Reusing the same string before inserting to a list seems to pass by value?

list<string> l;
string s;

getline(cin, s);
l.push_back(s);
getline(cin, s);
l.push_back(s);

开发者_Go百科

Using input

123
test

Printing the list 'l' results:

123
test

Shouldn't this print:

test
test

I'm a little confused as to why the string is being passed by value.

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push_back copies the string object s passed into it. So wahetever modification you do to s is not affecting the pushed back value.

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It shouldn't, because the string is copied when you add it to the list l. string argument is passed by reference to getline:

istream& getline ( istream& is, string& str );

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Your title is misleading; you don't have a loop anywhere in the code that you posted.

The string is being passed by value to push_back, which makes a copy in the list. The results you're getting are exactly what I would expect.

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Pseudocode:

void list::push_back(string& s)
{
   list_.add(new string(s));   // copy string
}

You see, the string is copied in push_back method. It is passed by reference just to avoid a redundant copying.

void list::push_back(string s) // copy string
{
   list_.add(new string(s));   // copy string
}