《c++入門經典》筆記11
《c++入門經典》筆記11
第十一章 開發高階指標
11.1在堆中建立物件
實際上,類就是物件的型別,物件也是一種變數,所以你可以在堆中建立int型變數,自然也就能建立自定義型變數。
Cat *pCat = new Cat;
這將呼叫預設建構函式(無參建構函式),每當在堆或棧中建立物件時,都將呼叫建構函式。
11.2刪除物件
對指向堆中物件的指標呼叫delete時,將呼叫物件的解構函式,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()函式結束時,simpleCat物件不再在作用域中,所以編譯器呼叫其解構函式。
11.3使用指標訪問成員
方法一(解引用運算子):
(*pRags).getAge();
方法二(指向運算子->):
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堆中的資料成員
類可能有一個或多個數據成員為指標,並指向堆中的物件。可在建構函式或成員函式中分配記憶體,並在解構函式中釋放記憶體。
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指標
每個類成員函式都有一個隱藏的引數——this指標,它指向用於呼叫函式的物件。
通常,在成員函式中,無需使用this指標來訪問當前物件的成員變數,如果願意,可以顯示地使用this指標。
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懸垂指標
懸垂指標又稱為野指標或者迷失指標,指的是對指標呼叫了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)就開始被使用的指標。
實際上上章筆記中delete關鍵字時就已經提到野指標的危害。所以進行delete之後應該重新new賦值或者設定為nullptr。
11.7const指標
宣告指標時,可在型別前、型別後或者兩個地方都使用const。
const int *pOne;//指向常量的指標 int * const pTwo;//常量指標 const int * const pThree;//指向常量的常量指標
三條語句意義各不相同,三個指標型別也各不相同。
pOne是指向整型常量的指標,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標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,表示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標,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標,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
完整程式碼:(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指標與const成員函式
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常量型物件的指標
Rectangle *const pConstPtr = new Rectangle; //pConstPtr為指向Rectangle型物件的常量指標
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;
}