How to Correctly Call Methods on Interface Pointers in Go?

Calling Methods on Interface Pointers in Go

In Go, when programming against interfaces, you may encounter situations where you need to be able to call methods on a pointer to an interface value. This can arise in cases where you want to access underlying methods that are not directly exposed by the interface itself.

Consider the following scenario:

package main

import "fmt"

// SqlExecutor interface
type SqlExecutor interface {
    Get(i interface{}, key interface{}) (interface{}, error)
}

// GorpDbMap and GorpTransaction implement SqlExecutor
type GorpDbMap struct{}
type GorpTransaction struct{}

func (db GorpDbMap) Get(i interface{}, key interface{}) (interface{}, error) {
    return nil, nil
}

func (tx GorpTransaction) Get(i interface{}, key interface{}) (interface{}, error) {
    return nil, nil
}

func main() {
    // Initialize a GorpDbMap or GorpTransaction
    dbMap := GorpDbMap{}
    transaction := GorpTransaction{}

    // Create a repository that uses the SqlExecutor interface
    repo := Repository{
        // This will result in an error
        Gorp: &dbMap,
    }

    // Try to call Get method on the pointer to the SqlExecutor interface
    obj, err := repo.GetById(1, 2)
    if err != nil {
        fmt.Println(err)
    }
    fmt.Println(obj)
}

// Repository demonstrates calling methods on interface pointers
type Repository struct {
    Gorp SqlExecutor
}

func (r Repository) GetById(i interface{}, key interface{}) interface{} {
    obj, err := r.Gorp.Get(i, key)
    if err != nil {
        panic(err)
    }
    return obj
}

When running the above code, you will encounter the following error:

r.Gorp.Get undefined (type *gorp.SqlExecutor has no field or method Get)

This error occurs because the pointer to the SqlExecutor interface value is not being properly dereferenced before calling the Get method.

To resolve this issue, you need to dereference the pointer to the interface value by using the asterisk operator (*). This can be accomplished by modifying the code as follows:

func main() {
    // Initialize a GorpDbMap or GorpTransaction
    dbMap := GorpDbMap{}
    transaction := GorpTransaction{}

    // Create a repository that uses the SqlExecutor interface
    repo := Repository{
        Gorp: &dbMap,
    }

    // Try to call Get method on the pointer to the SqlExecutor interface
    obj, err := (*repo.Gorp).Get(1, 2)
    if err != nil {
        fmt.Println(err)
    }
    fmt.Println(obj)
}

By dereferencing the pointer to the interface value, you are now able to access the underlying methods of the struct that implements the interface. In this case, the Get method of the GorpDbMap struct is successfully called.

It's important to note that in Go, it is generally recommended to favor passing values by value over passing pointers to values. This helps to prevent unintentional modifications to the original values. In the case of interfaces, you should typically pass the interface value itself, rather than a pointer to the interface value.

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