Notes, continuation, escape and other symbols

Annotation symbols

basic dos and don'ts

There are two kinds of annotation symbols ：

• C The style is /* Content */
• C++ The style is // Content

Look at the four situations above , Only the fourth is wrong .

The commented content will be replaced with spaces in the preprocessing stage .

So the fourth situation becomes in t j;, And put keywords int Separation is not supported by the compiler , So there's an error .

You can also see the second situation , Annotation symbols are not valid in strings .

# and define Can there be spaces between ？

The answer is yes .

C Styles cannot be nested

You can see in the 140 There's a mistake , This is because the compiler will only look for the first /* And the first */, therefore C Style annotations cannot be nested .

y = x/*p

When divided by the pointer , It is very easy to make such mistakes , There are two solutions

The second bracketed method is strongly recommended .

Basic requirements of notes

1. The notes should be accurate 、 Understandability , Prevent ambiguity . Wrong annotation is not only useless but also harmful .

2. Write code and comment , Modify the corresponding comments while modifying the code , To ensure the consistency of comments and code . Comments that are no longer useful should be deleted in time .

3. Comments are about code “ Tips ”, Not documents . The notes in the procedure should be simple and clear , Too many notes can be dazzling .

4. A clear statement without comments .

   for example ：i++; //i Add 1,i Add 1 Completely superfluous comments .

5. For global data ( Global variables 、 Constant definition, etc ) It must be annotated .

6. The notes are in English , Try to avoid abbreviations in comments , Especially the less commonly used abbreviations .

   Some compilers cannot display Chinese , So when the code that appears in Chinese is opened by the compiler, it will be a mess （ Such compilers are rare nowadays ）.

   For people with poor English , Or in Chinese , Otherwise, it's better not to write wrong notes in English .

7. The position of the comment should be adjacent to the code being described , It can be on the same line as the statement , You can also go up , But it can't be placed below . The notes of different fields in the same structure should be aligned .

8. When the code is long , Especially when there are multiple nesting , There should be a note at the end of some paragraphs , Easy to read .

9. The indentation of comments should be consistent with that of code .

10. When commenting code snippets, pay attention to “ Why do （why）”, instead of “ How do you do it? （how）”.

    Notes explaining how to do it usually stay at the level of programming language , Not to illustrate the problem . Try to explain “ How do you do it? ” The comments of generally do not tell us the intention of the operation , And specify “ How do you do it? ” Comments for are usually redundant .

11. The unit of value must be noted .

    The note should explain what the unit of a certain value means , such as ： For length, it must be stated that the unit is millimeter 、 rice 、 Or kilometers .

12. Comment on the scope of the variable , Especially parameters .

13. Comment on a series of numerical numbers , Especially when writing the underlying driver （ Such as pin number ）.

14. For function entry / Export data 、 Conditional statements 、 Branch statements give comments .

    Conditions and branch statements are often the key to a program to achieve a specific function . For maintenance personnel , Good comments can help better understand the program , Sometimes even better than looking at design documents .

15. Avoid inserting comments in the middle of a line of code or expression .

    Unless necessary , Comments should not be inserted in between code or expressions , Otherwise, it is easy to make the code incomprehensible .

16. In complex functions , In branch statement 、 Appropriate comments are required after the loop statement ends , It is convenient to distinguish each branch or cycle .

    

17. For areas that do not need to be compiled, use conditional compilation .（ Not recommended ）

Another is based on if Judge , Processing during code run . Seriously not recommended

Continuation and escape characters

Continuation function

C Use backslash in language \ Identify line breaks . The compiler will remove the backslash , The character following the backslash is automatically continued to the previous line . But notice ： No space after backslash . Let's test it ：

Be careful. ： stay VS2022 in , Sometimes even without \, Line breaks are also effective . But this coding style is not recommended , When such code appears , It is impossible to clearly tell the user whether the code is connected or written incorrectly . therefore , When we need to connect , Be sure to use a continuation \.

It is incorrect to add a space after the continuation , The effect of not adding a connector is exactly the same as that of adding a connector , But the coding style without connectors is not recommended ！

escape

The backslash can be used as a continuation , It can also be used as the start identifier of escape characters .

C in , There are some characters , Is its literal meaning , such as a 、b、c. There are also some characters , Ability means something special , such as “、'、\.

The essential meaning of escape is ： Literally to special , Or special to literal .

Single and double quotes

Basic understanding

We know that double quotation marks are all string constants , Single quotation marks are all character constants . To be more precise , Single quotation marks are all integer character constants , Why do you say that , Here's an example .

For the size of the string , We must be very clear , There is a hidden at the end of the string \0, So when you calculate the size, you actually calculate two characters 1 and \0, Size is 2. So why is the size of characters not 1 It is 4 Well ？

Actually , When the compiler allocates memory for characters , The allocated size is 4 Bytes , The size of characters is 4 Bytes . So we often say that characters are actually integers .

But character variables are not 4 It is 1, because char The memory size of type is 1, you 're right , We are in the process of assigning character constants to character variables , In fact, they are intercepting , Intercept the low byte of the four bytes of the integer , Then assign it to the character variable . Look at the next example .

Because when intercepting , Is to intercept from the low byte , and char Type variables can only intercept one byte , Therefore, only the character of the low byte of the character constant will be intercepted , There will also be the phenomenon of the above figure .

