Dereferencing vs Indexing on const string reference

Question

For const string reference variable str in the following piece of code:

Works well: for(int i=0; str[i]; i++)

Throws error: for(int i=0; *(str+i); i++)

Error: error: no match for 'operator+' (operand types are 'const string {aka const std::basic_string}' and 'int')

// Return true if str is binary, else false
bool isBinary(const string &str)
{
   for(int i=0; *(str+i); i++){
       if(str[i]!='0' && str[i]!='1')
           return false;
   }
   return true;
}

P.S.: I can understand this might be a naive question, but then I would be happy to be redirected to useful sources too!

@πάνταῥεῖ I get where I am wrong, thanks for the help — devautor, Jun 20 '16 at 07:01
Try [the book guide and list](http://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list). (Found at the [C++ info page](http://stackoverflow.com/tags/c%2b%2b/info).) — molbdnilo, Jun 20 '16 at 07:32

fluter · Accepted Answer · 2016-06-20T07:30:54.993

2

str is of type std::string, it is not char*, and there is no operator+(int) defined for it, what you can do is to get its size with length member function:

bool isBinary(const string &str)
{
   for(int i=0; i < str.length(); i++){
       if(str[i]!='0' && str[i]!='1')
           return false;
    }
    return true;
}

On the other hand, if you got a c string, you can do the second form, since c strings are just null terminated char arrays.

bool isBinary(const char *str)
{
   for(int i=0; *(str+i); i++){
       if(str[i]!='0' && str[i]!='1')
           return false;
    }
    return true;
}

Also, you could get c string from c++'s std::string with its c_str() member function.

const char *s = str.c_str();

edited Jun 20 '16 at 07:30

answered Jun 20 '16 at 06:58

fluter

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Got it! Indeed, too naive to ask, thanks! – devautor Jun 20 '16 at 06:59
You could also mention, that you can get underlying `const char*` from `std::string` using [`std::string::c_str`](http://www.cplusplus.com/reference/string/string/c_str/) – Zereges Jun 20 '16 at 07:29
@Zereges Updated, thanks. – fluter Jun 20 '16 at 07:31

Dereferencing vs Indexing on const string reference

1 Answers1