double free while using vector and strings - c++

Question

I wrote the following function and a simple class, while trying to understand how expensive a work with vector can be.
void gen_random(string & str, const int len)
{
static const char alphanum[] =
"0123456789"
"ABCDEFGHIJKLMNOPQRSTUVWXYZ"
"abcdefghijklmnopqrstuvwxyz";
srand((unsigned int)time(NULL));
str.reserve(len);
for (int i = 0; i < len; ++i)
{
str += alphanum[rand() % (sizeof(alphanum) - 1)];
}
}
class Person
{
public:
//CTOR with parameter
Person(u_int32_t Id)
{
std::cout << "\033[1;32mPerson CTOR: " << Id << "\033[0m" << std::endl;
m_Id = Id;
m_RandSid = new string;
gen_random(*m_RandSid, 10);
}
//CCTOR
Person(const Person & p)
{
std::cout << "\033[1;31mPerson CCTOR: " << p.m_Id << "\033[0m" << std::endl;
m_Id = p.m_Id;
m_RandSid = p.m_RandSid; //trigger string operator=()
}
//MCTOR
Person(Person&& p)
{
std::cout << "\033[1;34mPerson MCTOR: " << p.m_Id << "\033[0m" << std::endl;
m_Id = p.m_Id;
m_RandSid = p.m_RandSid;
p.m_RandSid = nullptr;
}
//DTOR
~Person()
{
std::cout << "\033[1;33mPerson DTOR: "<<m_Id <<"\033[0m"<< std::endl;
if (nullptr != m_RandSid)
{
delete m_RandSid;
}
}
u_int32_t m_Id;
string * m_RandSid;
};
and driver :
int main()
{
int a;
vector<Person> v;
for (int i = 0; i < 2; ++i)
{
std::cout <<std::endl<< "inserting person #" << i << std::endl;
std::cout << "Vector size = " << v.size()<< " Vector capacity = " << v.capacity() << std ::endl;
v.emplace_back(i);
std::cout << *v[i].m_RandSid << std::endl;
std::cout << "Vector size = " << v.size()<< " Vector capacity = " << v.capacity() << std ::endl;
}
std::cout<<std::endl<<"--------------------------------------------------------"<<std::endl;
return 0;
}
when I run this program, I the following output:
inserting person #0
Vector size = 0 Vector capacity = 0
Person CTOR: 0
07QoUmgEe6
Vector size = 1 Vector capacity = 1
inserting person #1
Vector size = 1 Vector capacity = 1
Person CTOR: 1
Person CCTOR: 0
Person DTOR: 0
07QoUmgEe6
Vector size = 2 Vector capacity = 2
--------------------------------------------------------
Person DTOR: 0
free(): double free detected in tcache 2
I don't understand where else I perform another free :\
The other problem is that the string is randomized per execution and not per object,
If srand is performed on each execution, why all the string look the same ?

Answer 1

In your copy and move constructor you simply copy raw pointers, which makes 2 pointers to point to the same memory, and when both objects destroyed you get double deallocation:
Person(const Person & p)
{
std::cout << "\033[1;31mPerson CCTOR: " << p.m_Id << "\033[0m" << std::endl;
m_Id = p.m_Id;
m_RandSid = p.m_RandSid; // now both pointers point to the same memory
}
It is not clear why you need dynamically allocated string objects, you should just store objects by value, but if you do need that you should use smart pointers (either std::shared_ptr or std::unique_ptr depends of what ownership you need). That will not only make your problem disappear but you would not have to provide copy and move constructor manually, compiler generated ones would be sufficient.
Note, your class is also missing proper copy and move assignment operator, though it is not exposed in code shown it still violates the rule of three/five/zero and you may have problems with your code later.

Answer 2

Your copy constructor copies the pointer value, and should be doing a deep copy (allocate a new string). In a simple design like this I would avoid using new/free for the string.
You initialize (actually reset) rand with time with second precision - time(). Your app probably finishes in less than a second, hence the similar strings. Initialize rand only once, eg when starting the app.