vector question

[Babis Haldas]

Can you please tell me whats wrong with the follow code
Its crasing in the line
cout << v[0]->x1 << endl;
when accesing the first element of the vector
If i define the vector as
vector<foo*>v ;
everything is ok

why it doesnt like v(10) ?

thanks

#include <iostream>
#include <vector>

using namespace std;

struct foo {
int x1;
int x2;
};

int main()
{
vector<foo*> v(10);
foo * p;
p = new foo;
p->x1 = 111;
p->x2 = 222;
v.push_back(p);

cout << v[0]->x1 << endl;
}

 

[Victor Bazarov]

Can you please tell me whats wrong with the follow code
Its crasing in the line
cout << v[0]->x1 << endl;
when accesing the first element of the vector
If i define the vector as
vector<foo*>v ;
everything is ok

why it doesnt like v(10) ?

Because if you don't specify the size, it creates an *empty* vector,
in which you later insert the *real* (non-null) element, which you
later access.

thanks

#include <iostream>
#include <vector>

using namespace std;

struct foo {
int x1;
int x2;
};

int main()
{
vector<foo*> v(10);

That creates a ten-sized vector of pointers to 'foo', each null (IIRC).

foo * p;
p = new foo;
p->x1 = 111;
p->x2 = 222;
v.push_back(p);

That inserts *yet another* pointer to 'foo' at the end of the vector.

cout << v[0]->x1 << endl;

v[0] is a null pointer. You try dereferencing it. Undefined behaviour.

}

V

 

[Babis Haldas]

Thanks

Can you please tell me whats wrong with the follow code
Its crasing in the line
cout << v[0]->x1 << endl;
when accesing the first element of the vector
If i define the vector as
vector<foo*>v ;
everything is ok

why it doesnt like v(10) ?

Because if you don't specify the size, it creates an *empty* vector,
in which you later insert the *real* (non-null) element, which you
later access.

thanks

#include <iostream>
#include <vector>

using namespace std;

struct foo {
int x1;
int x2;
};

int main()
{
vector<foo*> v(10);

That creates a ten-sized vector of pointers to 'foo', each null (IIRC).

foo * p;
p = new foo;
p->x1 = 111;
p->x2 = 222;
v.push_back(p);

That inserts *yet another* pointer to 'foo' at the end of the vector.

cout << v[0]->x1 << endl;

v[0] is a null pointer. You try dereferencing it. Undefined behaviour.

}

V

 

[utab]

That creates a ten-sized vector of pointers to 'foo', each null (IIRC).

Why are these pointers are all NULL. Is that because of the ctors of
the vector class value-initializes them. If that is, is not value
initializating to 0(NULL for pointers) wrong because it is not a
built-in type?

Or I am mistaken at some point?

Regards,

 

[Mark P]

Why are these pointers are all NULL. Is that because of the ctors of
the vector class value-initializes them. If that is, is not value
initializating to 0(NULL for pointers) wrong because it is not a
built-in type?

The vector ctor will default initialize them. For pointers, default
initialization means that they are set to zero. (This should not be
confused with other situations where uninitialized variables are given
garbage values. Default initialized is not the same as uninitialized.)

Mark