{Beginner}{G++} Having problems with some "char"s

[madmaxsantana]

I have written this program which demonstrates how to access files using C++. But when I compile it (using g++) I get a few errors which are out of my understanding. Please help me with it. I am not asking to debug, but to please explain what those errors mean and how would I remove them.
Here is the source code.

#include <iostream>
#include <cstdlib>
#include <cstdio>
#include <string>
#include <fstream>
using namespace std;


const char NULLSTRING = ' ';

char filename[255] = NULLSTRING;
char text[16384]=NULLSTRING;
void Prompt();
void New();
void Existing(char*);
void Clear();
void Exit();


int main()
{
	Prompt();
	return 0;
}


void Clear()
{
	system("clear");
}

void Welcome()
{
	Clear();
	cout<<"Welcome."<<endl;
	cout<<"This is a demonstration of reading and writing to files using C++"<<endl;
	cout<<"For better understanding read the source code."<<endl;
	cout<<"Hit any key to launch the demo."<<endl;
	cout<<"\n..."<<endl;
	getchar();
	Prompt();
}

void Prompt()
{
	char choice;
	cout<<"Please choose from following:\n"<<endl;
	cout<<"1: Create New File"<<endl;
	cout<<"2: Display Contents of an Existing File"<<endl;
	cout<<"0: Quit program\n"<<endl;
	choice = getchar();
	switch (choice)
	{
	case '1':
		New();
	break;
	case '2':
		Existing(filename);
	break;
	case '0':
		Exit();
	break;
	default:
		cout<<"Wrong choice entered."<<endl;
		Prompt();
	break;
	}
}

void New()
{
	char readoption;
	Clear();
	cout<<"Please enter name of the file."<<endl;
	gets(filename);
	cout<<"Now you can enter text for the file."<<endl;
	cout<<"You text can include all characters except ~"<<endl;
	cout<<"When finished, type ~ and press Return."<<endl;
	cout<<"\n"<<endl;
	cout<<"~"<<endl;
	getline(text, 16384,'~');
	ofstream fout;
	fout.open(filename);
	fout<<text<<'~'<<endl;
	fout.close();
	Clear();
	cout<<"Do you want to read contents of your file now? (y/n)\t:";
	readoption=getchar();
	switch(readoption)
	{
	case 'y':
		Existing(filename);
	break;
	case 'n':
		filename = NULLSTRING;
		Prompt();
	break;
	default:
		cout<<"Invalid Option, Operation Cancelled, Back to Menu."<<endl;
		filename = NULLSTRING;
		Prompt();
	break;
	}
}

void Existing(char nameasarg)
{
	if (nameasarg = NULLSTRING)
	{
		cout<<"Please specify filename."<<endl;
		gets(filename);
		Existing(filename);
	}
	else
	{
		ifstream fin;
		fin.open(nameasarg);
		if(fin.fail())
		{
			cout<<"Sorry! This file doesn't exist. Check Spelling."<<endl;
			filename = NULLSTRING;
			Existing(filename);
		}
		else
		{
			getline(fin, text, '~');
			cout<<"Press any key to return to main menu."<<endl;
			getchar();
			Prompt();
		}
	}

}

void Exit()
{
	exit(0);
}

All of the errors are about char variables and conversions... For convenience I am also posting the O/P of G++.

main.cpp:11: error: array must be initialized with a brace-enclosed initializer
main.cpp:12: error: array must be initialized with a brace-enclosed initializer
main.cpp: In function ‘void New()’:
main.cpp:81: error: cannot convert ‘char*’ to ‘char**’ for argument ‘1’ to ‘__ssize_t getline(char**, size_t*, FILE*)’
main.cpp:95: error: incompatible types in assignment of ‘const char’ to ‘char [255]’
main.cpp:100: error: incompatible types in assignment of ‘const char’ to ‘char [255]’
main.cpp: In function ‘void Existing(char)’:
main.cpp:117: error: invalid conversion from ‘char’ to ‘const char*’
main.cpp:117: error: initializing argument 1 of ‘void std::basic_ifstream<_CharT, _Traits>::open(const char*, std::_Ios_Openmode) [with _CharT = char, _Traits = std::char_traits<char>]’
main.cpp:121: error: incompatible types in assignment of ‘const char’ to ‘char [255]’
main.cpp:126: error: invalid conversion from ‘void*’ to ‘char**’
main.cpp:126: error: cannot convert ‘char*’ to ‘size_t*’ for argument ‘2’ to ‘__ssize_t getline(char**, size_t*, FILE*)’

