How to Print the Values of Nested Structs with Pointers in Go?

Printing Struct Values with Pointers in Go

In Go, it's common to encounter situations where you need to print the value of a struct that contains pointers to other structs. However, the default behavior of the %v format specifier in fmt.Printf() displays the pointer address instead of the actual value.

Consider the following example:

package main

import "fmt"

type A struct {
    a int32
    B *B
}
type B struct {
    b int32
}

func main() {
    a := &A{
        a: 1,
        B: &B{
            b: 2,
        },
    }
    fmt.Printf("v ==== %+v \n", a)
}

The output of the above code is:

v ==== &{a:1 B:0xc42000e204}

As you can see, the B field is printed as the memory address of the B struct, not its actual value.

Custom Printing with Stringer Interface

One way to print the contents of nested structs is to implement the Stringer interface for both the A and B types. The Stringer interface requires a single method, String(), which returns a string representation of the value.

Here's an updated example with the Stringer interface:

package main

import "fmt"

type A struct {
    a int32
    B *B
}

type B struct{ b int32 }

func (aa *A) String() string {
    return fmt.Sprintf("A{a:%d, B:%v}",aa.a,aa.B)
}

func (bb *B) String() string {
    return fmt.Sprintf("B{b:%d}",bb.b)
}

func main() {
    a := &A{a: 1, B: &B{b: 2}}

    // using the Stringer interface
    fmt.Printf("v ==== %v \n", a)

    // or just print it yourself however you want.
    fmt.Printf("v ==== A{a:%d, B:B{b:%d}}\n", a.a, a.B.b)

    // or just reference the values in the struct that are structs themselves
    // but this can get really deep
    fmt.Printf("v ==== A{a:%d, B:%v}", a.a, a.B)
}

Now, the output is:

v ==== A{a:1, B:B{b:2}}

Manual Printing

If you don't wish to implement the Stringer interface, you can manually print the desired representation of the struct using the print statements. For example, you can access the nested struct's fields and print them separately:

fmt.Printf("v ==== A{a:%d, B:B{b:%d}}\n", a.a, a.B.b)

This approach gives you complete control over the format of the output.

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