刪除C++行使

Question

我在做練習，我不明白爲什麼下面的代碼返回：

Program start, before f() -- number of objects: 0 
After f(), before g()  -- number of objects: 0 
After g(), before h()  -- number of objects: -1 
After h(), program end -- number of objects: -1 

沒有什麼錯F（），我明白了一切發生的事情。但是，我無法弄清楚在g（）和h（）中如何調用構造函數和析構函數。謝謝:)

代碼：

class Counted { 
public: 
    Counted(); 
    ~Counted(); 
    static int getNbrObj(); 
private: 
    static int nbrObj; 
}; 

int Counted::nbrObj = 0; 

Counted::Counted() { 
    nbrObj++; 
} 

Counted::~Counted() { 
    nbrObj--; 
} 

int Counted::getNbrObj() { 
    return nbrObj; 
} 

void f() { 
    Counted c; 
    Counted* pc = new Counted; 
    delete pc; 
} 

void g() { 
    Counted c1; 
    Counted c2 = c1; 
} 

void h() { 
    Counted c1; 
    Counted c2; 
    c2 = c1; 
} 

using namespace std; 

void print_nbr_objects(const string& msg) { 
    cout << msg << " -- number of objects: " 
     << Counted::getNbrObj() << endl; 
} 

int main() { 
    print_nbr_objects("Program start, before f()"); 
    f(); 

    print_nbr_objects("After f(), before g() "); 
    g(); 

    print_nbr_objects("After g(), before h() "); 
    h(); 

    print_nbr_objects("After h(), program end "); 
} 

Answer 1

你剛纔儀器一個構造，但C++類有多個構造函數。特別是，你需要儀器拷貝構造函數，這是一個有類似的簽名如下：

Counted(Counted const& other); 

拷貝構造函數被調用至少一個語句代碼：

Counted c2 = c1; 

Answer 2

在g() c2是用複製構造函數構造的（它沒有實現，所以它得到的默認編譯器生成一個），但通過您定義的析構函數銷燬。

Answer 3

在你的G功能，線路

Counted c1 = c1; 

調用不同的構造函數：拷貝構造函數。因爲這個構造函數不會增加nbrObj，所以當c1和c2解構時，它們會將nbrObj遞減兩次。

然而，你的g函數使用了默認的構造函數和解構器，所以對數字沒有影響。

Answer 4

添加一個拷貝構造函數，你會得到預期的結果：

class Counted { 
public: 
    Counted(); 
    Counted(const Counted& other) 
    { 
     nbrObj++; 
    } 
    ~Counted(); 
    static int getNbrObj(); 
private: 
    static int nbrObj; 
}; 

int Counted::nbrObj = 0; 

Counted::Counted() { 
    nbrObj++; 
} 

Counted::~Counted() { 
    nbrObj--; 
} 

int Counted::getNbrObj() { 
    return nbrObj; 
} 

void f() { 
    Counted c; 
    Counted* pc = new Counted; 
    delete pc; 
} 

void g() { 
    Counted c1; 
    Counted c2 = c1; 
} 

void h() { 
    Counted c1; 
    Counted c2; 
    c2 = c1; 
} 

using namespace std; 

void print_nbr_objects(const string& msg) { 
    cout << msg << " -- number of objects: " 
     << Counted::getNbrObj() << endl; 
} 

int main() { 
    print_nbr_objects("Program start, before f()"); 
    f(); 

    print_nbr_objects("After f(), before g() "); 
    g(); 

    print_nbr_objects("After g(), before h() "); 
    h(); 

    print_nbr_objects("After h(), program end "); 
}