From: roger tunnicliffe
Sent: 05 October 2022 23:40
To: fltk.general
Subject: [fltk.general] Why do I need to convert a string to a string for Fl_Input

 

I have the following definitions:-

 

 Fl_Input            *win2_unitprice;
  win2_unitprice        = new Fl_Input(x+50,y+200,100,20,"Unit Price..:");

and the following code:-

double z;

.

.
  string s1 = to_string(z);
  string s2 = s1.c_str();
// win2_unitprice->value(s1);
    win2_unitprice->value(s1.c_str());

If I uncomment line 3 I get the following error:-
error: no matching function for call to ‘Fl_Input::value(std::__cxx11::string&)’
   win2_unitprice->value(s1);

but if i look at the data I see that:-

s1 holds the address of the data 0x31 0x2e 0x31 0x30 ....

and 

s2 holds the address of the data 0x31 0x2e 0x31 0x30 ....

 

so why do I need to use c_str() to make this work ??

<START>

Hi Roger,

 

The short answer is the difference between strings in C and C++ languages. A C-string has type char* - that is a pointer to a series of 8-bit characters in memory, ended with the NUL (x00) character. A C++-string (type string) is a class with some overheads to manage access to it and memory allocation.

 

FLTK generally uses char* to pass string data around.

 

The class string method string::c_str() accesses the raw char* string within the class and this is used to pass the string content to FLTK widgets.

 

The statement

string s2 = s1.c_str()

uses a feature of C++ that overloads the assignment operator (=) so that you can create a new C++ string from the C string.

 

Regards Phil.