Argument checking - c++

Question

#include <iostream>
#include <fstream>
#include <string>
using namespace std;
int main (int argc, char* argv[])
{
string STRING;
ifstream infile;
STRING = argv[1];
infile.open(argv[1]);
if (infile.fail())// covers a miss spelling of a fail name
{
cout << "ERROR. Did you make a mistake in the Spelling of the File\n";
return 1;
}
else
{
while(!infile.eof())
{
getline(infile,STRING); // Get the line
cout<<STRING + "\n"; // Prints out File line
}
infile.close();
return 0;
}
}
I have got this program working fine apart from one problem
if the user only runs the program with no file name (what I believe to be called arguments) e.g ./displayfile then I get a Segmentation fault
How would I amend my code so that the program would exit with an error message along the lines of "Add a file name"
My first thought is something along the lines of
if (!argc=2)
{
cout << "ERROR. Enter a file name";
return 1;
}
ADDED:
just in case this matters am compiling using
g++ displayfile.cpp -o displayfile

Answer 1

#include <iostream>
#include <fstream>
#include <string>
using namespace std;
int main (int argc, char* argv[]) {
if(argc != 2) {
cout << "You need to supply one argument to this program.";
return -1;
}
string STRING;
ifstream infile;
STRING = argv[1];
infile.open(argv[1]);
if (infile.fail())// covers a miss spelling of a fail name {
cout << "ERROR. Did you make a mistake in the Spelling of the File\n";
return 1;
}
else {
while(!infile.eof()) {
getline(infile,STRING); // Get the line
cout<<STRING + "\n"; // Prints out File line
}
infile.close();
return 0;
}
}

Answer 2

Besides the obvious check for argc != 2 I couldn't help fixing some of the worse code and the obvious error:
#include <iostream>
#include <fstream>
#include <string>
using namespace std;
int main (int argc, char* argv[])
{
if (argc != 2)
{
cout << "ERROR. Invalid number of arguments\n";
return 1;
}
ifstream infile(argv[1]);
if (!infile) // covers a miss spelling of a fail name
{
cout << "ERROR. Did you make a mistake in the Spelling of the File\n";
return 1;
}
string STRING;
while(getline(infile, STRING))
cout << STRING << '\n'; // Prints out file line
return 0;
}
You don't need to call ifstream::open, just use the constructor, likewise don't you need to declare STRING so early and neither to initialize it to the file name, since you don't use it. Don't forget, this is not C, you don't need a whole mess of declarations at the beginning of each function.
Second, checking for the flags of a stream is often a bad idea, just check for !infile to find any errors. But the real error is in checking for infile.eof in the while condition, since it only gets set once getline has tryed to read over the end of the file, so you would actually print one (probably empty) line too much. Just check for getline's return value to find any errors or the end of file.
And don't add the newline onto the string when outputting, just put it out after the string. And last but not least, no need for infile.close, since the destructor calls it anyway.

Answer 3

change STRING = argv[1]; to if (argv[1] != null) STRING = argv[1]; not sure though, so you'll have to test it out first.