How to convert a std::string into a different class of string?

Question

I read data from a file and I wanted to use on of its values in a function. This is a custom function, I am programming TOOLKIT for Creo Parametric.

Well, that function is expecting a 'xrstring' but I have a std::string.

How to I convert one into another?

I've tried writing xrstring test_var = xrstring(std_var);

And it doesn't work.

//definition of xrstring

typedef const char          *xrstring;
#define xstringuninit   ((xrstring) 2)
#define xstringnil      ((const char *) 1)
#define xwstringnil     ((const wchar_t *) 1)
#define xwstringuninit     ((const wchar_t *) 2)

//definition of pfcCreateStringParamValue

pfcParamValue_ptr        pfcCreateStringParamValue (xrstring Value);

error C2664: 'pfcParamValue_ptr pfcCreateStringParamValue(xrstring)': cannot convert argument 1 from 'std::string' to 'xrstring'

You can't implicitly convert a std::string object to a pointer to char (const or not), and you have never been able to do that. You need to explicitly do it using the c_str function. This should be well known to all who studied a little. — Some programmer dude
– Some programmer dude, Commented Oct 30, 2019 at 15:09
Note that xrstring test_var = std_var.c_str(); the test_var is only valid for as long as the life span of std_var, and only as long as std_var is not modified. If test_var needs to live longer than that, std::strdup will make a copy, but then you need to std::free that memory when done with test_var. — Eljay
– Eljay, Commented Oct 30, 2019 at 15:18
Thanks everyone, c_str work. However, if std_var has special characters, test_var becomes some weird characters. How can I fix this? — Luís F.
– Luís F., Commented Oct 30, 2019 at 15:18
This is a custom function, I am programming TOOLKIT for Creo Parametric. -- I can't think of a single "toolkit" that has their own string type to not be able to take a char * to a buffer of characters. The std::string::c_str() function provides such a pointer. — PaulMcKenzie
– PaulMcKenzie, Commented Oct 30, 2019 at 15:18

MrProgrammer · Accepted Answer · 2019-10-30 15:20:49Z

2

The xrstring you speak of is const char * or a C-style string.

std::string comes with a convenient function to convert a std::string to C-style string - std::string::c_str()

Hence, here is the right way to do what you want to do:

xrstring test_var = std_var.c_str();

Also, there does not exist a direct const char * to std::string cast. The Standard Library string is not a mere pointer-to-char. It contains other information like string-length as well.

I personally would recommend you to declare xstring as a class if you want to do a conversion like xrstring test_var = xrstring(std_var);, and make a suitable constructor for the conversion.

answered Oct 30, 2019 at 15:20

MrProgrammer

4613 silver badges15 bronze badges

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

4 Comments

Luís F. Over a year ago

Thanks, that works. However, if std_var has special characters, test_var becomes some weird characters. How can I fix this?

MrProgrammer Over a year ago

Can you show us one such case here (a std_var with special characters and what it's test_var looks like)?

Luís F. Over a year ago

I was trying to write in a logfile to check if I was retrieving data correctly from my .csv file. I kept getting weird results whenever I started my program. I figured out through my notepad++ that it was sometimes encoding in ANSI and sometimes UTF-8. In ANSI It comes out good "Construção", in UTF-8, instead of a "çã" i have 2 black rectangles with xE7 and xE3 (can't copy it here). When I use this string in my program, I get this: ඗翷.

MrProgrammer Over a year ago

I guess, you might want to take a look at wchar_t and wstring to get the Unicode support in C++ and define typedef const wchar_t *xrstring; instead. I am not aware of any implementations of C++ where normal char and string are interpreted as UTF-8 by the compiler.

Collectives™ on Stack Overflow

How to convert a std::string into a different class of string?

1 Answer 1

4 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

4 Comments

Your Answer

Sign up or log in

Post as a guest

Related