Using a string to open a file..

Well let's say i have :

Code:

---


#include <iostream>
#include <fstream>
#include <string>

int main(){

string filename;

cout<<"Give the name of the file you want to write"<<endl;
cin>>filename;

ofstream filepointer(filename);

return 0;
}

---

It doesn't work.. But how can i use the string to be the filename.. ?

use filename.c_str().

great, another thing is from what i know you can declare variables anywhere you want in c++ but i get a compile error when i try ..

e.g :

Code:

---

int main(){
int choice=0;

cout<<"Give a number from 1 to 3"<<endl;
cin>>choice;

 switch(choice){
 case 1:
  break;
 case 2:
  break;
 case 3:
  string whatever;  //I get an error here
  cin>>whatever;
  cout<<"You wrote"<<whatever<<endl;
  break;

 }
return 0;
}

---

but when i put it in separate brackets like

Code:

---

{
string whatever;
cin>>whatever;
cout<<"You wrote"<<whatever<<endl;
}

---

it works !

> string whatever; //I get an error here
Well paste your error message.

Since the whatever variable has scope throughout the switch statement (as you've written it), it could be possible that code could access the variable without the variable being declared/initialized due to the code responsible for doing such being skipped as follows:

Code:

---

int value = 2;
switch( value )
{
case 1:
    int foo = 5;
    break;
case 2:
    cout << foo << endl;
    break;
default:
    break;
}

---

The variable foo in the above switch statement is declared within the scope of the switch statement but it is possible that (as demonstrated above) that the int foo = 5; line of code can be skipped which would mean that foo was uninitialized/undeclared at the time we reach the cout statement. I know your code doesn't do anything like that (at least the small bit you've decided to share with us) but the compiler probably isn't smart enough to know that and wants you to make sure you understand what could happen and ensure you do something that in no uncertain term keeps that possibility from ever being realized. You can do that in two ways:

1) The first way you've already discovered is to create the variable in a seperate block of code within brackets {}. This works because now there is no possible way you could potentially use that whatever variable outside of that block.

2) The second way to resolve this would be to declare the whatever variable outside of the switch statment so that any potential use of said variable within any of the case's occurs with that variable fully declared and in scope.

Code:

---

string whatever;
switch(choice)
{
case 1:
    break;
case 2:
    break;
case 3:
    cin>>whatever;
    cout<<"You wrote"<<whatever<<endl;
    break;
}

---

I get this error without the brackets... I get it even if i declare the string outside the switch..

Code:

---

main.cpp: In function ‘int main(int, char**)’:
main.cpp:370: error: jump to case label
main.cpp:365: error:  crosses initialization of ‘std::ofstream outputf’
main.cpp:378: error: jump to case label
main.cpp:365: error:  crosses initialization of ‘std::ofstream outputf’
main.cpp:383: error: jump to case label
main.cpp:365: error:  crosses initialization of ‘std::ofstream outputf’
make: *** [cplus] Error 1

---

This is the part of the program i get the errors ..

Code:

---

  case 5:    //LINE 360
        string filename;
        cout<<"\033[2J\033[1;1H";
        cout<<"Give the name of the file you want the lists to be saved : ";
        cin>>filename;
        ofstream outputf(filename.c_str());  //LINE 365
        fifolist.save_list(outputf);
        lifolist.save_list(outputf);
        orderlist.save_list(outputf); 
      break;
    case 6:          //LINE 370
      cout<<"\033[2J\033[1;1H";
      cout<<fifolist;
      cout<<lifolist;
      cout<<orderlist;
      cout<<"Press ENTER to continue..";
      while(cin.get()!='\n');
      break;
    case 7:                //LINE 378
      cout<<"\033[2J\033[1;1H";
      cout<<"Press ENTER to exit..";
      while(cin.get()!='\n'); //i need to add exit(-1) after this line
      break;
    default:                //LINE 383
      break;    //LINE 384

---

If you use the second solution, you have to declare all your variables outside the switch, including outputf.

I think the first solution is better, though. If you only need the variables to exist for a single case, you should limit their scope with the exrta brackets.

A third solution is to create a separate function for each complicated case. Opening an ofstream is complicated enough IMO to warrant a separate function. Then you can just call the function from your case without extra brackets.

you are right i had to put both ofstream and string declaration outside the switch statement to work...i guess the solution with the brackets is better but at least now i know what was wrong :)

thanks guys