如何将Delphi中的UTF-8字符串传递给DLL C extern函数？

Question

I am calling from Delphi a C extern function in a Visual Studio compiled DLL. The DLL method in turn calls a C++ method that takes as argument a C++ string type. The string at the Delphi end is UTF-8 encoded (without the BOM). I need to make sure the C++ method that takes the string type gets the UTF-8 encoded string.

I can modify the DLL source code. My question:

My UTF-8 string on the Delphi side is of type string. What type should the C extern method take? PChar, PWideChar? and how do I convert that to a C++ string type?

Note: I can't convert the UTF-8 string into an AnsiString first because the encoding stores some Greek letters that must be preserved. The C++ end will make a copy of the Delphi string and handle the disposal of any allocated memory.

Delphi end (Using XE6):

mystr : string;

callCExternMethod (mystr) // cast to what?

C++ End (Using VS 2013):

void callCExternMethod (????? mystr) {

  // convert mystr to C++ string type

  callCPlusPlusMethod (takes C++ string type)
}

Answer 1

On the Delphi side the parameter is PAnsiChar which you pass like this: PAnsiChar(Utf8String(str)) .

On the C++ side you receive the parameter as const char* .

Obviously you need to ensure that the calling convention matches.

Answer 2

Another option would be to use the UTF8String type:

mystr : string;
u8: UTF8String;

u8 := UTF8String(mystr);
callCExternMethod(PAnsiChar(u8));

Note: the UTF8String type is not available for use on mobile platforms in Delphi XE5 through 10.0 Seattle, unless you use a patch to enable it:

http://andy.jgknet.de/blog/2013/10/the-return-of-the-byte-strings/

UTF8String has been re-enabled for use in mobile starting in Delphi 10.1 Berlin.