question

Can I do this:

class A
{
    public:
    void print()
    {
         std::cout<<"A"<<std::endl;
    }
};

class B
{
    public:
    void print()
    {
         std::cout<<"B"<<std::endl;
    }
};

template <class T>
class C 
{
     public:
     void say()
     {
          T.print();
     }
};

int main()
{
     A a;
     B b;
     C<a> c;
     c.say();
     C<b> d;
     d.say();

     return 0;
 }

If I don't want to use a base class for A and B , what is the best way (fast running speed) way of doing this?

Answer 1

There are multiple things wrong with your code.

You seem to be mixing up types with instances (or classes and objects).

This code should actually compile and work as intended.

#include <iostream>

class A
{
    public:
    void print()
    {
         std::cout<<"A"<<std::endl;
     }
};

class B
{
    public:
    void print()
    {
         std::cout<<"B"<<std::endl;
     }
};

template <class T>
class C 
{
     public:
     void say(T obj)
     {
          obj.print();
          // If T::print() were static, you could call it like that instead.
     }
};

int main()
{
     A a;
     B b;
     C<A> c; // template for C demands a class, not an object!
             // in practice this would mean you write 'A' instead of 'a' here
     c.say(a);
     C<B> d; // same here
     d.say(b);
}

Answer 2

Where T is a type T.print(); is not valid C++. Instead you have the following options:

If you make print a static method you could do:

void say()
{
     T::print();
}

This might be a good route if the print method does not need any state. It's nothing more that than a straight (possibly inlined) function call. So not much scope for performance problems.

Otherwise you have to pass the object :

void say(T object)
{
    object.print();
}

There are some options for how you would pass the object. Whether by value as above (assuming T isn't a reference/pointer type) or by reference. These may or may not have significance on the performance depending on the particular case.

Or create an instance within the say method:

void say()
{
    T().print();
}

For simple cases this is likely to optimise out to be essentially the same as the static print method case. Though of course the constructing a T could be potentially costly for less trivial cases.

Answer 3

At first you forgot the semicolons behind the class declarations and somehow your main does not return an integer. So assuming thats fixed there is only a small amount of code there, which is still wrong, but is a huge impact. You mixed up run time and compile time.

 A a;
 B b;
 C<a> c;
 C<b> d;

You try to use a non-const instance as a template parameter, which makes no sense. It will be created on runtime, but on compiletime you have no clue about those. A template parameter does not take a non-const value, most of time it simply takes a type , like in your code. template<class T> or template<typename T> is expecting some type, not an instance of something. However you can recieve instances by doing things like template<A myConstA> . Anyways you need to rewrite it, such that it recieves the type , as it is known at compile-time.

A a;
B b;

//you can do C<decltype(a)>, too
C<A> c;
C<B> d;

You have your print function like this:

void print()
{ T.print(); }

But you cannot call a non-static function without an instance! Thus you have a few possibilities to fix this. Either take a reference of type T and call from that, like so:

void print(T& t)
{ t.print(); }

or simply, if ok, do this:

void print()
{ T().print(); }

or make the print functions of both A and B static! Try it online !

---

By the way whatever you do, if you decide not to make it static, make it a const function and noexcept , since it never throws and does change nothing from the instance.