Running a program from command prompt and using argv in C++

I have written a program that takes the filename from argv[1] and do operations on it . When debugging from visual studio I pass the filename from project options>>debugging>>command arguments and It works fine and prints all results correctly .

But when trying from the command prompt , I go to the dir of project/debug the I type

program

It works fine and prints "No valid input file" in the same window (Which is my error handling technique)

but when i type

program test.txt

It just does nothing . I think no problem in code because it works fine from the debugger .

Code :

int main(int argc, char *argv[]) 
 { 
int nLines;
string str;

if(argv[1]==NULL)
{
    std::cout << "Not valid input file" << endl;
    return 0 ;

}
ifstream infile(argv[1]);

getline(infile,str);
nLines = atoi(str.c_str());//get number of lines

for(int line=0 ;line < nLines;line++)
{
    //int currTime , and a lot of variables ..
            //do a lot of stuff and while loops
      开发者_如何学Go    cout << currTime <<endl ;

}
    return 0 ;
    }

You don't check if file was successfully opened, whether getline returned error code or not, or if string to integer conversion didn't fail. If any of those error occur, which I guess is the case, nLines will be equal to 0, no cycles will be performed and program will exit with return code 0.

This code worked correctly for me running on the command line.

#include <string>
#include <algorithm>
#include <functional>
#include <vector>
#include <iostream>
using namespace std;

int main(int argc, char *argv[]) 
{ 
    int nLines;
    string str;

    if(argv[1]==NULL)
    {
        std::cout << "Not valid input file" << endl;
        return 0 ;

    }
    else
        std::cout << "Input file = " << argv[1] << endl;
}

Output :

C:\Users\john.dibling\Documents\Visual Studio 2008\Projects\hacks_vc9\x64\Debug>hacks_vc9.exe hello
Input file = hello

By the way, this code is dangerous, at best:

if(argv[1]==NULL)

You should probably be checking the value of argc before attempting to dereference a possibly-wild pointer.

The file probably contains an invalid numeric first line (perhaps starting with a space or the BOM).

That would explain no output, since if nLines == 0 no output should be expected