c++ : char* invariably stores the word vector

This is although a code specific question but the output is quite bizarre.

I am aware of STL string etc. I was fooling around when I noticed something strange, and could not find a reason for it. :(

See the Two Codes below and the output.

[Code #1] (https://ideone.com/ydB8sQ)

#include <iostream>
#include <vector>
#include <cstdlib>
#include <cstdio>

using namespace std;


class str
{

private:
    vector<char> A;

public:

    str(const char *S) {

        int sz = sizeof(S);

        cerr << sz << endl;

        for (int i = 0; i < sz; ++i) {
            cout << S[i];
            //A.push_back(S[i]);   //!-- Comment --!//
        }
    }
};

int main(int argc, char const *argv[])
{
    str A("");

    return 0;
}

In this, An Empty String is passed and is printed. The Vector A does nothing but is relevant to this problem. In the first version, A is untouched, and the code prints garbage value. (see ideone O/P)

In this second version ( see A.push_back is now uncommented )

[Code #2] (https://ideone.com/PPHGZy)

#include <iostream>
#include <vector>
#include <cstdlib>
#include <cstdio>

using namespace std;


class str
{

private:
    vector<char> A;

public:

    str(const char *S) {

        int sz = sizeof(S);

        cerr << sz << endl;

        for (int i = 0; i < sz; ++i) {
            cout << S[i];
            A.push_back(S[i]);
        }
    }
};

int main(int argc, char const *argv[])
{
    str A("G");

    return 0;
}

The Output is :

Gvector

This is across GCC / MinGW x64. This one never prints garbage value but always contains the word 'vector'.

Where is the char* in the function pointing to?
Why would 'vector' be there anyways?
Also, the size of char * is 8.

EDIT : This does not happen if it isn't wrapped around a 'class'.

The word 'vector' appears always. I supposed it was random garbage value but then how come ideone still has the same word in its memory?

Solution

The main problem in your code is in line int sz = sizeof(S);. sizeof(S) is always equal to sizeof(char *) which seems to be 8 on your system. sizeof gives you number of bytes for variable itself. If you want to know number of bytes in string to which your char pointer points, you should use strlen function instead.

You get that vector string in output randomly, as you are accessing memory which is not in allocated space. Accessing such memory is undefined behavior, so you get your undefined result.