[cdl-studenti] [OOP] Eliberare automata de memorie.

[marius Dragus]

Nu e ok. Varianta asta e gresita din mai multe motive (produce double free,
copiaza memorie inexistenta).

z= x+y;

0. Se apeleaza operatorul + pentru x
1. Se aloca y  in operator+.
2. Se apeleaza destructorul pentru y
3. Se apeleaza operatorul de asignare pentru z ( dar partea dreapta a fost
eliberata deja la pasul 2 -- deci e gresit)
4. Se apeleaza destuctorul pentru z ( double free).



2010/3/6 Dan-George Filimon <dangeorge.filimon at gmail.com>

> Atunci, de ce nu:
>
> mine operator+ (const mine x) {
> mine y;
> memcpy(y.u, x.u, 10000 * sizeof(int));
> return y;
> }
>
> Ideea fiind ca la final, y fiind un obiect va fi distrus automat cand iesi
> din main prin apelul destructorului. Ce faci tu e sa creezi un non-obiect in
> operator+ si banuiesc ca de-aia nu merge.
>
> On Mar 6, 2010, at 4:29 PM, marius Dragus wrote:
>
> Pai operatorul + nu ar trebui sa modifice obiectul curent.
> De exemplu pe stringuri:
>
> string x ="ana";
> string y ="are";
> string z = x+y;
>
> cout<< x; //afiseaza "ana", x nu e modificat.
>
>
> 2010/3/6 Dan-George Filimon <dangeorge.filimon at gmail.com>
>
>> Nu inteleg de ce ai in definitia operatorului + o noua alocare de memorie.
>> De ce nu folosesti pointerul this?
>>
>> Cand faci un operator ca + m-as gandi ca ai ceva de genul:
>> mine operator+(const mine x) {
>>         memcpy(this->u, x.u, 10000 * sizeof(int));
>>        return *this;
>> }
>>
>> Asta nu ar face ce vrei?
>>
>> On Mar 6, 2010, at 4:20 PM, marius Dragus wrote:
>>
>> > Salut,
>> >
>> > Incerc sa scriu o clasa "mine" in C++ care sa permita operatorul +.
>> > Problema este ca fiecare obiect aloca dinamic un array, iar o operatie
>> de genul x =a +b+c ar cauza memory leak-uri.
>> >
>> > Cum s-ar putea face asta?
>> >
>> > Mai jos este codul. Mentin un contor al constructorilor/destructorilor,
>> si se vede evident ca se obtin memory leakuri.
>> >
>> > Mersi,
>> > Marius
>> >
>> > /*************************CODE*********************/
>> >
>> > #include <iostream>
>> >
>> > int count = 0;
>> > using namespace std;
>> >
>> > struct mine
>> > {
>> >     int * u;
>> >     mine()
>> >     {
>> >     u = new int[10000];
>> >     count++;
>> >     }
>> >
>> >     ~mine()
>> >     {
>> >     delete[] u;
>> >     count--;
>> >     }
>> >
>> >     mine & operator=(const mine & x)
>> >     {
>> >     //cout << "assign" << endl;
>> >     u = x.u;
>> >     }
>> >
>> >
>> > };
>> >
>> > mine& operator+(const mine & x, const mine &y)
>> > {
>> >     mine * ret = new mine;
>> >     return *ret;
>> > }
>> >
>> > int main()
>> > {
>> >
>> >     mine a, b, c, d;
>> >     mine e;
>> >
>> >
>> >     for (int i = 0; i<10; i++)
>> >     {
>> >     e = a + b + c + d;
>> >
>> >     }
>> >
>> >     cout << count << "\n"; // afiseaza 35 desi ar trebui sa fie 5
>> >
>> >     return 0;
>> > }
>> >
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