[Bazzy]

const char NULLSTRING = ' ';

char filename[255] = NULLSTRING;
char text[16384]=NULLSTRING;

NULLSTRING isn't a string, it's a single character

Line 81,126 getline(text, 16384,'~'); http://www.cplusplus.com/reference/iostream/istream/getline/

Line 95 filename = NULLSTRING, Line 100,121 filename = NULLSTRING;, NULLSTRING is a character, you can't use assignment with arrays

Line 117 fin.open(nameasarg); nameasarg is a single char

[madmaxsantana]

Ok... I got what the errors mean. But I am not able to manage a workaround. I have changed NULLSTRING to a string but it returns a similar error that conversion from string to character-array is not possible. (By the way what is the difference between a string and an array of characters?)

I also tries to change NULLSTRING form const char to char[16] but it says the arrays should be initialized inside braces. That cannot be done here since I want to keep the scope of NULLSTRING global... Enclosing it in braces would limit the scope.

Well I read the getline() manual but the problem is still the same. I don't get where am I doing the mistake... char* is a pointer I understand that. But what is a char**? I have seen this type of variable first time in the definition of getline().

The further errors in Line 95, 100, 121 are associated with the variable type of NULLSTRING and I have already mentioned what I have tried.

The last error, again, how do I use namesarg[] as an argument to the function?

Will be really thankful if you could help.

[Bazzy]

By the way what is the difference between a string and an array of characters?

This may be a point to clarify.
In C, strings are pointers to null-terminated sequences of characters ( character arrays ) Which are quite cumbersome to use.
http://www.cplusplus.com/doc/tutorial/ntcs/
C++ standard library defines std::string which is much more flexible
http://www.cplusplus.com/reference/string/string/
I would strongly recommend you to switch to std::strings, your code would look like this:

string filename; // you don't have to worry about the size
string text; // you don't need to initialize it to an empty string as it's done automatically

//...

cout<<"Please enter name of the file."<<endl;
getline(cin,filename);

//...

  case 'n':
    filename = "";
    Prompt();

//...

void Existing(const string &nameasarg) // by const reference = faster than by value with objects

//...

fin.open( nameasarg.c_str() ); // c_str retursn a C string from the std::string 

I also tries to change NULLSTRING form const char to char[16] but it says the arrays should be initialized inside braces. That cannot be done here since I want to keep the scope of NULLSTRING global... Enclosing it in braces would limit the scope.

The compiler means this:
char NULLSTRING[16] = { ' ' };

[madmaxsantana]

Hi there buddy... I tried the tricks you told me. I must tell you that strings have always freaked me out... I don't know most people consider them to be one of the easiest parts of C++ but since the time I learnt C++, I have been dreading strings. And the problem is, one way or the other, I always come across strings in my practice. So I really need to get over stringophobia very soon, LOLxxx.

Anyway, I did as you told. Then compiled it. Last time, before I had followed your latest advice, the compiler said I can't convert from char to char* in functions New() and Existing()... Now, as the char[] have been changed to string, the compiler is returning a similar error saying that I cannot convert from std::string to char*... I have changed the argument of Existing() to const string &var but it is still taking it as char*... I am not getting much of it. I don't want to annoy you actually, but would be glad if you could help me work it thoroughly. I have read the manuals from the links that you gave me but it's all about classes and objects, not that I don't know about them, but I need an overview of strings, not an interview, :) lolxxx.

Again apologizing if I am bothering you.

[Bazzy]

The best way would be changing everything to a string. You can ignore the const& thing if you want.

void Existing(string nameasarg);

string s = "foobar";
Existing ( s );

If you really want to have C string parameters, you can do like this:

void Existing(char *nameasarg);

string s = "foobar";
Existing ( s.c_str() );

[madmaxsantana]

Can #include string handle this or do I have to #include cstring ?

[Bazzy]

I think <string> already includes <cstring> but anyway, you don't need cstring

[PanGalactic]

Bazzy, you cannot rely on such include file dependencies. Header dependencies are implementation specific. For maximum portability, one should excplicitly include every header from which you use symbols.

[madmaxsantana]

Got it... I think structmem.c_str() works without #include <cstring> ... But I have included it anyway.