different insert position with stringstream

Question

I struggle to understand the different behaviours of stringstream from the following code. Can someone shed a light on how stream internally works?

int main(){
    string str = "123";  
    stringstream ss(str);
    cout << ss.str() <<endl;  
    ss << str;
    cout << ss.str() <<endl;
} 

output:

123

123

int main(){
    string str = "123";  
    stringstream ss;
    ss << str;
    cout << ss.str() <<endl;  
    ss << str;
    cout << ss.str() <<endl;
} 

output:

123

123123

Answer 1

It's because the actual write position isn't updated, so when you in the first example do

ss << str;

you overwrite the current string.

Use the flag std::ios_base::ate to the constructor of the string stream

std::stringstream ss(str,
    std::ios_base::in | std::ios_base::out | std::ios_base::ate);

to position the read/write pointers at the end.

See the example in this reference.

And it's like that for all streams, when you open a file stream the positions are also at the start.