c++ - initializing pre-allocated struct instances within a loop -

i want allocate memory number of instances of structure, using malloc(). then, want initialize each instance within loop. but, each iteration, observ constructor, , destructor after, called... why? more suprising me fact each of instance exists after loop despite of call of destructor... , instances initialized same values! i'm definitively missing important... grateful if can me because now, can't explain happen. here c++ code :

struct mystruct{    int* a;    int* b;    mystruct(int x, int y)    {     std::cout << "the constructor called" << std::endl;     = (int*)malloc(x*sizeof(int));     b = (int*)malloc(y*sizeof(float));    }    ~mystruct()    {        std::cout << "the destructor called" << std::endl;        delete[] a;        delete[] b;    } };  int main(int argc, char** argv){ int nb = 3; mystruct *s = (mystruct*)malloc(nb*sizeof(mystruct)); for(int i=0 ; i<nb ; i++) {     *(s+i) = mystruct(1,2); } std::cout << std::endl; for(int i=0 ; i<nb ; i++) {     std::cout << "instance " << << " :" << std::endl;     std::cout << (unsigned int)(*(s+i)->a) << std::endl;     std::cout << (unsigned int)(*(s+i)->b) << std::endl << std::endl; } system("pause");} 

my command window display :

the constructor called

the destructor called

the constructor called

the destructor called

the constructor called

the destructor called

instance 0 : 1608524712 4277075694

instance 1 : 1608524712 4277075694

instance 2 : 1608524712 4277075694

press key continue . . .

best regards

in expression

*(s+i) = mystruct(1,2) 

what happens create temporary instance of structure (the mystruct(1,2) part) copy instance s[i]. copy shallow, , pointers copied , no new data allocated or copied. after expression done temporary instance not needed anymore , destructed.

this destruction of course cause freeing of data members, , pointers in structure in array (the copy) no longer valid, , accessing them undefined behavior. also, don't initialize allocated memory anything, memory contain whatever in there before allocation , contents seem random.

i suggest read std::vector, std::shared_ptr, , importantly the rule of three.