C++翁恺学习17-const

2022-02-07 20:02:13 阅读：229 来源： 互联网

标签：const 17 int error ok 翁恺 day size

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

	int i;	const int ci=3;
int *ip;	ip=&i;	ip = &ci; //error
const int *cip; 对象是 const	cip=&i;	cip = &ci;

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
    ...
}

标签：const,17,int,error,ok,翁恺,day,size
来源： https://blog.csdn.net/hymn1993/article/details/122808885

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