struct type embedded fields access

I am trying to learn golang and at moment I am trying to understand pointers. I defined three struct type

type Engine struct {      
	power int             
}                         
                          
type Tires struct {       
	number int            
}                         
                          
                          
type Cars struct {           
	*Engine               
	Tires                 
}       

As you can see, in the Cars struct I defined an embedded type pointer *Engine. Look in the main.

func main() {                    
                                     
	car := new(Cars)             
	car.number = 4               
    car.power = 342     
	fmt.Println(car)            
} 

When I try to compile, I've got the following errors

panic: runtime error: invalid memory address or nil pointer dereference
[signal 0xb code=0x1 addr=0x0 pc=0x23bb]    

How can I access the power field?

For example,

package main

import "fmt"

type Engine struct {
	power int
}

type Tires struct {
	number int
}

type Cars struct {
	*Engine
	Tires
}

func main() {
	car := new(Cars)
	car.Engine = new(Engine)
	car.power = 342
	car.number = 4
	fmt.Println(car)
	fmt.Println(car.Engine, car.power)
	fmt.Println(car.Tires, car.number)
}

Output:

&{0x10328100 {4}}
&{342} 342
{4} 4

The unqualified type names Engine and Tires act as the field names of the respective anonymous fields.

> The Go Programming Language Specification
>
> Struct types
>
> A field declared with a type but no explicit field name is an
> anonymous field, also called an embedded field or an embedding of the
> type in the struct. An embedded type must be specified as a type name
> T or as a pointer to a non-interface type name *T, and T itself may
> not be a pointer type. The unqualified type name acts as the field
> name.

Try this:

type Engine struct {      
    power int             
}                         

type Tires struct {       
    number int            
}                         


type Cars struct {           
    Engine               
    Tires                 
}

and than:

car := Cars{Engine{5}, Tires{10}}
fmt.Println(car.number)
fmt.Println(car.power)

http://play.golang.org/p/_4UFFB7OVI

If you want a pointer to the Engine, you must initialize your Car structure as:

car := Cars{&Engine{5}, Tires{10}}
fmt.Println(car.number)
fmt.Println(car.power)

One more example, in case of not unique field names

package embeded

import "fmt"

type Engine struct {
	id    int
	power int
}

type Tires struct {
	id     int
	number int
}

type Cars struct {
	id int
	Engine
	Tires
}

func Embed() Cars {
	car := Cars{
		id:     3,
		Engine: Engine{id: 1, power: 5},
		Tires:  Tires{id: 2, number: 10},
	}
	fmt.Println(car.number)
	fmt.Println(car.power)
	fmt.Println(car.id)
	fmt.Println(car.Engine.id)
	fmt.Println(car.Tires.id)
	return car
}