Char pointers and strings through ifstream

Question

So I have this recursive descent parser that works fine recognizing and using values through command line arguments but I am not sure how to port this to reading from a .dat file, using the proper char pointer, printing the string, and working for multiple strings.

Parser using command line args:

#include <iostream>
#include <fstream>

using namespace std;

bool A(void);
bool E(void);
bool T(void);
bool F(void);
bool P(void);
bool I(void);
bool L(void);

char *c;

int main(int argc, char *argv[]){

    c = argc == 2 ? argv[1] : (char *)"";

    if (A() && *c == '\0') {
        cout << "The string \"" << argv[1] << "\" is in the language." << endl;
    }
    else {
        cout << "The string \"" << argv[1] << "\" is not in the language." << endl;
    }

    return 0;
}

bool A(void){

    if( I() )
    {
        if ( *c == '=' ){
            ++c;
            if ( E() )
            return true;
        }
    }
    return false;
}

bool E(void){

    if( T() ){
        if ( *c == '+' || *c == '-' ){
                ++c;
                return E();
        }
        return true;
    }
    return false;
}

bool F(void){

    if( P() ){
        if( *c == '^'){
            ++c;
            return F();
        }
    return true;
    }
    return false;
}

bool I(void){

    if ( *c >= 'a' && *c <= 'z'){
        ++c;
        return true;
    }
    return false;
}

bool L(void){

    if ( *c >= '0' && *c <= '9' ){
        ++c;
        return true;
    }
    return false;
}

bool P(void){

    if ( I() ){
        return true;
    }
    else
    if ( L() ){
        return true;
    }
    else
    if ( *c == '(' ){
            ++c;
            if ( E() ){
                    if ( *c == ')' ){
                        ++c;
                        return true;
                    }
            }
    }
    return false;
}

bool T(void){

    if( F() ){
        if ( *c == '*' || *c == '/' ){
                ++c;
                return T();
        }
        return true;
    }
    return false;
}

I don't know what I can replace argv[1] with to print the string. To get the proper char pointer could I just do this?

ifstream fin("input.dat");
        while (fin >> *c)

when I try that I get segmentation fault.

Answer 1

You are asking the stream to store a value into what is represented by the value that c points to. Instead of that, consider:

char ch;
char *c = &ch;

ifstream fin("input.dat");
while (fin >> ch) {
    // whatever
}

This would not segfault immediately. However, bad things would happen on the "next" iteration, as you change the value of c inside the parsing functions.

You need to restructure the program and abstract the intention of reading the next character from the method of reading the next character (by incrementing a pointer, or reading from a stream, or whatever).

Or you could also read the file into one big memory buffer and then proceed as if that buffer was a string passed from the command line.

Answer 2

Assuming your file is a text file with one entry to parse per line you could do this:

ifstream fin("input.dat")
if (fin.is_open())
{
    std::string line;
    while (fin.good())
    {
       getline(fin, line);
       c = line.c_str();
       ... go and parse c
    }
}

Update : apparently c is non-const, so using c_str() won't work just like that. You have two options:

1. Change your parser to work with std::string (use an index to point at the current character).
2. Allocate some memory and copy the c_str() in there.

Number 1 is probably the better way.