One 、string

1.char * and string The difference between

char* Is a pointer to the address of the character array , The system provides a string.h The header file declares many string manipulation functions ,strlen strcat strcmp strcpy…
string Is a class , This class encapsulates the operation functions , Make the operation of string more flexible , More ways , More reasonable management ;
string This class , We do not need to consider the allocation and release of memory , Don't worry about cross-border collapse ,string Have done relevant processing .

2.string The definition of

typedef basic_string<char,char_traits<char>,allocator<char>> string;

3. Constructors

string();
string(size_type length,char ch);
string(const char *str);
string(const char *str,size_type length);//str The first few 
string(string &str,size_type index,size_type length);// In the middle 
string(const string &str);// Copy structure 

4. Common functions

size();

Two 、vector

1.C++ STL And vector Of capacity and size Attribute difference

size It is the present. vector The actual size of the container , That is, how many containers the container currently has .
capacity It means that realloc The maximum number of elements allowed before , That is, the pre allocated memory space .
Of course , These two properties correspond to two methods ：resize() and reserve().
Use resize() The object memory space in the container really exists .
Use reserve() Just modified capacity Value , The objects in the container have no real memory space ( Space is " wild " Of ).
Remember to use [] Operator to access the objects in the container , There is likely to be an array out of bounds problem .

in the light of capacity This attribute ,STL Other containers in , Such as list map set deque, Because the memory of these containers is hashed , So it won't happen realloc() Call status of , So we can think of capacity Attributes are meaningless for these containers , Therefore, these containers do not have this attribute at design time .
stay STL in , Have capacity The only container for attributes is vector 、deque and string.deque The elements of are stored continuously in memory , Same ownership capacity attribute ,deque Have vector All functions of , And you can insert data in the head and tail .

2.vector data(num); When defining, use num Specify the number of elements , This allows random access to these elements later .

#include <iostream>
#include <vector>
using namespace std;    // using namespace std  std
int main()
{
    
	vector <int> data;    // Define vector objects  int n;
	int temp;
	for (int i = 0;i < 5;++i)
	{
    
		cin >> temp;// Input with auxiliary variables 
		data.push_back(temp);
	}
	for (int i = 0;i < 5;++i)
	{
    
		cout << data[i] << endl;
	}
}

#include <iostream>
#include <vector>
using namespace std;    // using namespace std  std
int main()
{
    
	vector <int> data(5);    // Define vector objects  int n;
	for (int i = 0;i < 5;++i)
	{
    
		cin >> data[i];// Direct access to input 
	}
	for (int i = 0;i < 5;++i)
	{
    
		cout << data[i] << endl;
	}
}

3.vector The definition of

template <class T, class Allocator = allocator <T>>// Vector template 
class vector {
    
    ...
}