《c++入門經典》筆記11

第十一章 開發高級指針

11.1在堆中創建對象

實際上，類就是對象的類型，對象也是一種變數，所以你可以在堆中創建int型變數，自然也就能創建自定義型變數。

Cat *pCat = new Cat;

這將調用預設構造函數（無參構造函數），每當在堆或棧中創建對象時，都將調用構造函數。

11.2刪除對象

對指向堆中對象的指針調用delete時，將調用對象的析構函數，然後釋放記憶體。

程式清單11.1 HeapCreator.cpp

#include <iostream>

class SimpleCat
{
public:
    SimpleCat()
    {
        std::cout << "Constructor called\n";
        itsAge = 1;
    }
    ~SimpleCat()
    {
        std::cout << "Destructor called\n";
    }

private:
    int itsAge;
};

int main()
{
    std::cout << "SimpleCat simpleCat ...\n";
    SimpleCat simpleCat;

    std::cout << "SimpleCat *pRags = new SimpleCat ...\n";
    SimpleCat *pRags = new SimpleCat;

    std::cout << "delete pRags ...\n";
    delete pRags;

    std::cout << "Exiting, watch simpleCat go ...\n";
    return 0;
}

這裡最後一個Destructor called是因為main()函數結束時，simpleCat對象不再在作用域中，所以編譯器調用其析構函數。

11.3使用指針訪問成員

方法一（解引用運算符）：

(*pRags).getAge();

方法二（指向運算符->）：

pRags->getAge();

程式清單11.2 HeapAccessor.cpp

#include <iostream>

class SimpleCat
{
public:
    SimpleCat()
    {
        itsAge = 2;
    }
    ~SimpleCat()
    {
        std::cout << "Destructor called\n";
    }
    int getAge() const { return itsAge; }
    void setAge(int age) { itsAge = age; }

private:
    int itsAge;
};

int main()
{
    SimpleCat *simpleCat = new SimpleCat;

    std::cout << "simpleCat is " << (*simpleCat).getAge() << " years old"
              << "\n";
    simpleCat->setAge(5);
    std::cout << "simpleCat is " << simpleCat->getAge() << " years old"
              << "\n";

    return 0;
}

11.4堆中的數據成員

類可能有一個或多個數據成員為指針，並指向堆中的對象。可在構造函數或成員函數中分配記憶體，併在析構函數中釋放記憶體。

程式清單11.3 DataMember.cpp

#include <iostream>

class SimpleCat
{
public:
    SimpleCat()
    {
        itsAge = new int(2);
        itsWeight = new int(5);
    }
    ~SimpleCat()
    {
        delete itsAge;
        delete itsWeight;
    }
    int getAge() const { return *itsAge; }
    void setAge(int age) { *itsAge = age; }
    int getWeight() const { return *itsWeight; }
    void setWeight(int weight) { *itsWeight = weight; }

private:
    int *itsAge;
    int *itsWeight;
};

int main()
{
    SimpleCat *simpleCat = new SimpleCat;

    std::cout << "simpleCat is " << simpleCat->getAge() << " years old"
              << "\n";
    simpleCat->setAge(5);
    std::cout << "simpleCat is " << simpleCat->getAge() << " years old"
              << "\n";

    return 0;
}

11.5this指針

每個類成員函數都有一個隱藏的參數——this指針，它指向用於調用函數的對象。

通常，在成員函數中，無需使用this指針來訪問當前對象的成員變數，如果願意，可以顯示地使用this指針。

程式清單11.4 This.cpp

#include <iostream>

class Rectangle
{
private:
    int itsLength;
    int itsWidth;

public:
    Rectangle();
    ~Rectangle();
    void setLength(int length) { this->itsLength = length; }
    int getLength() const { return this->itsLength; }
    void setWidth(int width) { this->itsWidth = width; }
    int getWidth() const { return this->itsWidth; }
};

