Simple function returns junky string
Author
(1) why does the function below returns a junk string instead of "123" ?!
const char* getChar()
{
char buf[5];
sprintf(buf, "%d", 123);
std::string s = buf;
return s.c_str();
}
(2) how to make the function return the string as wchar_t ?
thanks for your help
Ask yourself what happens to the local function variable std::string s when you exit from the function and function scope?
Hint: s gets stored on the stack when the function begins.
Hint2: s.c_str() points to memory within the s variable
Hint: s gets stored on the stack when the function begins.
Hint2: s.c_str() points to memory within the s variable
Quote:Original post by silvia_steven_2000
(1) why does the function below returns a junk string instead of "123" ?!
because "s" is a local variable and is automatically deleted when the function's scope ends.
Quote:Original post by silvia_steven_2000
(2) how to make the function return the string as wchar_t ?
std::string is a type alias for std::basic_string which is class template typedef'ed with char and defaults to using std::char_traits<char>, there exists another type alias for wchar_t known as std::wstring.
(1) a. use global variables
or b. declare your local variables as static
(2)
You need to convert the string to unicode first. Standard C library function "mbtowc" does it.
An example:
static wchar_t wbuf[5];
mbtowc(wbuf,buf,5);
return wbuf;
or b. declare your local variables as static
(2)
You need to convert the string to unicode first. Standard C library function "mbtowc" does it.
An example:
static wchar_t wbuf[5];
mbtowc(wbuf,buf,5);
return wbuf;
Return a std::string instead of char *. Or std::wstring (I think it,s the right name), if you want wide-chars.
Quote:Original post by persil
Return a std::string instead of char *. Or std::wstring (I think it,s the right name), if you want wide-chars.
Quoted for being the right answer. Do the conversion on the receiving end, and only if you really have to. Better yet, stick with std::string basically everywhere else in your program, and wrap calls to library functions expecting/returning char*. For example:
char* LibraryFunction(char* input);// possibly modifies the pointed-to data, but obviously cannot change the 'input'// pointer itself, since it's a C function and thus there's no pass-by-ref.std::string wrapper(std::string& input) { // create a mutable copy of the string data char* tmp = new char[input.length() + 1]; // If the library function ends up writing beyond that allocation, then it's // the library's fault :P Meanwhile, std::strings can contain embedded \0's, // but the library probably wouldn't handle those properly *regardless*. strcpy(tmp, input.c_str()); // Call function on the buffer and save result char * result = LibraryFunction(tmp); // Wrap both buffers into strings and return appropriately input = tmp; return std::string(result);}
Author
declaring the variable as static works fine and makes the function return a non junk value but still I can not convert the char* to wchar_t*. how is that done ?
I tried this but it did not work:
static wchar_t wbuf[5];
mbtowc(wbuf, buf,5);
return wbuf;
basically I need to do :
wchar_t* x = convert_to_wide_char("my title")
SetWindowTextW(x)
I tried this but it did not work:
static wchar_t wbuf[5];
mbtowc(wbuf, buf,5);
return wbuf;
basically I need to do :
wchar_t* x = convert_to_wide_char("my title")
SetWindowTextW(x)
"mbtowc" converts only a single character into unicode format. To covert a sequence of chars use "mbstowcs". Sorry for this mistake. (Their names are so similar.) Use
static wchar_t wbuf[5];
mbstowcs(wbuf, buf,5)
SetWindowTextW(wbuf)
This should work.
static wchar_t wbuf[5];
mbstowcs(wbuf, buf,5)
SetWindowTextW(wbuf)
This should work.
correct me if I'm wrong but isn't it a bad thing if that variable is static and consumes memory throughout the whole program whether you use the function once or twice?
This topic is closed to new replies.
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