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 ??

Breakpoint 1, w2_browser_Callback (w=0x7dc1b0, data="" at fV3.04.cxx.267
(gdb) x/8b &s1
0x7fffffffdc30:             0x40 0xdc 0xff 0xff 0xff 0x7f 0x00 0x00
(gdb) x/8b 0x7fffffffdc40
0x7fffffffdc40:                0x31 0x2e 0x31 0x30 0x30 0x30 0x30 0x30
(gdb) x/8b &s2
0x7fffffffdc50:                0x60 0xdc 0xff 0xff 0xff 0x7f 0x00 0x00
(gdb) x/8b 0x7fffffffdc60:
0x7fffffffdc60:                0x31 0x2e 0x31 0x30 0x30 0x30 0x30 0x30