为什么分配给指针会导致恐慌

Question

I have the following code:

type box struct {
    val int
}

var p *box

This works fine:

p = &box{val: 2}

But this results in error:

*p = box{val: 2}

Error:

panic: runtime error: invalid memory address or nil pointer dereference 
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x5e59d9]

Why does the 2nd form of assignment result in panic?

Answer 1

Because p points to nothing (nil). You must first allocate some memory and make p point to it before being allowed to deference p. You can use the new builtin to allocate memory initialised to the zero value of the pointed type.

p = &box{val: 2}

has the same result as:

p = new(box)
*p = box{val: 2}