Rectangle::Rectangle()
{
    itsWidth = 5;
    itsLength = 10;
}

Rectangle::~Rectangle()
{
}

int main()
{
    Rectangle theRect;
    std::cout << "theRect is " << theRect.getLength() << " feet long." << std::endl;
    std::cout << "theRect is " << theRect.getWidth() << " feet wide." << std::endl;
    theRect.setLength(20);
    theRect.setWidth(10);
    std::cout << "theRect is " << theRect.getLength() << " feet long." << std::endl;
    std::cout << "theRect is " << theRect.getWidth() << " feet wide." << std::endl;
    return 0;
}

11.6懸垂指針

懸垂指針又稱為野指針或者迷失指針，指的是對指針調用了delete（釋放其指向的記憶體）之後，沒有重新賦值（即沒有重新初始化）就開始被使用的指針。

實際上上章筆記中delete關鍵字時就已經提到野指針的危害。所以進行delete之後應該重新new賦值或者設置為nullptr。

11.7const指針

聲明指針時，可在類型前、類型後或者兩個地方都使用const。

const int *pOne;//指向常量的指針
int * const pTwo;//常量指針
const int * const pThree;//指向常量的常量指針

三條語句意義各不相同，三個指針類型也各不相同。

pOne是指向整型常量的指針，也就是編譯器預設它指向的是一個常量（雖然可能不是），所以不能通過這個指針來更改所指向的常量（編譯器認為是常量但不一定是）的值，比如*pOne = 5;編譯器就會報錯。

 int one = 10;
 const int * pOne = &one;
 *pOne = 5;//報錯，表達式必須是可修改的左值，但此時*pOne被認為不可修改

pTwo是指向整型的常量指針，可以修改指向的整型變數，但是pTwo不能指向其他變數。

 int two = 20;
 int * const pTwo = &two;
 *pTwo = 15;
 pTwo = &one;//報錯，不能指向別的變數

pThree是一個指向整型常量的常量指針，不能修改它指向的值，也不能讓它指向其他變數。

	int three = 30;
 const int * const pThree = &three;
 pThree = &one;//報錯，不能指向別的變數
 *pThree = 25;//報錯，此時*pThree被認為不可修改

完整代碼：(註釋起來的是報錯的)

#include <iostream>

int main()
{
    int one = 10;
    const int * pOne = &one;
//    *pOne = 5;

    int two = 20;
    int * const pTwo = &two;
    *pTwo = 15;
//    pTwo = &one;

    int three = 30;
    const int * const pThree = &three;
//    pThree = &one;
//    *pThree = 25;

    std::cout<<"one: "<<one<<" *pOne: "<<*pOne<<std::endl;
    std::cout<<"two: "<<two<<" *pTwo: "<<*pTwo<<std::endl;
    std::cout<<"three: "<<three<<" *pThree: "<<*pThree<<std::endl;
    return 0;
}

11.8const指針與const成員函數

程式清單11.5 ConstPointer.cpp

#include <iostream>

class Rectangle
{
private:
    int itsLength;
    int itsWidth;

public:
    Rectangle();
    ~Rectangle();
    void setLength(int length) { itsLength = length; }
    int getLength() const { return itsLength; }
    void setWidth(int width) { itsWidth = width; }
    int getWidth() const { return itsWidth; }
};

Rectangle::Rectangle() : itsWidth(5), itsLength(10) //初始化列表
{
}

Rectangle::~Rectangle() {}

int main()
{
    Rectangle *pRect = new Rectangle;
    const Rectangle *pConstRect = new Rectangle; //pConstRect為指向Rectangle常量型對象的指針
    Rectangle *const pConstPtr = new Rectangle;  //pConstPtr為指向Rectangle型對象的常量指針

    std::cout << "pRect width: " << pRect->getWidth() << " feet\n";
    std::cout << "pConstRect width: " << pConstRect->getWidth() << " feet\n";
    std::cout << "pConstPtr width: " << pConstPtr->getWidth() << " feet\n";

    pRect->setWidth(10);
    //pConstRect->setWidth(10);
    pConstPtr->setWidth(10);

    std::cout << "pRect width: " << pRect->getWidth() << " feet\n";
    std::cout << "pConstRect width: " << pConstRect->getWidth() << " feet\n";
    std::cout << "pConstPtr width: " << pConstPtr->getWidth() << " feet\n";

    return 0;
}