Does the implementation of an interface in Go language have to explicitly use an interface to be considered to implement the interface?

Problem discussion: Implementation method and inspection mechanism of interfaces in Go language

In Go programming, the implementation of an interface is accomplished by implicitly satisfying all methods of the interface definition in the structure. So, regarding the implementation of an interface in Go, does it necessary to use an interface explicitly to be considered to implement the interface? We will use the code provided by the questioner as an example to discuss this issue in detail.

The questioner gave two code snippets, namely main1 and main. A Apple structure is defined in main1 and instantiated, but no interface usage is involved. Main not only defines the Apple structure, but also defines a Fruit interface, and implements the interface method through the Apple structure, and then assigns an instance of Apple type to a variable of Fruit type in the main function.

In Go language, the method of implementing an interface in a structure is implicit, and there is no need to explicitly declare that an interface is implemented in the structure definition. As long as the structure implements all methods of the interface definition, it is considered to implement the interface. Therefore, in the example provided by the questioner, the Apple structure implements the Fruit interface because it defines the Grow() method, which is the method required by the Fruit interface.

However, further explanation is needed regarding the interface implementation problem in main1 and main. Although the Fruit interface is not explicitly used in main1, it actually implements the Fruit interface because the Apple structure defines the Grow() method. The interface check of Go language is carried out as needed. Since there is no interface usage involved in main1, the interface check will not be performed. Therefore, although there is no interface involved in main1, the Apple structure still implements the Fruit interface.

In contrast, the Apple type instance in main is assigned to a Fruit type variable, which will trigger the Go language interface check mechanism. Because the Apple structure implements all methods of the Fruit interface, this assignment operation is legal.

Therefore, in Go, the structure implementation interface is implicit and can be implemented without explicitly using the interface. Although there is no interface check involved in main1, the Apple structure still implements the Fruit interface; and in main, the interface check is triggered by assigning the Apple instance to a variable of type Fruit, confirming that Apple has implemented the Fruit interface.

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