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I would like to share with you what is the use of the pointer in cCompact +. I believe most people don't know much about it, so share this article for your reference. I hope you will gain a lot after reading this article. Let's learn about it together.

One, memory and address

We know that every byte of computer memory has a unique address, and each CPU address is addressed by jumping through fixed steps (which explains why memory alignment is required). For example, we can think of memory as a row of houses on a long street. each house has its own fixed house number, and each house can hold data. In order to read the data in a house, we must know the house number of the house, open the room according to this house number and take away the data. Similarly, the computer must number each byte of memory, just like the house number, the number of each byte is unique, and a byte can be accurately found according to the number.

Second, the essence of pointer is address.

What does the compiler do when we declare a variable in a program and assign a value to it? In fact, the variable name represents a storage unit in memory, and the system assigns an address to the variable when the compiler compiles and connects the program:

Int a = 10

In the above line of code, we define and initialize this variable a, and the system allocates a memory unit to a. An is just an alias for this memory unit. To take a value from a variable in the program is actually to find the corresponding memory unit through the variable name and read data from it.

If the memory address assigned to the variable an is 0xFF00, then we can say that the address is the house number of the variable a. The address of a variable is called the pointer to the variable. So, the essence of the pointer is the address, and the pointer variable is a special variable that specifically holds the pointer (that is, the address). When we say the memory unit corresponding to this address, we can say that the pointer points to this memory unit.

For example:

Int a = 10; int* p = & a; / / define pointer variable p = 20; / / modify the value of pointer p to 20.

In the above two lines of code, we first define an integer variable a, and then a pointer variable p points to a. In the second line of code, the symbol & represents the address, which is equivalent to assigning the address of the variable a to the pointer variable p (p points to a), and * adding to the pointer variable represents dereference, which means to find the value that the pointer p points to. Therefore, the third line of code means that the value pointed to by p is changed to 20. In short, it is important to remember that the symbol & represents the value, and the symbol * represents dereference:

Symbol

Meaning &

Take the address

*

Dereference

The memory model of these three lines of code is as follows:

We assume that the memory header address assigned to variable an is 2000, and we declare another pointer variable p, which also takes up memory space (4 bytes for 32-bit systems and 8 bytes for 64-bit systems. Please think about why), but this variable p holds the address of variable a, that is, 2000, when we want to manipulate a through p First of all, it is necessary to find the content it points to according to the saved address of p, that is, dereference * p. When the content of * p is released and changed, the value stored in the memory cell with the first address 2000 will also change, so when the variable * p is reassigned to 20, the value of the variable a will also change to 20.

This extends to the second-level pointer, and if we define another pointer variable Q to point to p, then Q is a second-level pointer, because the object it points to is still a pointer, only one level lower than himself, it is a first-level pointer. So how to define the second-level pointer, please see the following code:

Int a = 10 intact * p = & an intact * Q = & p

The third line of code above defines a second-level pointer Q, which points to the first-level pointer p, which in turn points to the variable a, whose memory model is shown in the following figure:

The content of the second-level pointer Q is the address of the first-level pointer p rather than the content. Note that the address of p is 2008 and the content of p is 2000. So dereferencing Q means that * Q gets p, which is 2008, and dereferencing (* Q) means that the value of variable an is obtained by dereferencing (* Q). Because the operator associativity is from right to left, parentheses can be omitted, that is, * Q is the value of a. We can write code to try:

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