Because the character constant is actually four bytes , in other words , A character constant can only have four characters .

Empty characters and empty strings

Null character , That is, the character constant is empty , This in itself is meaningless , So the compiler reports an error .

An empty string , But in fact, there is a hidden \0, So empty characters make sense , And the size is 1.

Why do computers need characters

The essence of computer only knows binary , So why do computers need characters ？ Doesn't it smell good directly ？

Because computers are designed to solve human problems , But , How do people know that computers solve people's problems ？ When the computer outputs binary results , Ordinary people can't understand it directly .

therefore , Why do computers need characters ？ The essence is to make people understand .

Then why is it in English ？ Doesn't Chinese smell good ？

The earliest computer was invented by Americans , Their language is English , Only by 26 English letters and a bunch of punctuation marks , And computers are just beginning to be invented , Americans only need to solve their problems , So there are now simple characters .

Because computers only know binary , And people only know characters . therefore , There must be a set of rules , Used for binary and character conversion , This is called ASCII clock .

Logical operators

&&

Cascade two （ Multiple ） Logical expression , It has to be true at the same time , The result is true . And &（ Bitwise AND ） It's a completely different concept .

||

Cascade two （ Multiple ） Logical expression , At least one must be true , The result is true . And |（ Press bit or ） It's a completely different concept .

A short circuit

&& The number of operations （ sentence ）, As long as one is false , Other operands （ sentence ） No longer .

|| The number of operations （ sentence ）, As long as one is true , Other operands （ sentence ） No longer .

This is the short circuit .

Use the characteristics of short circuit , The above can be achieved similar to if The function of .

An operator

Basic concepts

& The corresponding bits of the binary of two numbers are 1 when , The result is 1; conversely , The result is 0.

| At least one of the corresponding bits of the binary of two numbers is 1 when , The result is 1; conversely , The result is 0.

^ Is when the corresponding bits of two binary numbers are different , The result is 1; Phase at the same time , The result is 0.

~ Is to negate all the binary bits of the number .

&& 、|| VS &、 |

&&,|| Cascading are multiple logical expressions , What is needed is the true and false results .

&,| Cascading is multiple data , Bit by bit operation .

Logical result ： Did you have your meal? ？

data structure ： How much did you eat ？

About ^

Yes ^ Three understandings of

• Bit by bit , Same as false , Difference is true
• Any number and 0 Exclusive or , It's all about itself
• Support the law of association and the law of exchange

The first understanding is shown in the figure above , Next, let's look at the last two understandings

First look at the first three lines , Any number and 0 Exclusive or is itself , It's easy to understand , because 0 Of 32 Every bit is 0, and 3 Of the bits is 0 Is the same after XOR , The result is still the 0, and 3 Of the bits is 1 After XOR of , The result is 1, In fact, the overall result is 3 In itself . So we can draw this conclusion .

Look at the last three lines , First, 3 and 3 Exclusive or , Two identical numbers , The bits also correspond to the same , The result is 0, and 0 and 4 Exclusive or , The result is still 4 了 . Look at the last two lines , See the result is still 4, It is also verified that the XOR operator satisfies the commutative law and the associative law .

Swap two integers

There are three ways to exchange two numbers , The first is the most common ：

Next is the second one ：

But this method is likely to go wrong , Because when two variables of the same type are added , The result may exceed the maximum value of this type , The result is the result after interception , And then something will go wrong .

Finally, method 3 ：

This method combines the right ^ Three understandings of , And it's a method that won't make any mistakes .

Bit operators need to be defined with macros before using .

The specified bit position is 1

The specified bit position is 0

【 The rules 2-19】 In fact, it is an integer improvement problem

Regardless of any bitwise operator , The goals are calculated by the computer , And there are only CPU Have computing ability （ It's easy to understand ）, But the calculated data , It's all in memory . so , Before calculation （ No matter what operation ）, Must get the data from memory CPU in , Get CPU Where ？ without doubt , stay CPU In the register of .

And the register itself , With the number of bits in the computer , The number of bits in the register is also different . commonly , stay 32 Place below , The number of bits in the register is 32 position .

But char The type data is only 8 A bit , Read to register , Can only fill the low 8 position , And the rest is high 24 position , It needs to be done “ Improve the overall shape ” 了 .

Move left and right

<< Move left ： The highest bit is discarded , The lowest point is made up of 0

>> Move right ：

1. An unsigned number ： The lowest bit is discarded , Highest complement 0（ Logical shift right ）
2. Signed number ： The lowest bit is discarded , The highest complement sign （ Arithmetic shift right ）

How to understand “ discarded ”？

Basic understanding chain ：

<< perhaps >> It's all calculation , All in CPU In the middle of , But the variables involved in the movement , It's in memory . So you need to move the data to CPU And then move it in the register of . In the process of actual movement , Is done in a register , That is, in a unit with a fixed size , that , Move left and right, there must be a place to run ==“ outside ”== The situation of .

Deep understanding of shift left and right

Look at the picture above , We can conclude that ：

Move left ： No brain tonic 0

Move right ： First judge whether it is arithmetic right shift or logical right shift . The basis of judgment ： Look at your own type , It has nothing to do with the content of the variable .