C++翁恺学习31-模板

why templates?

• suppose you need a list of X and a list of Y

  • the lists would use similar code
  • they differ by the type shored in the list

• choices

  • require common base class   方案1
    • may not be desirable
  • clone code   方案2
    • preserves type-safety
    • hard to manage  难以管理维护
  • untyped lists  方案3  没有类型的list
    • type unsafe   不安全

templates  方案4

函数模板(用来做出函数)   类模板   模板函数  模板类

• reuse source code
  • generic programming
  • use types as parameters in class or function definitions  类型成为参数
• template functions   
  • example : sort function
• template classes   
  • example:contains such as stack,list, queue...
    • stack operations are independent of the type of items in the stack
  • template member functions

function templates

• perform similar operations on different types of data

• swap function for two int arguments:

  void swap(int& x,int& y){
      int temp = x;
      x = y;
      y = temp;
  }

• what if we want to swap flats ,strings,Currency,Person?

example:swap function template

template    // 声明而不是定义
void swap(T& x,T& y){  // 函数模板  T代表一个类型
    T temp = x;
    x = y;
    y = temp;
}

• the template keyword introduces the template
• the class T specifies a parameterized type name
  • class means any built-in type or user-defined type
• inside the template,use T as a type name

function template Syntax

• parameter types represent:
  • types of arguments to the function
  • return type of the function
  • declare variables within the function

template instantiation

• generating a declaration from a template class/function and template arguments:
  • types are substituted into template
  • new body of function or class definition is created
    • syntax errors,type checking
  • specialization -- a version of a template for a particular argument(s)

example:using swap

int i = 3;
int j = 4;
swap(i,j);	//use explicit int swap

float k = 4.5;	float m = 3.7;
swap(k,m);	//instanstiate float swap  编译器给制造出来了一个相应函数
std::string s("hello");
std::string t("world");
swap(s,t);	//std::string swap

• a template function is an instantiation of a function template

interactions  

• only exact match on types is used

• no conversion operations are applied

  swap(int, int);	//ok
  swap(double, double);	//ok
  swap(int, double)	//error

• even implicit conversions are ignored

• template functions and regular functions coexist

overloading rules

• check first for unique function match

• then check for unique function template match

• then do overloading on functions

  void f(float i,float k)();
  template 
  void f(T t,T u){};
  f(1.0,2.0);
  f(1,2);
  f(1,2.0)

function instantiation

• the compiler deduces the template type from the actual arguments passed into the function

• can be explicit:

  • for example,if the parameter is not in the function signature(older compilers won't allow this)

    template 
    void foo(void){...}
    foo();//type T is int
    foo();//tyoe T is float

class templates

• classes parameterized by types

  • abstract operations form the types being operated upon
  • define potentially infinite set of classes
  • another step towards reuse

• typical use:container classes

  stack 
  //is a stack that is parameterized over int
  list  
  queer 

example:Vector

template 
class Vector{
public:
    Vector(int);
    ~Vector();
    Vector(const Vector&);
    Vector& operator=(const Vector&);
    T& operator[] (int);
private:
    T* m_elements;
    int m_size;
};

usage

Vector v1(100);
Vertor v2(256);

v1[20] = 10;
v2[20] = v1[20];	//ok if int -> Comlex defined

Vector members

template   // 每一个都是函数模板
Vector::Vector(int size):m_size(size){
    m_elementds = new T[m_size];
}
template 
T& Vertor::operator[](int indx){
    if(indx < m_size && indx > 0){
        return m_elments[indx];
    }else{
        ...
    }
}

类模板里面的每一个函数都是函数模板

a simple sort function

//bubble sort -- don't use it

template 
void sort(vector& arr){  // 函数模板  声明  
    const size_t last = arr.size()-1;
    for(int i = 0;i < last; i++){
        for(int j = last; i < j; j--){
            if(arr[j] < arr[j-1]){
                //which swap?
                swap(arr[j], arr[j-1]);
            }
        }
    }
}

sorting the vector

vector vi(4);
vi[0] = 4;vi[2] = 7; vi[1]=3; vi[3] = 1;
sort(vi);	//sort(vector&)

vector vs;
vs.push_back("Fred");
vs.push_back("Willma");
vs.push_back("Barney");
vs.push_back("Dino");
vs.push_back("Prince");
sort(vs);	//sort (vector&)
//note:sort uses operator < for comparation

templates

• templates can use multiple types

  template
  class HashTable{
  	const Value& lookup(const Key&) const;
  	void install(const Key& ,const Value&);
  	...
  };

• templates nest ---they're just new types!

  Vector< Vector >	//note sapce > >  空格

• type arguments can be complicated

  Vector&,int)>//内层是函数

expression parameters

• template arguments can be constant expressions

• non-Type parameters

  • can have a default argument

  template 
  class FixedVector{
  public:
      FixedVector();
      //...
      T& oprator[](int);
  private:
  	T elements[bounds];//fixed size array
  }

usage:Non-type parameters

• usage

  FixedVector v1;
  FixedVector v2;
  FixedVector v3;//uses default

• summary

  • embedding sizes not necessarily
  • can make code faster
  • makes use more complicated
    • size argument appears everywhere
  • can lead to (even more) code bloat

templates and inheritance

• templates can inherit from non-template classes

  template 
  class Deried:public Base{...}

• templates can inherit form template classes

  template
  class Derived : pubic List{...}  

• non-templates classes can inherit from templates

  class SupervisorGroup:public List

writing templates

• get a non-template version working first
• establish a good set of test cases
• measure performance and tune