I am trying to convert a const char pointer to a constant wide string pointer. Here is my code.
double ret;
const wchar_t* file;
file = const wchar_t* (file_old);
I get something that says this: "Error, type name not allowed." over the const.
Any help is greatly appreciated!
You need to actually convert the contents to which the pointer points to wide characters. The easiest way to do that is probably to put the narrow version into a string and convert using wstring_convert as detailed in this answer: https://stackoverflow.com/a/18374698/82320
Related
I am trying to pass a const char* to a windows SetThreadDescription() function.
int pthread_setname_np(HANDLE thread, const char* name) {
return SetThreadDescription(thread, name);
}
and I am getting an error
'HRESULT SetThreadDescription(HANDLE,PCWSTR)': cannot convert argument 2 from 'const char *' to 'PCWSTR'
Of course I've stumbled into this and checked the character set and it was already set to Use Multi-Byte Character Set. Is there a way to fix this?
edit: add properties
#someprogrammerdude provided the answer in a comment:
The SetThreadDescription function doesn't seem to exist in an "ASCII" (non-wide character) version. You need to convert the string to wide-character string and use that converted string in the call.
I am now using C++ to program a robot using PROS. Pros has a print function, which is taking in a const char*. Now, I'm using lvgl to create my own screen, and I want to replicate the print function. Like the printf() functions, I want it to include variadic params to do the %d effect (so it converts all the %? to the corresponding values). The problem now is about the conversions between functions. I wanted to make a convert function to convert a string and the variadic params into a complete string. I need to input is a string which is like "hey" and I'm unsure what the type name should be. I need to be able to get size, search in it for %ds but I need the function to return a const char* to pass onto the lvgl to pring on the screen. I am having a bad time trying to convert a string into an const char* for the out put of the convert function.
Also, I tried using the input type as a char*, and when I input a string like "hello" is says a error [ISO C++11 does not allow conversion from string literal to 'char ' [-Wwritable-strings]]. But instead, when is use a const char, the error disappears. Anyone knows why?
Thanks everyone for your kind help!
char* and const char* are two flavours of the same thing: C-style strings. These are a series of bytes with a NUL terminator (0-byte). To use these you need to use the C library functions like strdup, strlen and so on. These must be used very carefully as missing out on the terminator, which is all too easy to do by accident, can result in huge problems in the form of buffer-overflow bugs.
std::string is how strings are represented in C++. They're a lot more capable, they can support "wide" characters, or variable length character sets like UTF-8. As there's no NUL terminator in these, they can't be overflowed and are really quite safe to use. Memory allocation is handled by the Standard Library without you having to pay much attention to it.
You can convert back and forth as necessary, but it's usually best to stick to std::string inside of C++ as much as you can.
To convert from C++ to C:
std::string cppstring("test");
const char* c_string = cppstring.c_str();
To convert from C to C++:
const char* c_string = "test";
std::string cppstring(c_string);
Note you can convert from char* (mutable) to const char* (immutable) but not in reverse. Sometimes things are flagged const because you're not allowed to change them, or that changing them would cause huge problems.
You don't really have to "convert" though, you just use char* as you would const char*.
std::string A = "hello"; //< assignment from char* to string
const char* const B = A.c_str(); //< call c_str() method to access the C string
std::string C = B; //< assignment works just fine (with allocation though!)
printf("%s", C.c_str()); //< pass to printf via %s & c_str() method
I know there are a lot of questions like this out there on StackOverflow, but I haven't been able to find any that help resolve my case. Whenever I try to do something like this:
// str = some string or char array
// some magic to get around fpermissive errors
stringstream convert;
convert << str;
// capture the stream's temporary string
const string store = convert.str();
// get a manageable array
const char* temp = store.c_str();
and then try to do something like atoi(temp[0]), I keep getting the classic conversion error that char couldn't be converted to const char. In the documentation for atoi and many other functions, const char is a required parameter. How can a char be sent in if there's only a const one? Does retrieving a char at a specific position auto-cast to char?
I'm not sure if this is what is causing the error, but atoi takes as its parameter not a char, but the pointer to one. So instead of
atoi(temp[0])
try this
atoi(&temp[0])
as that is a pointer.
This is a basic understanding concepts related question.
Working using: Embarcadero C++ Builder
What is the difference between:
opendir("C:\\XYZ")
and
String file = "C:\\XYZ";
opendir(file);
Aren't both strings?
The first one works but the sexond gives me error:
E2034 Cannot convert Unicode String to ' const char*'
In a case where I take input from the user I can only pass a string. How do i pass the whole path?
first one is a const char*, second one is a std::string. The opendir function accepts only const char* in your case and thus cannot convert std::string to const char* on its own. you can get the function to work by opendir(file.c_str()); .
No. A String is not a char array. opendir needs a char array.
opendir() expects an 8bit narrow const char* as input. When you pass a narrow literal to opendir(), you are passing it a const char[], which implicitly degrades to const char*, and all is fine.
String is System::String, which is a typedef for System::UnicodeString, which is Embarcadero's UTF-16 encoded string class (similar to std::wstring, but with different semantics). When you pass a String to opendir(), you get a conversion error.
To pass a String value to opendir() (or any other function that expects char*), you need to first convert it to a System::AnsiString, and then use AnsiString::c_str() to get a char* from it, eg:
String file = "C:\\XYZ";
opendir(AnsiString(file).c_str());
I need to pass one of my parameters to a write() function. It is asking for a type of 'const void*' I am a PHP guy and don't know C++ very well.
Here is my parameter:
const fmx::Text& distance = dataVect.AtAsText(3);
I don't know of any other way to pull in that field. I would love to just declare it const void* but I don't know how.
I guess just converting it would be easier than trying to pull it in the correct way??
The error message: cannot convert const fmx::Text to const void* for argument 2
write(fd, distance, 4);
I know this worked so can I just convert?
const void* s = "5000";
This is for a plugin in FileMaker so I don't really get c++ here.
Is there more anyone would need to help me solve this??
Thanks so much!
If fmx::Text was a pointer type, the compiler would automatically convert a reference to it into a void*. You need to use the address-of operator to give the function a pointer to work with:
write(fd, &distance, 4);
I don't really know filemaker, but this link shows that fmx::Text has a GetBytes function. You can then pass the pointer to the buffer filled with this function.
I'm assuming you actually want the text string.
I think you need to check the api for fmx::Text to get the string you want. Here is something I found to get the string out.
Looks like the type stores the data as UTF16, so you have to run a bit of code to get a string out, then pass it to your write function:
//a function to convert to a normal string
std::string getString(fmx::Text& Text)
{
char buffer[512] = {0}; //NOTE YOU HAVE A STRING SIZE LIMIT
// convert original text to ASCII text
outText.GetBytes( buffer, sizeof(buffer)-1, 0, Text.GetSize(), fmx::Text::kEncoding_Native );
return buffer;
}
Then call the function
std::string myString = getString(distance);
write(fd, myString.c_str(), myString.size());
Note I'm assuming a lot here...that you want a string in the current encoding, and not the raw UTF16 data from 'distance'. AND that GetBytes will not mangle the null characters in buffer....
You'll also need to include <string> in your c++ file.