[C++复习]08|Templates and STL

[note 1]

Function templates

template <class T>
return-type function-name (arguments of type T)
{
	function body
}

在这里插入代码片

[example 1]

#include <iostream>
using namespace std;

template<class T>
void Swap(T &x, T &y)
{
	T temp=x;
	x=y;
	y=temp;
}

void fun(int m, int n,float a,float b)
{
	cout << "m and n before swap: " << m << " " << n << "\n";
	Swap(m,n);
	cout << "m and n after swap: " << m << " " << n << "\n";
	cout << "a and b before swap: " << a << " " << b << "\n";
	Swap(a,b);
	cout << "a and b after swap: " << a << " " << b << "\n";
}

int main()
{
	fun(100,200,11.22,33.44);
	return 0;
}

[note 2]

• C++ will not perform any implicit conversion for the template arguments.

[solution 1]
explicit type conversion

cout<<"the min of 2 and 3.2 is:"<<min(double(2),3.2)<<endl;

[solution 2]
specify the argument’s data type explicitly

cout<<"the min of 2 and 3.2 is:"<<min<double>(2,3.2)<<endl;

[solution 3]
specify multiple parameters for function template.

#include<iostream>
#include<cstring>
using namespace std;

template<class T1, class T2>
void display(T1 x, T2 y){cout << x << " " << y << "\n";}

int main()
{
    display(1999,"EBG");
    display(12.34,1234);
    
    return 0;
}

[note 3]

The return type depend on the template parameter T1 (the first parameter).

[note 4]

class template

template<class T, int size>
class array
{
	T a[size];
}

to create a object:
array<int, 10> a1;

[note 5]

shows how a template function is overloaded with an explicit function.

#include<iostream>
#include<string>
using namespace std;

template<class T>
void display(T x){cout<<"Template display: "<<x<<"\n";}
void display(int x){cout<<"Explicit display: "<<x<<"\n";}	//Overloads the generic display()

int main()
{
	display(100);											//Invokes the ordinary version of display() and not the template version.
	display(12.34);
	display('C');

	return 0;
}

[note 6]

Member function templates

template<class T>
return-type classname<T>::functionname(arglist)
{
     //
}

[note 7]

Three key components of STL:

1. containers
2. algorithms
3. iterators

[note 8]

STL-vector

#include<iostream>
#include<vector>									//Vector header file
using namespace std;

void display(vector<int> &v)
{
	//Size(): The member function gives the current size of the vector.
	for(int i=0;i<v.size();i++){cout << v[i] << "  ";}	
	cout << "\n";
}

int main()
{
	vector<int> v;									//Create a vector of type int
	cout << "Initial size = " << v.size() << "\n";	//Gives the current size of the vector
	//Putting values into the vector
	int x;
	cout << "Enter five integer values: ";
	for(int i=0;i<5;i++)
	{
		cin >> x;
		//member function used to put five values into the vector
		v.push_back(x);								//Takes a value as its argument and adds it to the back end of the vector.								 
	}
	cout << "Size after adding 5 values: ";
	cout << v.size() << "\n";

	//Display the contents
	cout << "Current contents: \n";
	display(v);

	//Add one more value
	v.push_back(6.6);								//float value truncated to int and then puts it into the vector at its back end(Since the vector v is of type int, it can accept only integer values )

	//Display size and contents
	cout << "\nSize = " << v.size() << "\n";
	cout << "Contents now: \n";
	display(v);

	//Inserting elements
	vector<int>::iterator itr = v.begin();			//iterator(point to the first position of the vector)
	itr = itr + 3;									//itr points to 4th element
	v.insert(itr,1,9);

	//Display the contents
	cout << "\nContents after inserting: \n";
	display(v);

	//Removing 4th and 5th elements
	v.erase(v.begin()+3,v.begin()+5);				//Removes 4th and 5th element from the vector

	//Display the contents
	cout << "\nContents after deletion: \n";
	display(v);
	cout << "END\n";
	return(0);
}

output:

Initial size = 0
Enter five integer values: 
1
2
3
4
5
Size after adding 5 values: 5
Current contents: 
1  2  3  4  5  

Size = 6
Contents now: 
1  2  3  4  5  6  

Contents after inserting: 
1  2  3  9  4  5  6  

Contents after deletion: 
1  2  3  5  6  
END

[note 9]

STL-list

#include<iostream>
#include<list>
#include<cstdlib>                //For using rand() function
using namespace std;

void display(list<int> &lst)
{
    list<int>::iterator p;
    for(p=lst.begin(); p!=lst.end(); ++p)
        cout << *p << ", ";
    cout << "\n\n";
}

int main()
{
    list<int> list1;            //Empty list1 of zero length
    list<int> list2(5);            //Empty list2 of size 5
    
    for(int i=0; i<3; i++)        //List1 filled with three values using the member function push_back() and math function rand()
        list1.push_back(rand()/100);
    
    list<int>::iterator p;
    //List2 filled using a list type iterator p and a for loop
    for(p=list2.begin(); p!=list2.end(); ++p)    //list2.begin() gives the position of the first element and list2.end() gives the position immediately after the last element.
        *p=rand()/100;
    
    cout << "List1 \n";
    display(list1);
    cout << "List2 \n";
    display(list2);
    
    //Add two elements at both the ends of list1
    list1.push_front(100);
    list1.push_back(200);
    
    //Remove an element at the front of list2(Similarly, we may use pop_back() to remove the last element.)
    list2.pop_front();
    
    cout << "Now list1 \n";
    display(list1);
    cout << "Now list2 \n";
    display(list2);
    
    list<int> listA, listB;
    //The objects of list can be initialized with other list objects illustrated as follows
    listA=list1;
    listB=list2;
    
    //Merging two lists(unsorted)
    list1.merge(list2);                    //Adds the list2 elements to the end of list1.
    cout << "Merged unsorted list2 \n";
    display(list1);
    
    //Sorting and merging
    listA.sort();        //The element in a list may be sorted in increasing order using sort() member function.
    listB.sort();        //Note that when two sorted lists are merged, the elements are inserted in appropriate locations and therefore the merged list is also a sorted one.
    listA.merge(listB);
    cout << "Merged sorted lists \n";
    display(listA);        //Display the contents of various lists.(Note the difference between the implementation of display() in Program 14.1 and Program 14.2.)
    
    //Reversing a list
    listA.reverse();
    cout << "Reversed merged list \n";
    display(listA);
    
    return(0);
}

[note 10]

STL-maps

#include<iostream>
#include<string>
#include<map>
using namespace std;

int main()
{
	string name[]={"aa","bb","cc"};
	double salary[]={1200,2000,1450};

	map<string,double>sal;
	map<string,double>::iterator p;

	for(int i=0;i<3;i++)
		sal.insert(make_pair(name[i],salary[i]));

	sal["tom"]=3400;
	sal["bob"]=2000;

	for(p=sal.begin();p!=sal.end();p++)
		cout<<p->first<<"\t"<<p->second<<endl;

	string person;
	cout<<"Name of the person to be searched: ";
	cin>>person;
	for(p=sal.begin();p!=sal.end();p++)
		if(p->first==person)
			cout<<p->second<<endl;

    return 0;
}