19.2 container classification, array and vector container refinement

stl Component part
Containers 、 iterator 、 Algorithm ( function )、 distributor ( Allocate memory )、 other ( Adapter 、 functor / Function objects, etc ).

One ： Classification of containers

vector、list、map, Containers are used to store data
STL Containers in can be divided into three categories
<1> Sequence containers （Sequence containers）： Where to put it , Where this element is placed . such as ：array、vector、deque、list、forward_list.

<2> Associate container （Associative Containers）： Element is a key / It's worth it , Use the key （key） To find the value （value） Especially suitable for searching . Automatically sort internally by key , Be able to control the inserted content , However, the insertion position cannot be controlled . such as ：set、multiset、map、multimap、hash_set、hash_map、hash_multiset、hash_multimap. Internal may use trees 、 Hash table .

<3> Unordered container （Unordered Containers）：c++11 Launched in . The position of the element is not important , What matters is whether the element is in the container . Unordered containers are also related containers . With the insertion of elements , The position of each element in the container may change as it changes . such as ：unordered_set、unordered_multiset、unordered_map、unordered_multimap. Internally, hash tables may be used to implement .

There is an official saying ：c++ The standard does not stipulate that any container must use any specific means of implementation .

Two ： Description of the container and examples of application matters

<1>array： Is a sequential container , It's actually an array , Memory space is continuous , The size is fixed ; How old were you when you first applied , He's just how old , No more size . In essence, it is an array with continuous memory space , Fixed size , You can't increase its size . Objects are continuous , But the memory space of the string pointed to by the object doesn't matter .

#include <array>
#include <string>
#include <iostream>

using namespace std;

void func()
{
	// contain 5 Array of elements 
	array<string, 5> mystring = { "I", "Love1Love2Love3Love4Love5Love6Love7", "China" };
	cout << "myString.size() = " << mystring.size() << endl;  //5
	mystring[0] = "It is very long~~~~~~~~~~~~~long~~~~~~~~~~~~long";
	mystring[4] = "It is very long~~~~~~~~~~~~~long~~~~~~~~~~~~long";
	cout << "sizeof(string) = " << sizeof(string) << endl;
	for (size_t i = 0; i < mystring.size(); ++i)
	{
		const char* p = mystring[i].c_str();
		cout << "-----------------begin---------------------" << endl;
		cout << " Array element value  = " << p << endl;
		printf(" Object address  = %p\n", &mystring[i]);
		printf(" The string address to  = %p\n", p);
		cout << "-----------------end---------------------" << endl;
	}

	const char* p1 = "Love1Love2Love3Love4Love5Love6Love7";
	const char* p2 = "Love1Love2Love3Love4Love5Love6Love7";
	printf("p1 Address  = %p\n", p1);
	printf("p2 Address  = %p\n", p2);
}

int main()
{
	func();
	return 0;
}

myString.size() = 5
sizeof(string) = 40
-----------------begin---------------------
 Array element value  = It is very long~~~~~~~~~~~~~long~~~~~~~~~~~~long
 Object address  = 000000244C30F9C0
 The string address to  = 0000018DE25E5CE0
-----------------end---------------------
-----------------begin---------------------
 Array element value  = Love1Love2Love3Love4Love5Love6Love7
 Object address  = 000000244C30F9E8
 The string address to  = 0000018DE25F1180
-----------------end---------------------
-----------------begin---------------------
 Array element value  = China
 Object address  = 000000244C30FA10
 The string address to  = 000000244C30FA18
-----------------end---------------------
-----------------begin---------------------
 Array element value  =
 Object address  = 000000244C30FA38
 The string address to  = 000000244C30FA40
-----------------end---------------------
-----------------begin---------------------
 Array element value  = It is very long~~~~~~~~~~~~~long~~~~~~~~~~~~long
 Object address  = 000000244C30FA60
 The string address to  = 0000018DE25E40B0
-----------------end---------------------
p1 Address  = 00007FF655DAF398
p2 Address  = 00007FF655DAF398

<2>vector
(1) Adding elements to the back and deleting elements from the back are both quick ;push_back().
(2) Inserting elements into the middle may cause many subsequent elements to be reconstructed , destructor . The efficiency will be very low .
(3) The search speed should not be too fast .

vector Container memory is also next to , vector The container has a “ Space ” The concept of , Each space can contain an element ;
How many elements in the container can be used size() Look at , And how much space there is in this container , Sure use capacity();
capacity() It must not be less than size();vector The number of spaces in the container must not be less than the number of elements ;
use reverse Space can be reserved , The premise is that you know in advance how many elements this container can hold ; It can greatly improve the running efficiency of the program .