Pipe an input to C++ cin from Bash - c++

Question

I'm trying to write a simple Bash script to compile my C++ code, in this case it's a very simple program that just reads input into a vector and then prints the content of the vector.
C++ code:
#include <string>
#include <iostream>
#include <vector>
using namespace std;
int main()
{
vector<string> v;
string s;
while (cin >> s)
v.push_back(s);
for (int i = 0; i != v.size(); ++i)
cout << v[i] << endl;
}
Bash script run.sh:
#! /bin/bash
g++ main.cpp > output.txt
So that compiles my C++ code and creates a.out and output.txt (which is empty because there is no input). I tried a few variations using "input.txt <" with no luck. I'm not sure how to pipe my input file (just short list of a few random words) to cin of my c++ program.

Answer 1

You have to first compile the program to create an executable. Then, you run the executable. Unlike a scripting language's interpreter, g++ does not interpret the source file, but compiles the source to create binary images.
#! /bin/bash
g++ main.cpp
./a.out < "input.txt" > "output.txt"

Answer 2

g++ main.cpp compiles it, the compiled program is then called 'a.out' (g++'s default output name). But why are you getting the output of the compiler?
I think what you want to do is something like this:
#! /bin/bash
# Compile to a.out
g++ main.cpp -o a.out
# Then run the program with input.txt redirected
# to stdin and the stdout redirected to output.txt
./a.out < input.txt > output.txt
Also as Lee Avital suggested to properly pipe an input from the file:
cat input.txt | ./a.out > output.txt
The first just redirects, not technically piping. You may like to read David Oneill's explanation here: https://askubuntu.com/questions/172982/what-is-the-difference-between-redirection-and-pipe