Embeded interface : struct doesn't implement method

Question

I don't understand why interface composition doesn't " inherit" the methods from the parent interface when it is satisfied in the implementation.

package main

import (
    "fmt"
)

type TestRepository interface {
    FindById(int) (error)
}

type TestService interface {
    TestRepository
    Method(id int) error
}



type testService struct {
    implRepository TestRepository
}

func NewTestService(r TestRepository) TestService {
    return &testService{implRepository: r}
}

When compiling this code i'm getting : *testService does not implement TestService (missing FindById method) while i'm actually expecting it to implement it since the type of "implRepository" is TestRepository.

It works if i do :

func NewTestService(r TestRepository) *testService {
    return &testService{implRepository: r}
}

But it kind of defeat the purpose of the interface ( as far as the service one goes at least )

What am i missing / how should it be done ?

Tarun Khandelwal · Accepted Answer · 2020-05-31 21:04:15Z

1

The answer to your question is simply "you aren't composing the interface the way you need to." You have made TestRepository a struct member - that is not going to "inherit" the methods on that interface. What you need is the following:

type testService struct {
    TestRepository
}

func NewTestService(r TestRepository) TestService {
    return &testService{TestRepository: r}
}

Of course, the struct testService still need to implement Method(id int) error to actually work.

answered May 31, 2020 at 21:04

Tarun Khandelwal

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