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STL value string learning
2022-07-27 15:32:00 【51CTO】
Write it at the front
We finally came into contact with C++ Of STL 了 , This is a C++ A very important part of the module , You can say that , If you learn C++ I haven't learned STL, So your C++ There is a high probability that it is incomplete , We learn STL There are two levels of learning , The first is that you can use , The second is to know their bottom , As for the higher level , Now I haven't reached , I won't share it with you . Let's have a brief understanding first STL, Have a general understanding , Today's main content is string.
What is? STL
STL(standard template libaray- Standard template library )： yes C++ An important part of the standard library , Not just a reusable component library , and It is a software framework including data structure and algorithm . I have shared the knowledge of data structure with you before , Also with big family C I have written in English , But their usability is a little low , You're writing OJ Question time , Use C Language , You will find that sometimes you have to implement the data structure yourself , Big guys also have such troubles , So appear STL, Generic programming is used , This is also the reason why we talked about templates earlier .
STL edition
We learn STL There are three versions , The functions inside are roughly the same .
- The original version Alexander Stepanov、Meng Lee The original version completed in HP Labs , This is the original STL
- P. J. edition Inherit HP edition , Slightly VS Series use , Code readability is low
- SGI edition Inherited from HP edition Ben . By GCC(Linux) use , Good portability , Reading is very high . We learn from the back STL To read part of the source code , The main reference is this version .
STL Components
STL Six components of , Let's look at the picture directly , The following content will involve , Here, let's first meet .
string
Let's get to know what is string, Simply speaking ,string stay C String we know in language , stay OJ in , Questions about strings are basically based on string Class , And in routine work , For simplicity 、 convenient 、 quick , Basically use string class , Few people use C String manipulation functions in the library . Because we often use , So the bosses turned this into a template class , In a sense ,string Do not belong to STL, It appeared much earlier .
know string
Have a look first STL On the inside string The classification of , We may wonder , How can there be four kinds here string, What kind of ,SLT Isn't it a template , Why here string It doesn't look like ... We solve these problems one by one .
Character encoding table
Talking about why there are four string Before ? We need to talk about what is a character encoding table . We all know that in the computer world , They only know 0 and 1, But in the real world , We have something similar to Chinese , English language , How can we connect the real world with the computer world ? This is the function of character coding table . The original character coding table was proposed by the United States , be called ASCII code , Chinese is not supported .
In order to encode the character table belonging to their respective countries , China has made up GBK, It uses 16 Binary of bit , But there is no unified standard in all countries , At this time, international organizations ISO Formulated the Unicode, It's also called the universal code ．
We already know the history of the character table , The so-called four string It is to support characters with different bits , Yes , We're not the only ones char This is a character type , What we mainly learn seems string, Their four usages are roughly the same , Learned a , Just check the other documents .
string Is the template code
Yes , What we see string yes typedef Yes , That is, the parameter is char Template of type .
string The bottom of the
string The bottom layer of is a character array that can be dynamically developed , We must keep this in mind , Later, when we simulate the implementation, we follow this .
Use string
Now we can use string, We need the header file when using , But also need to release std Inside string, Say less and do more , Let's see .
Constructors
Now we can officially contact string What's in it , Be careful C98 in string There are seven constructors , But not all of us use it , Here I come to Jianshao with four .
Constructors | Function description |
string() | Construct an empty string |
string(const string& str) | Copy structure , Deep copy |
string(const char* s) | Construct by a string |
string(size_t n, char c) | Construct a structure that contains n individual c String of characters |
Now let's demonstrate .
string()
string(const char* s)
string(size_t n, char c)
string(const string& str)
length & Capacity
We know string The bottom layer is a dynamically opened array , that string Support the calculation we string Let's take a direct look at the functions of the effective length and capacity of objects .
size() & length()
Both of these are calculations string The effective length of the object , The only difference is the function name ,length() Is the initial method , The effective length of our string can be described as length, But the following hash table is a little unreasonable , So it adds size() This function , In order to keep the naming standard , That's it .
capacity()
Calculation is the capacity of the current object , That's the length of the array .
max_size()
This function is the maximum length of our string , Generally, it is not used by anyone , I'll just mention it here .
operator[] & at()
string Reload the [\] This operator , Can support subscript access , there at() Function and [\] It's the same thing , It's just an early version , There is no difference in function .
A cross-border visit
If we visit the next position of a valid character , Back to a \0, You don't know the bottom yet , But we know that the empty string contains a \0 Of , This is for compatibility C Language .
But if we cross the border ,VS The compiler will report an error .
Traverse string
Traverse string We have three ways , These three methods are also often used .
- Use Subscript
- Use iterator Let's focus on
- Use Range for
### The subscript access
string This class is overloaded [\] Of this operator , Plus string The bottom layer is an array , It supports random access ,
Iterators access
Maybe you're a little confused about iterators , I don't know what it is , Let's do that. , Let me just say that , The so-called iterator is that we traverse STL The most common method . Yes , You're not mistaken , Not all STL The bottom floor is a continuous space , In other words, subscript access is not universal .
string The iterator in is a native pointer , There are four kinds of iterators . Let's see .
