add doc for pointer

Author: xlj

120-Pointers/pointer-1.gop

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+// A pointer is a variable whose value is a memory address. Pointers are defined using an ampersand (the & character), known as the address operator, followed by
+// the name of a variable
+//
+// A pointer’s type is fixed, which means that when you create a pointer
+// to an int, for example, you change the value that it points to, but you can’t use it to point to the memory
+// address used to store a different type, such as a float64. This restriction is important, pointers are
+// not just memory addresses but, rather, memory addresses that may store a specific type of value.
+//
+// The type of a pointer is based on the type of the variable from which it is created, prefixed with an
+// asterisk (the * character). The type of variable named second is *int, because it was created by applying
+// the address operator to the first variable, whose value is int. When you see the type *int, you know it is a
+// variable whose value is a memory address that stores an int variable.
+//
+
+first := 100
+var second *int = &first
+first++
+println("First:", first)
+println("Second:", second)

120-Pointers/pointer-2.gop

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+# Following a Pointer
+// The phrase following a pointer means reading the value at the memory address that the pointer refers
+// to, and it is done using an asterisk (the * character). The asterisk tells Go+ to follow the pointer and get the value at the memory location. This is known as dereferencing the pointer.
+
+first := 100
+second := &first
+first++
+*second++
+var myNewPointer *int
+myNewPointer = second
+*myNewPointer++
+println("First:", first)
+println("Second:", *second)
+
+// The first new statement defines a new variable, which I have done with the var keyword to emphasize
+// that the variable type is *int, meaning a pointer to an int value. The next statement assigns the value of
+// the second variable to the new variable, meaning that the values of both second and myNewPointer are the
+// memory location of the first value. Following either pointer accesses the same memory location, which
+// means incrementing myNewPointer affects the value obtained by following the second pointer.
+//
+// A common misconception is that the first and second variables have the same value, but that’s not
+// what is happening. There are two values. There is an int value that can be accessed using the variable
+// named first. There is also an *int value that stores the memory location of the first value. The *int value
+// can be followed, which will access the stored int value. But, because the *int value is, well, a value, it can be
+// used in its own right, which means that it can be assigned to other variables, used as an argument to invoke a
+// function, and so on.

120-Pointers/pointer-3.gop

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+# Understanding Pointer Zero Values
+// Pointers that are defined but not assigned a value have the zero-value nil.
+//
+// The pointer second is defined but not initialized with a value and is written out using the println
+// function. The address operator is used to create a pointer to the first variable, and the value of second is
+// written out again.
+//
+// A runtime error will occur if you follow a pointer that has not been assigned a value
+
+first := 100
+var second *int
+println(second)
+second = &first
+println(second)

120-Pointers/pointer-4.gop

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+# Pointing at Pointers
+// Given that pointers store memory locations, it is possible to create a pointer whose value is the memory
+// address of another pointer.
+
+first := 100
+second := &first
+third := &second
+println(first)
+println(*second)
+println(**third)
+
+// The syntax for following chains of pointers can be awkward. In this case, two asterisks are required.
+// The first asterisk follows the pointer to the memory location to get the value stored by the variable named
+// second, which is an *int value. The second asterisk follows the pointer named second, which gives access
+// to the memory location of the value stored by the first variable. This isn’t something you will need to do
+// in most projects, but it does provide a nice confirmation of how pointers work and how you can follow the
+// chain to get to the data value.