C++ I need to understand where to use pointers and double pointers - c++

Question

I have this basic multi tool program which goal is to complete four functions on a file that contains some strings.. take a string, for example, take a line and put it in uppercase.
I get that it's not perfect yet, but i need to understand why my char* and char** are doing some mess.
thanks a lot for your time
#include <iostream> //to use enter and exit
#include <cstring>
#include <string>
#include <fstream> //to use files
using namespace std;
/*--------------------------FONCTION PART-----------------------------------*/
//define one fonction for each transformation
//FONCTION 1
void fonctionu (char* argv){
char pattern = argv[2];
char file = argv[3];
fstream data(file, ios::in);
if (data){
string line;
while (getline (data ,line)){
unsigned found = line.find(pattern);
if (found != string::npos) {
for (int i = 0; line[i]!='\0'; i++) {
if(line[i] >= 'a' && line[i] <= 'z') {
line[i] = line[i] - (32);// (ASCII)
cout<<line[i];
}} }
if (found == string::npos) {
for (int i = 0; line[i]!='\0'; i++) {
cout<<line[i];
}}}}}
//FONCTION 2
void fonctiond( char* argv){ //remove line with xyz
char pattern = argv[2];
char file = argv[3];
fstream data(file, ios::in);
if (data){
string line;
while (getline (data ,line)){
unsigned found = line.find(pattern); //as a reminder argv[2] is the pattern (warning : it need to be an unsigned to compare two unsigned)
if (found != string::npos) {
//delete the line of the file when there is the pattern
cout<<'\0'; }
if (found == string::npos){
for (int i = 0; line[i]!='\0'; i++) {
cout<< line[i];
}}}}}
//FONCTION 3
void fonctionc( char* argv){
char pattern = argv[2];
char file = argv[3];
fstream data(file, ios::in);
if (data ){
string line;
while (getline (data ,line)){
unsigned found = line.find(pattern); //as a reminder argv[2] is the pattern
if (found != string::npos) {
for (int i = 0; line[i]!='\0'; i++) {
cout<< "\033[31m"<< line[i]; //the line will be red
}
}
if (found == string::npos){
for (int i = 0; line[i]!='\0'; i++) {
cout<< line[i];
}}}}}
//FONCTION 4
//replace the pattern xyz by abc
void fonctions ( char* argv){
char pattern = argv[2];
char file = argv[3];
fstream data(file, ios::in);
if (data ){
string line;
while (getline (data ,line)){
unsigned found = line.find(pattern);
if (found != string::npos) {
for (int i = 0; line[i]!='\0'; i++) {
if(line[i] >= 'a' && line[i] <= 'z') {
line[i] = line[i] - (97); // (ASCII) //we creat a shift that allow to make the x became a 'a' and the y a 'b'...etc
cout<<line[i];}}
}
if (found == string::npos) {
for (int i = 0; line[i]!='\0'; i++) {
cout<<line[i];
}}}}}
/*--------------------------MAIN PART---------------------------------------*/
int main(char* argv){
// ipsacs : argv[0]
//option :
string a = argv[1]; //string ! to compare line 103,108 and 112
// pattern : argv[2];
//file : argv[3];
if (argv[1]=='\0' || argv[2]=='\0'){
cout <<"ERROR"<<'\n';}
if (a == "u"){
char fonctionu (argv);}
if (a== "d"){
char fonctiond(argv);}
if (a == "c"){
char fonctionc(argv);}
if (a == "s"){
char fonctions(argv);}
return 0;
}
/*--------------------------TEST PART--------------------------------------*/
/*
pour compiler : g++ -Wall ipsacs.cpp -o ipsacs
./ipsacs u xyz foo //how the program would receive arguments, basically
Le programme ne renvoie rien.
