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This article mainly introduces "how to use the C language keyword auto register". In the daily operation, I believe many people have doubts about how to use the C language keyword auto register. The editor consulted all kinds of materials and sorted out simple and easy-to-use operation methods. I hope it will be helpful to answer the doubts about "how to use the C language keyword auto register". Next, please follow the editor to study!

One: auto

Before we learn the keyword auto, we need to understand two concepts: scope and lifecycle.

Scope

Scope is a programming concept. Generally speaking, the name used in a piece of program code is not always valid / available.

And the scope of the code that limits the availability of the name is the scope of the name.

Scope of local variables: variables whose scope is contained in the code block. Where it is defined is valid only within its scope.

Scope of global variables: variables defined outside all functions. It works throughout the project.

Let's use simple code to understand:

Int a = 20 int main () {if (1) {int b = 10; printf ("% d", b);} return 0;}

In this code we can see that we have created two variables, one is the global variable a, and the other is the local variable b.

According to the definition of scope, we can know:

The scope of the global variable an is the entire project:

The scope of the local variable b is only enclosed in if parentheses:

In order for us to better understand this code, we write several kinds of code to deepen our understanding:

Int a = 20 int int main () {if (1) {int b = 10;} printf ("% d", a); printf ("% d", b); return 0;}

We found an error in this code.

Although we created b, but b is a local variable, the scope is only inside the code block (inside if parentheses), does not include the line printf, so the program reported an error, can not access b.

Special:

When the local variable conflicts with the global variable, the local variable takes precedence.

When the defined variable name is the same, the local variable takes precedence.

Int a = 20 int main () {a = 10; printf ("% d", a); return 0;}

When we print it, we can find:

When the global variable conflicts with the local variable, the local variable takes precedence. To avoid this problem, try not to use the same variable name definition.

Life cycle

The life cycle of a variable refers to a period of time between the creation of a variable and its destruction.

The life cycle of a local variable is: the beginning of entering the scope of the local variable-> the end of the scope of the local variable.

The life cycle of a global variable is the life cycle of the entire program. Life cycle main function enters-> ends the life cycle of the entire program

For a more convenient understanding: we can popularly understand that the survival time of a variable is the life cycle of the variable.

Ex.: an ancient emperor died from birth. The period from birth to death was the life cycle of the emperor.

/ / * Life cycle * / / int g = 100 / / Life cycle of global variables: / Life cycle main function enters-- > ends the life cycle of the entire program / / int main () {/ / {/ / Local variable life cycle: / enters the scope of the local variable starts-> ends the scope of the local variable / / int a = 100 / / printf ("% d\ n", a); / /} / return 0politics /} auto

Auto claims to be the most magnanimous keyword

The use of auto: generally defined in the code block variables, local variables, the default is auto modified. Using auto only in this code block can be omitted.

The explanation in the code is:

Auto int g_val = 100int witch main () {for (auto int i = 0persi)

< 10; i++) { printf("i = %d\n", i); if (1) { auto int j = 1; printf("before: %d\n", j); j++; printf("after: %d\n", j); } } return 0;} 由于auto只可修饰局部变量 因此第一句auto int g_val = 100；是错误的

In the global variables within the later code, the auto in the local variables modified by auto can be omitted, so we also say that auto is the oldest keyword, and we can also omit auto in our usual programming.

Two: register

Register-register-the fastest keyword

First of all, we need to understand the storage level.

The closer the storage unit to the CPU, the higher the efficiency and the higher the unit cost.

The farther away the storage unit is from the CPU, the lower the efficiency and the lower the unit cost.

For any kind of hardware, the upstream hardware is the cache of the downstream hardware, so we can say that the register is the cache of the downstream storage device.

The essence of register: the essence is to improve the computing efficiency of the computer at the hardware level.

What variables can be used with register

1. Local variables (global variables cause CPU registers to be occupied for a long time)

two。 Will not be written (write back to memory, reload, meaningless)

3. Be used at high frequency (improve efficiency, put it in register, do not need to access memory and read, improve efficiency)

Int main () {register int val = 100; / the register variable has no address and cannot take the address val = 200; / / it can be written to printf ("% d\ n", val); / / the address return 0 is not allowed under the gcc compiler;}

Tip: don't use register a lot because the number of registers is limited.

Because the register keyword is used to store the val directly in the register, the val does not have a memory address, so the address operation cannot be performed.

At this point, the study of "how to use the C language keyword auto register" is over. I hope to be able to solve your doubts. The collocation of theory and practice can better help you learn, go and try it! If you want to continue to learn more related knowledge, please continue to follow the website, the editor will continue to work hard to bring you more practical articles!

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