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C++ Constructor And Destructor

发布时间 2023-08-16 18:13:47作者: 名白

if you have problems with constructors and destructors, you can insert such print statements in constructors for your real classes to see that they work as intended. For larger programs, this exact kind of tracing becomes tedious, but similar techniques apply.

For example, you can determine whether you have a memory leak by seeing if the number of constructions minus the number of destructions equals zero. Forgetting to define copy constructors and copy assignments for classes that allocate memory or hold pointers to objects is a common — and easily avoidable — source of problems.

when does those happen? A good way to get a feel for that is to add print
statements to constructors, assignment operations, and destructors and then just
try.

For example:

struct X {
    int val;
    // simple test class
    void out(const string& s, int nv)
    { cerr << this << "–>" << s << ": " << val << " (" << nv << ")\n"; }
    X(){ out("X()",0); val=0; }
    // default constructor
    X(int v) { val=v; out( "X(int)",v); }
    X(const X& x){ val=x.val; out("X(X&) ",x.val); }
    // copy constructor
    X& operator=(const X& a)
    // copy assignment
    { out("X::operator=()",a.val); val=a.val; return *this; }
    ~X() { out("~X()",0); }
    // destructor
};

X glob(2); // a global variable
X copy(X a) { return a; }
X copy2(X a) { X aa = a; return aa; }
X& ref_to(X& a) { return a; }
X* make(int i) { X a(i); return new X(a); }
struct XX { X a; X b; };

int main(){
    X loc {4};    // local variable
    X loc2 {loc}; // copy construction

    loc = X{5};  // copy assignment
    loc2 = copy(loc);  // call by value and return
    loc2 = copy2(loc); 
    X loc3 {6};
    X& r = ref_to(loc); // call by reference and return
    delete make(7);
    delete make(8);
    vector<X> v(4);     // default values
    XX loc4;
    X* p = new X{9};    // an X on the free store
    delete p;
    X* pp = new X[5];  // an array of Xs on the free store
    delete[ ] pp;
}

Try executing this.