Implementing cat in Linux system

Question

This code was for implementing cat in Lin, and when I compiled it, it returned me "tp undefined". When I asked my professor, he said the way I used getline is in C language. I'm confused.

int main(int argc, char*argv[]) {
  FILE* file;
  for (int i = 1; i < argc; i++) {
    file = fopen(argv[i],"w");//

    if (file < 0) {
      perror("Error, Can't open file!");
      return -1;
    }

    if (file.is_open()) {
      string tp;
      while (getline(file, tp)) {  // c type
        printf("%d\n", tp);
      }
    }
  }
  fclose(file);
  return 0;
}

Answer 1

Problem:

1. printf is incorrectly formatted. %d is for signed integers. %s is for strings of characters (More info here). Additionally, if you want to work with printf() you will need a C string or to call the std::string::c_str() function.
2. You're doing things in C style (using FILE*, fopen(), etc).

Solution:

1. If you still want or need to use a C style, replace with printf("%d\n", tp); with printf("%s\n", tp.c_str());.
2. Use a C++ style instead:

• FILE* -> std::ifstream.
• fopen() -> std::ifstream::is_open().
• file < 0 -> std::ifstream::fail().
• perr -> std::cerr.
• printf() -> std::cout.
• fclose() -> std::ifstream::close().

Additional information:

1. using namespace std; is considered a bad practice (More info here).

Full code:

#include <iostream>
#include <fstream>

int main(int argc, char*argv[])
{
    for(int i = 1; i < argc; i++)
    {
        std::ifstream file(argv[i]);
        if(file.fail())
        {
            std::cerr << "Error opening file.\n";
            return 1;
        }
        std::string tp;
        while(std::getline(file,tp))
            std::cout << tp;
    }
    return 0;
}