Opening a file through argv[1] C++ Question

Question

Hi,

I have check to see if the argument in argv[1] is valid for my file.

i have a menu system in its own sub-routine called in main.

Reading the file and so on is done in another sub-routine sepearte from the first, also called in main.

How can i transfer the argv[1] to the first sub-routine? Or even the second?

string sArgInit = argv[1]

This way i can open the file using a C-String.

But i cant get the string to any function outside of main..

is there a way to do this without: global varaibles, passing the string as an argument paramter to the sub-routine.

Answer 1

const char *filename = argv[1];

Then pass the pointer to your subroutine. argv[1] remains valid for the whole execution of the program.

Answer 2

First of all, you may want to check that argv[1] is not null before you try to assign it to the C++ string.

You can move the C++ string object around to functions.

Eventually, you can use c_str() to make a C string

For example: char* sData = (char*)str.c_str()

Answer 3

The following code shows how to do exactly what you want, checking that argv[1] exists before passing it to functions as C char-pointers or C++ strings and using the value within that function.

using namespace std;
#include <iostream>
#include <string>

static void f1 (char *s) {
    cout << "1: " << s << endl;
}

static void f2 (string *s) {
    cout << "2: " << (*s) << endl;
    cout << "3: " << s->c_str() << endl;
}

int main (int argc, char *argv[]) {
    if (argc < 2) {
        cout << "Usage: " << argv[0] << " <test_string>" << endl;
    }

    char *s1 = argv[1];
    string *s2 = new string (argv[1]);

    f1 (s1);
    f2 (s2);

    return 0;
}

The output is, as expected:

1: hello
2: hello
3: hello

As for your edit, you cannot access argc/argv without storing them in a global or passing them to a function. That's because they're passed to the main function as arguments so inherently local to that function.

Answer 4

is there a way to do this without: global varaibles, passing the string as an argument paramter to the sub-routine.

No, you have to get it to the routine somehow.

What is wrong with passing it as a parameter?

void my_sub( char* s){}

void main(int argc, char* argv[])
{
    my_sub( argv[1] );
}

Answer 5

You would have to pass the argument as a parameter to the sub-routines.

void method1(char* filename) {
  // ..
}

void method2(char* filename) {
  // ...
}

void main(int argc, char* argv[]) {
  method1(argv[1]);
  method2(argv[1]);
}

I'm not sure if this answers your question, so please expand.

Answer 6

class myprog {
public:
    myprog(const string& filename) : m_filename(filename) {}

    void menufunc();
    void otherfunc();
private:
    string m_filename;
};

int main(int argc, char** argv)
{
    myprog prog(argv[1]);

    prog.menufunc();
    prog.otherfunc();
    return 0;
}