c++ method address vector - c++

Question

I'm new at using vector, I need a method that return the addres of the vector, this is the semplifide code:
class:
#include<vector>
class A
{
private:
std::vector<int> v;
public:
A()
{
for (int i = 0; i < 5; i++)
v[i] = i;
}
std::vector<int> give()
{
return v;
}
~A() {}
};
main:
#include <iostream>
#include "class.cpp"
int main(int argc, char **argv)
{
A a;
int *vptr = NULL;
vptr = &a.give()[0];
std::vector<int> b;
return 0;
}
this code compile but during the execution a segmentation fault is given

Answer 1

This constructor
A(){
for(int i = 0; i < 5; i++)
v[i] = i;
}
is invalid because the vector has no yet elements that you could use the subscript operator. More correctly would be to write
A() : v( 5 ) {
for(int i = 0; i < 5; i++)
v[i] = i;
}
As for your question then std::vector has member function data that returns the address of its internal buffer with elements.
So you could write
int * give(){ return v.data(); }
or
const int * give() const { return v.data(); }
In main you could write
int *vptr = a.give();

Answer 2

You have several big problems here.
First, in the constructor you're trying to access elements of the vector that don't yet exist:
A(){
for(int i = 0; i < 5; i++)
v[i] = i;
}
vector isn't the same as map where if the element you are looking for with operator[] doesn't exist a new one is created. In vector if you use operator[] with an index that doesn't exist you get Undefined Behavior, and a verly likely crash. You should be inserting items here. On way is push_back:
A(){
for (int i = 0; i < 5; ++i)
v.push_back (i);
}
Next, give() returns a vector by-value:
std::vector<int> give(){
return v;
}
Which makes a copy of v and returns that copy. Well that's technically OK, but when you call give() you don't assign the return value to a variable. What ends up happening is the copy of the vector becomes a temporary object. As soon as the so-called "full-expression" where the temporary was created is done, the temporary is destroyed.
That full-expression is this:
vptr = &a.give()[0];
So, vptr was assigned the address of the first element of that temporary vector, but as soon as the end of this expression is reached that vector is destroyed and vptr now points to hyperspace. If you attempt to dereference this wild pointer, you get Undefined Behavior and a very likely segmentation fault.
To fix this, you could let A tell you the address of the first element of the vector:
class A
{
// [...]
public:
void* give() { return v.data(); }
};