Cannot read in string from string class

Question

Just a short question and I will be on my way. Why am I getting errors when I try to read in string using the string class? The error I receive is: "no operator ">>" matches these operands/operand types are std::istream >> std::string *"

I am starting to use the class instead of cStrings. Unfortunately, I keep getting errors when I try to read in a string using cin. For example:

void ProcessEditMenuItems( int menu_choice, std::string fname[], std::string lname[], 
                    std::string phone[], std::string bday[]     /*char fname[][NAME_LENGTH], char lname[][NAME_LENGTH], char phone[][NAME_LENGTH], 
                       char bday[][NAME_LENGTH]*/ )
{
switch ( menu_choice )
{
    case 1:
        std::cout << "\nEnter new first name: ";
        std::cin >> fname;
    break;
    case 2:
        std::cout << "\nEnter new last name: ";
        std::cin >> lname;
    break;
    case 3:
        std::cout << "\nEnter new phone number: ";
        std::cin >> phone;
    break;
    case 4:
        std::cout << "\nEnter new birthday: ";
        std::cin >> bday;
    break;
}
}

The above code works for normal cStrings, but when I change the function declaration and definition to the string class it does not work.

i think you need to learn more. string is something like a char array. so string fname would be something like a char fname[19]. so no need of [] for using string in your case. use string fname[] when you need an array of strings!!! :) — Deamonpog
– Deamonpog, Commented Feb 12, 2013 at 17:11
I do need an array of strings. Hence why I was using string fname []. If you look at the commented out code, you will see I am trying to replace char fname[][NAME_LENGTH] -- a two dimensional array. — MrPickle5
– MrPickle5, Commented Feb 12, 2013 at 17:27
oh, then cin >> fname[i] will put the input to the 'i'th place of the fname array. You cannot do cin >> fname because fname is not a string but an array of strings. (operator >> of std::cin doesn't know what to do when the Right Hand Side of the >> operator is an array of strings!! so it should give compilation error.) Still you need to work on how you pass the parameters. i suggest passing them in as pointers or references. — Deamonpog
– Deamonpog, Commented Feb 12, 2013 at 17:33
@Deamonpog Thank you very much for the reply. So I change my function header to include "std::string& fname[]", correct? — MrPickle5
– MrPickle5, Commented Feb 12, 2013 at 17:39

Joseph Mansfield · Accepted Answer · 2013-02-12 17:07:17Z

3

Your arguments fname, lname, phone, and bday are declared as std::string arg[] which for an argument is transformed to std::string* arg. That is, they are pointers to std::string. You would only have an argument declared with std::string arg[] if you want to pass an array of std::strings.

If you're passing in a single std::string for each argument, then what you actually want is a reference type. For example, fname should be std::string& fname. The reference allows a std::string to be passed into the function without copying, so that when you read into it with >> you modify the original object.

answered Feb 12, 2013 at 17:07

Joseph Mansfield

111k22 gold badges249 silver badges329 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

ereOn · Accepted Answer · 2013-02-12 17:13:57Z

2

You are actually passing arrays of strings, but should pass non-const references to strings.

Change your function prototype to:

void ProcessEditMenuItems(
  int menu_choice,
  std::string& fname,
  std::string& lname,
  std::string& phone,
  std::string& bday
)

And it should be fine.

When converting from C-strings to std::strings, you may use the following conversion table:

Old parameter | New parameter      | Description
const char*   | const std::string& | Non-modifiable parameter
char*         | std::string&       | Modifiable string

The nicest thing being that you don't need to care about the string size when changing it while you would have to do with a char*. Nor do you have to handle deletion/reallocation if a size-increase was needed.

edited Feb 12, 2013 at 17:13

answered Feb 12, 2013 at 17:07

ereOn

56.4k42 gold badges174 silver badges251 bronze badges

Comments

simonc · Accepted Answer · 2013-02-12 17:06:38Z

1

fname is an array of std::string. Is the use of arrays a hangover from your earlier use of C strings - char fname[]? If so, you could change to passing std::string& instead

void ProcessEditMenuItems(int menu_choice,
                          std::string& fname,
                          std::string& lname,
                          std::string& phone,
                          std::string& bday)

answered Feb 12, 2013 at 17:06

simonc

42.3k12 gold badges91 silver badges105 bronze badges

2 Comments

MrPickle5 Over a year ago

If you look at the commented out code, the original cString was a two dimensional array (i.e. char fname[MAX_FRIENDS][NAME_LENGTH]). Does std::string& fname still work for this?

simonc Over a year ago

Ah, no, it won't work. If you want to have an array of strings, you'd have decide which index to use then stream into fname[index]. Or you could pass std::vector<std::string>& and push_back a new string to it if you want to add to the end of a variable array. (I'll change my answer if you confirm whether either option suits your needs.)

Shasha.Zhu · Accepted Answer · 2013-02-12 17:29:16Z

0

This error is similar to int arr[2]; arr = 1;
fname is an array which contains string objects. "std::cin >>" mean extract strings from the standard input. We can only assign string object to fname[0], fname[1] ....
try std::cin >> fname[0] instead;

answered Feb 12, 2013 at 17:29

Shasha.Zhu

3382 silver badges8 bronze badges

Collectives™ on Stack Overflow

Cannot read in string from string class

4 Answers 4

Comments

Comments

2 Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

4 Answers 4

Comments

Comments

2 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related