*/
here is the compiling log:
main.cpp:22:30: error: invalid conversion from ‘char’ to ‘const char*’ [-fpermissive]
fstream data(file, ios::in);
^
In file included from main.cpp:8:0:
/usr/include/c++/6/fstream:902:7: note: initializing argument 1 of ‘std::basic_fstream<_CharT, _Traits>::basic_fstream(const char*, std::ios_base::openmode) [with _CharT = char; _Traits = std::char_traits; std::ios_base::openmode = std::_Ios_Openmode]’
basic_fstream(const char* __s,
^~~~~~~~~~~~~
main.cpp: In function ‘void fonctiond(char*)’:
main.cpp:46:29: error: invalid conversion from ‘char’ to ‘const char*’ [-fpermissive]
fstream data(file, ios::in);
^
In file included from main.cpp:8:0:
/usr/include/c++/6/fstream:902:7: note: initializing argument 1 of ‘std::basic_fstream<_CharT, _Traits>::basic_fstream(const char*, std::ios_base::openmode) [with _CharT = char; _Traits = std::char_traits; std::ios_base::openmode = std::_Ios_Openmode]’
basic_fstream(const char* __s,
^~~~~~~~~~~~~
main.cpp: In function ‘void fonctionc(char*)’:
main.cpp:66:29: error: invalid conversion from ‘char’ to ‘const char*’ [-fpermissive]
fstream data(file, ios::in);
^
In file included from main.cpp:8:0:
/usr/include/c++/6/fstream:902:7: note: initializing argument 1 of ‘std::basic_fstream<_CharT, _Traits>::basic_fstream(const char*, std::ios_base::openmode) [with _CharT = char; _Traits = std::char_traits; std::ios_base::openmode = std::_Ios_Openmode]’
basic_fstream(const char* __s,
^~~~~~~~~~~~~
main.cpp: In function ‘void fonctions(char*)’:
main.cpp:88:29: error: invalid conversion from ‘char’ to ‘const char*’ [-fpermissive]
fstream data(file, ios::in);
^
In file included from main.cpp:8:0:
/usr/include/c++/6/fstream:902:7: note: initializing argument 1 of ‘std::basic_fstream<_CharT, _Traits>::basic_fstream(const char*, std::ios_base::openmode) [with _CharT = char; _Traits = std::char_traits; std::ios_base::openmode = std::_Ios_Openmode]’
basic_fstream(const char* __s,
^~~~~~~~~~~~~
main.cpp: At global scope:
main.cpp:109:5: warning: first argument of ‘int main(char*)’ should be ‘int’ [-Wmain]
int main(char* argv){
^~~~
main.cpp:109:5: warning: ‘int main(char*)’ takes only zero or two arguments [-Wmain]
main.cpp: In function ‘int main(char*)’:
main.cpp:113:20: error: conversion from ‘char’ to non-scalar type ‘std::string {aka std::basic_string}’ requested
string a = argv[1]; //string ! to compare line 103,108 and 112
~~~~~~^
main.cpp:121:26: error: invalid conversion from ‘char*’ to ‘char’ [-fpermissive]
char fonctionu (argv);}
^
main.cpp:123:25: error: invalid conversion from ‘char*’ to ‘char’ [-fpermissive]
char fonctiond(argv);}
^
main.cpp:125:27: error: invalid conversion from ‘char*’ to ‘char’ [-fpermissive]
char fonctionc(argv);}
^
main.cpp:127:27: error: invalid conversion from ‘char*’ to ‘char’ [-fpermissive]
char fonctions(argv);}
^

Answer 1

A char * is a pointer to a memory address holding a character, and is often used for c-strings.
A char ** is a pointer to a memory address holding a pointer to a memory address holding a character, and is often used for arrays of c-strings.
Your main signature if not a valid form, since there's no form of main which takes in only a character pointer. The form you are most likely looking for is int main( int argc, int **argv ). This takes in two parameters: argc being the number of arguments passed to the program, and argv containing the parameters, as c-strings, passed to the program.
In the main method, you would generally ensure that argc is correct, i.e. you have the correct number of parameters passed into your function (note: it's always at least 1 will argv[0] being the program name).
std::string a = argv[1] is fine, and it will cause the string pointed to by argv to be converted to a string. When calling your functions, you have char functionu(argv);. You will want to remove the char from that line; effectively, the way it is written, you are trying to create a char variable named functionu which has an initial value of argv.
Each of your functions, instead of taking in a char * should take in a char ** instead, then when you have something like char file = argv[3];, you would want to change this to char *file = argv[3];, or std::string file = argv[3];