C++翁恺学习17-const

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const

• declares a variable to have a constant value.

constants

• constants are variables 
  • observe scoping rules
  • declared with "const" type modifier(修饰)

compile time constants

const int bufsize = 1024;

• value must be initialized
• unless you make an explicit extern declaration

enxtern const int bufsize;

• complier won't let you change it
• compile time constants are entries in compiler symbol table,not really variables

run-time constants

• const value can be exploited

const int class_size = 12;
int finalGrade[class_size];//ok

int x;
cin>>x;
const int size = x;
double classAverage[size];//error

编译器不知道x的值，因为编译器不知道需要分配内存的大小。

pointer and const

char *const q = "abc";//q is const
*q = "c"//ok
q++;//error   指针是const 

const char *p = "abcd";//(*p) is a cosnt char   
*p = "b";//error 指针指的内存的东西是 const
p++; // ok 下一个内存的东西也是 const 了

quiz: what do these mean?

cosnt person* p = &p1; // 对象是 const
person const* = &p1;   // 对象是 const
person *const p= &p1; // 指针是 const

*的前面还是后面。 * 在 const 前， 指针是 const； * 在 const 后，对象是 const

pointers and constants

• remember

  *ip=54; //always legal since ip points to int
      
  *cip=54; //never legal since cip points to const int

string literals 字符串常量

char* s = "hello,world";

• s is a pointer initialized to point to string constant

• this is actually a "const char* s" but compiler accepts it without the const

• don't try and change the character values(it is undefined behavior)

if you want to change the string ,put it in an array:

char s[] = "hello,world!"; // 数组在栈中

C++中三种内存模型：

本地变量，在栈中；

全局变量在全局数据区里，全局变量中的这种常量（“hello，world”）在代码段里，代码段不可写的；

new出来的东西在堆中；

conversions

• can always treat a non-const value as const

  void f(const int* x){...}
  int a = 15;
  f(&a);//ok
  const int b = a;
  f(&b);//ok
  b=a+1;//error

• you cannot treat a constant object as non-constant without an explicit cast(const_cast)

passing by const value

void f1(const int i){
    i++;//illegal -- compile-time error
}

returning by const value?

int f3(){return 1;}
const int f4(){return 1;}
int main(){
    const int j = f3();//work fine
    int k = f4(); //but this works fine too
}

passing and returning addresses

• passing a whole object may cost you a lot.it is better to pass by a pointer.but it's possible for the programmer to take it and modify the original value

• in fact ,whenever you're passing an address into a function,you should make it a const if at all possible

constant objects

• what if an object is const?

  const Currency the_raise(42,38);

• what members can access the internals?

• how can the object be protected from change?

const member functions

• cannot modify their

  int Data::set_day(int d){
      //...error check d here...
      day = d;//ok,non-const so can modify
  }
  
  int Data::get_day() const{
      day++; //error modifies data member
      set_day(12); //error calls non-const member
      return day;//ok
  }

const member function usage

• repeat the const keyword in the definition as well as the declaration

  int get_day() const;//this ------ is const
  int get_day() const {return day;}

• functon members that do not modify data should be declared const

• const member function are safe for const objects

constant in class

class A{
    const int i;
};

• has to be initialized in initializer list of the cosnstructor

compile-time constants in classes

class HasArray{
    const int size;
    int array[size]; //error,不能作为数组的大小， 1.使用static  2. 枚举
    ...
}

​​​

• make the const value static:

  static const int size = 100;

  static indicates only one per class (not one per object)

• or use "anonymous enum" hack

  class HasArray{
      enum{size = 100};
      int array[size];//ok
      ...
  }

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