Passing contents of file as argument into C++ program - c++

Question

Using C++, need to pass the contents of a file to the program as an argument à la $ ./main <filename.txt or $ echo the contents of the file | ./main
I want to be able to access the contents of the file as a variable or something so I can manipulate further.
I have this basic listing including some of the stuff I've been trying. The data doesn't seem to be in argv (my runs always tell me I have one argument). What is the way to access a command line argument passed in this way?
#include <fstream>
#include <iostream>
int main(int argc, char **argv)
{
std::cout << "Have " << argc << " arguments:" << std::endl;
for (int i = 0; i < argc; ++i) {
std::cout << argv[i] << std::endl;
}
std::istream* input = &std::cin;
if(argc > 1) {
std::cout << input;
std::cout << "more than argument found\n";
}
std::string inputs;
std::cout<< "1:\n";
std::cout<< argv[0];
std::cout<< "\n2:\n";
std::cout<< argv[1];
std::cout<< "\n3:\n";
std::cout<< input;
std::cout<< "\nEnd\n";
return 0;
}
EDIT
I perhaps wasn't entirely clear, given it was a fairly fundamental question I was asking but didn't realize. Essentially, passing using $ ./main <filename.txt is non-negotiable. User RPGillespie's comment pointed me in the right direction. The pipe sends it to stdin (which I didn't realize). Thus I wanted to handle the data using cin. A minimal working example would be
int main() {
// read input
for (std::string line; std::getline(std::cin, line);) {
std::cout << line << std::endl;
}
return 0;
}

Answer 1

cat file | ./main will pipe the file content to the input of the ./main, not the argument.
If you want to pipe arguments in this way, use xargs:
cat file | xargs ./main

Answer 2

While #liliscent has a great solution, the following also works (I am mentioning it for the sake of completeness and since I feel it is more readable):
./main $(cat file)
This takes the contents of the file called file (since it interprets the command inside the $( and )), and passes them as arguments to ./main.