Why is this swap method not working?

Question

Why does the below two snippets of code have different results? I want to add a 1 in front of digits, which is a vector of integers. But the second snippet does not properly swap.

int tmpInt(1); for (int i=0; i<digits.size(); i++){ swap(tmpInt, digits[i]); } digits.push_back(tmpInt);

versus:

int tmpInt(1); for (auto it : digits){ swap(tmpInt, it); } digits.push_back(tmpInt);

Rhetorical: int x = 6; int y = x; y = 4; // why is x still 6 instead of 4???? — Stack Exchange Broke The Law
– Stack Exchange Broke The Law, Commented Mar 2, 2017 at 2:40
I think "it" is not an iterator so maybe you want to change the name — Petar Petrovic
– Petar Petrovic, Commented Mar 2, 2017 at 3:42

Sam Varshavchik · Accepted Answer · 2017-03-02 02:41:29Z

for (auto it : digits){

The range variable gets essentially copied by value, from the sequence, so

 swap(tmpInt, it);

all this is doing is swapping between tmpInt and a temporary range variable.

You need to use a reference, in order to get equivalent results with the first example:

for (auto &it : digits){ swap(tmpInt, it);

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