effective c++ 条款3 use const whereever you can

1 const 传达的意思应该是这个变量是常量不能更改

2 const 在 * 左边表示数据是const，在右边表示指针是const

//
char greeting[] = "hello";

char* p = greeting;

//const *: const data
//* const: const pointer
char const *p1 = greeting;    // const data
char * const p2 = greeting;   // const pointer
const char * const p3 = greeting; // const data and pointer

//not correct p1 is const data
//(*p1)++;

//correct p1 is const data
p1++;

//correct p2 is const pointer
(*p2)++;

//not correct p2 is const pointer
//p2++;

//not correct p3 is  const data and pointer
//(*p3)++;

//not correct p3 is  const data and pointer
//p3++;

　　3

//1 const return value avoid the error like this
// if( a+b = c) {...}
//2 const parameter
// it can avoid unintentioned change of parameter inside the function
const Rational operator+ (const Rational& lhs, const Rational& rhs)
{
Rational tmp;
tmp.real = lhs.real + rhs.real;
tmp.img = lhs.img + rhs.img;

return tmp;
}

class Rational
{
public:
float real;
float img;

Rational():real(0),img(0){};

};

　　4 const 成员函数

声明方式

作用

ConstMemberFunc.h

//---------------------------------------------------------------------------

#ifndef ConstMemberFuncH
#define ConstMemberFuncH
//---------------------------------------------------------------------------

#include <cstddef>    //for std::size_t

//1 const member function is used to operate const object
// eg. we have a class Textblock and it has a operator[] which is const
// when we have a const Textblock, the operator[] const will be used

//2
// a  const member function can change static class member
// b  const member function can not change non-static class member
// c  use mutable decleared variable can be changed in const member function

//3 const member function can only call const member function
//                        can not call non-const member function

//4 const object can only call const member function
//  const object can not call non const member function
class TextBlock
{
public:

static int TextBlockStaticNum;

char* text;

//1 const member function is used to operate const object
//  eg. we have a class Textblock and it has a operator[] which is const
//  when we have a const Textblock, the operator[] const will be used
const char& operator[](std::size_t position) const
{
return text[position];

//validate 3
//not correct
//3 const member function can only call const function
//                        can not call non-const  function
// print(); is the non-const function of vector

};

char& operator[](std::size_t position)
{
//return text[position];

//call const operator[] and remove constness for return
//not recommended
return const_cast<char&>(                                      // remove the const from the return of const []
static_cast<const TextBlock&>(*this) // add const to *this
[position] // call const []
);
};

//2
// a  const member function can change static class member
// b  const member function can not change non-static class member
// c  use mutable decleared variable can be changed in const member function
int nonStaticOrMutable;
mutable int textLen;
const void constMemberFunctionTest(int Num) const
{
//a change static member
TextBlockStaticNum = 1;

//b
//not correct
//can not change non-static class member
//nonStaticOrMutable = 1;

//c  use mutable decleared variable can be changed in const member function
textLen = 2;

};

//3 const member function can only call const member function
//                        can not call non-const member function
void print()
{
;
}

TextBlock( char charArray[])
{
text = charArray;
TextBlockStaticNum = 0;
textLen = 0;
nonStaticOrMutable=0;
};
} ;

//1 const member function is used to operate const object
// eg. we have a class Textblock and it has a operator[] which is const
// when we have a const Textblock, the operator[] const will be used
//Test function
extern void ConstAndNonConstObjCallConstMemberFunction();

//2
// a  const member function can change static class member
// b  const member function can not change non-static class member
// c  use mutable decleared variable can be changed in const member function
extern void ConstMemFunctionChangeClassMember();

//4 const object can only call const member function
//  const object can not call non const member function
extern void ConstObjCanNotCallNonConstMember(const TextBlock & ctb)
{
//not correct
//print is not a const member function
//ctb.print();

//correct
const char t = ctb[0];

};

#endif

　ConstMemberFunc.cpp　

//---------------------------------------------------------------------------

#pragma hdrstop

#include "ConstMemberFunc.h"
//---------------------------------------------------------------------------
#pragma package(smart_init)

#include "ConstMemberFunc.h"

void ConstAndNonConstObjCallConstMemberFunction()
{
TextBlock tb("test");
const TextBlock ctb("test");

//call non const operator[]
//correct
tb[0] = '1';

//call const operator[]
//not correct, since ctb is const operator
//ctb[0] = '1';
//normally this is used as a function call
//e.g. void print(const TextBlock ctb) //we do not change ctb in this function
}

void ConstMemFunctionChangeClassMember()
{
TextBlock tb("test");
tb.constMemberFunctionTest(5);

const TextBlock ctb("test");
ctb.constMemberFunctionTest(2);
}