Forward iterator
Here I directly use two ways to access , One is const Embellished , A modifiable .
The following is where the forward iterator points , Be careful end() The next position of the valid character pointed to .
You can modify what the iterator points to
If we want to modify what the iterator points to , Here you can see .
If we don't want to modify the data pointed to by the iterator , Can be called directly const Decorated iterators
The main application of this iterator is for those who don't want to modify string, Let's look at applications .
reverse iterator
Finished talking about positive iterators , Here we will talk about what is a reverse iterator , Its function is also traversal string, It's just anti ergodic .
This iterator is modifiable , There is no change here .
Now there are other things that can be reversed const Decorated iterators , We can use it , Don't talk about the specific .
Range for
Range for It's simpler , But there is one flaw , It must be traversed at one time
It looks tall , In fact, the bottom layer is also the reuse of iterators , The reusable iterator here can be modified
push_back()
string It supports inserting a character at the end , If the capacity is not enough , The compiler will automatically expand .
string Expansion mechanism
We know string The bottom layer of is a dynamic array , Now we want to see how it expands .
From here we can see that , The expansion rules of different compilers are different ,VS Next is 1.5 Double expansion ,g++ yes 2 Double expansion , And at first VS Opened up 15 Space ,g++ Didn't drive
reserve()
Everybody knows , Expansion comes at a price , Sometimes you need to open up space again and copy the array , If we know the total space opened , Wouldn't it be better to open up in advance , This is what this function does .
- Just change the capacity
- Don't change size, That is, you can understand that it is only capacity expansion .
Be careful ,reserve() The space opened up by function is not necessarily the same as what we want , Memory aligned , But it's not bad .
But here are two questions , If our N What will happen if it is larger than the existing capacity , What if it's smaller ? Here we discuss .
N > capacity
This is the same as the one above , It will automatically expand , Only change capacity, Don't change size.
N < capacity
If this happens , Nothing will change .
resize()
The function of this function can be seen from the name that it is reset size, in other words , Let's reassign the place of the trailing data , If there's not enough space , The compiler will automatically expand . But here are two functions , It is essentially a default function .
void resize (size_t n)
This is a reset N Space , Start counting from the beginning , Count to the subscript N The place of , This and all subsequent reset spaces become \0. There are three situations . Here we will discuss two common situations , The third is capacity expansion .
Situation 1
Situation two
Here I will give a summary , So-called resize Is to change the position after the last position of the effective element , Change the subscript to N, The distance from the original effective position , All become \0.
void resize (size_t n, char c)
This is even simpler , There is nothing to say , It also conforms to the above two rules , It also meets the insufficient expansion rules . Is to put the default \0 Change to the characters we want , There is no other explanation here .
clear()
Clean up valid characters ,VS There will be no volume reduction under , Look at the compiler's choice ．
append()
We found that ,push_back Tail insertion is ok , But it supports inserting a character , We want to insert a string , Can you ,string This interface is also provided . Let's briefly introduce the longest used ones .
append() | Function description |
string& append (const string& str); | Tail insert one string object |
string& append (const char* s); | Insert a string at the end |
template string& append (InputIterator first, InputIterator last); | Use iterators to tail |
Let me demonstrate these functions directly , The usage is relatively simple , The new iterator tail insertion here is something we haven't seen before , Use it here alone .
string& append (InputIterator first, InputIterator last)
This is quite easy , It is a tail insert first To last The content of , There is nothing magical , To the next few blogs , We'll focus on iterators .
operator+=
Relative to the above append, This is what we often use , And easier to understand .
We will often use this .
insert()
In general , We don't use this function , Are the use of tail insertion , But in order to let you know , Take it out here for everyone , Give you a brief introduction of the commonly used .
insert() | Function description |
string& insert (size_t pos, const string& str); | stay pos Position insert a string |
string& insert (size_t pos, const char* s); | |
iterator insert (iterator p, char c); |
If you can use it .
erase()
This is to delete the subscript from pos Count back len Characters , If you don't pass characters, delete npos individual , there npos Is a static constant , The data is very big .
As for the iterator, I won't show you .
swap()
string There's something inside swap Functional , Of course, we std The one in can also be used , Just need to take place deep copy , It's a little inefficient .
c_str()
This function returns the pointer of this array , At work , Some do not accept string The pointer to , It's here string The pointer inside , It may not be of much use now .
find()
find() Is to find string Is there any character or string we want in it , The default is from subscript 0 At the beginning , What is returned is the subscript of the character or the head position of the string , If you can't find it, you'll return npos.
One more rfind() function , This is from string Object to find , I won't share it with you here .
substr()
This can be said to be a function of string truncation , from pos The position starts to cut ,len Characters , Here is also a default function .
getline()
We all know ,>> The operator ends when it encounters a space , Here for string It's the same .
This function is not string Peculiar , stay std Inner seal .
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