Dialectics: c++ template class and class template, 1 minute

You have learned class and template class from previous days. (day-15: c++ template)

On day-15, I used template class to introduce template,

In most cases, class template is more accurate than template class.

Dialectics: what's the difference between them ? Why do we inverse the order by using class template ?

Let's think about some questions,

So then, we will call the template class class template, because it is template, used for creating classes.

Got:

class template

template <typename type_t00>
class my_class
{
};

c++ non-type template parameter, 2 minutes

On day-15, we have learned the template parameter is a type.

However, the template parameter (and argument) can be a value:

template <int value_v00>
class my_class
{
};

value_v00 is not a type, it is a value,

How to instantiate it:

my_class<23> object1;
auto object2 = my_class<32>{};

The non-type template parameter is called c++ nttp.

#include <iostream>

template <int value_v00>
class my_class
{
public:
	void print() const {std::cout << value_v00 << std::endl;}
};

int main()
{
	auto obj3 = my_class<32>{};
	obj3.print();
}

c++ template in inheritance, 15 minutes

class template inheritance

If A is derived from B, both A and B can be a class template, or a class (non-template).

#include <iotream>

template <typename type_t00>
class B
{
};

template <typename type_t00>
class A: public B<type_t00>	// A is derived from B
{
};

int main()
{
	A<int> obj1;
	auto obj2 = A<float>{};
}

template parameter overgoing in inheritance

parameter overgoing :::->

A<int>

You pass the template type parameter int to A, A will pass the template parameter int to B .

parameter overgoing :::-> pass itself

A passes a template parameter to B, can A pass A itself to B? Absolutely, it can.

template <typename type_t00>
class B {};

template <typename type_t00>
class A: public B<A<type_t00>>
{
};

Using the derived class as the template argument of the base class is a c++ idiom, it is called CRTP, coined by Jim Coplien in 1995.

template <typename type_t00>
class base_class {};

class derived_class: public base_class<derived_class> {};

parameter overgoing :::->

You pass paramter to A, A is derived from B, ...

Is it possible that A is derived from its parameters ? Yes !

#include <iostream>

template <typename type_t00>
class A: public type_t00
{
};

class B
{
};

int main()
{
	auto obj2 = A<B>{};
}

Multiple parameters work too:

#include <iostream>

template <typename type_t00, typename type_t01, typename type_t02>
class A: public type_t00, public type_t01, public type_t02
{
};

class B {};

template <typename type_t00>
class C {};

class D {};

int main()
{
	auto obj1 = A<B, C<int>, D>{};
}

Written on Nov 13, 2024

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std::cout << std::endl;

caught:

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