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This article introduces the knowledge of "how to declare the template type as formal parameter or friend". In the operation of actual cases, many people will encounter such a dilemma. Next, let the editor lead you to learn how to deal with these situations. I hope you can read it carefully and be able to achieve something!

In the new standard, we can declare template type parameters as friends:

Here we will use to instantiate the type of Bar declared as a friend. So for a certain type name Foo,Foo will become a friend of Bar.

Just look at the above instructions, it is estimated that many people are still confused, the following through an example close to the actual development to further illustrate.

Application example

Demand

We want to design an object counter that can meet the following requirements:

Use a counter class to count most unrelated types

I hope it can automatically prevent the misuse of the counter.

Counter class

If you remove part of the template, this class is a simple singleton pattern. What is special is that all members of this class are declared private and can only be used through Type types that are declared as friends.

Counter class usage example

Define two completely unrelated classes, instantiate the ObjectCounter class and count the objects.

Output result

The counting result correctly records the process of creation and destruction of the three objects of a1gma a2reb.

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