C++ read file from stdin

I've started to learn c++ and I need a way to read a file line by line directly from stdin. Now what I've tried is this:

freopen("input.txt", "r", stdin);
string buffer;
vector<int> temp = {};
getline(cin, buffer);
for(int elem : buffer) 
  if(elem != ' ')
    temp.push_back(elem - '0');
*dest = temp;

but when I run this code, the console waits for me to insert data via cli instead of reading it from stdin.
What can I do?

Comments

[Paul Melekhov]

When you didn't define any file as input or pipe from another app, keyboard input is your stdin.

[Sylvie Fiquet]

I'm not super familiar with modern C++ but I believe stdin's purpose is for getting input from the console/terminal. So I don't understand why you would read a file from stdin and in any case why you would redefine stdin. Maybe you're supposed to type something like this on the CLI:

myprogram < input.txt

so that file input.txt ends up in the stdin for the program? That's all I can think of.