WTF String

1.KURL:
it's the type of String in WTF,
we use

KURL.string().utf8().data();

to return the char * in the KURL.
And how does it come out?     a KURL is defined in WebCore/Platform/KURL.h, it has a member function string().
    b KURL.string(), return a class of String, which was defined in wtf/text/WTFString.h, it has a member function utf8()
    c String.utf8() return a class of CString , which was defined in wtf/text/CString.h, it has a member function data().
    d CString.data() return the type of char* , which can be printed directly.
to be continued
2. String in WTF
a. StringImplBase
defined in "WTF/text/StringImplBase.h"
it has no concrete implementation
b.StringImpl
defined in "WTF/text/StringImpl.h"
StringImplBase<--StringImpl
it hold a member UChar (wchar_t)//16 or 32 bit.
c.String
defined in "WTF/text/WTFString.h"
String hold a member of "StringImpl".
it has function to return it's data in CString

    CString ascii() const;
    CString latin1() const;
    CString utf8(bool strict = false) const;

d. CString
was defined in "WTF/text/CString.h"
it has a function "data()"to return the type of "char*".
how is the UChar come to char *
the most imporant function was String.utf8() in "WTF/text/WTFString.h"
using "convertUTF16ToUTF8".//so UChar is 16 bit.
which was defined in "WTF/unicode/utf8.cpp"

ConversionResult convertUTF16ToUTF8(
    const UChar** sourceStart, const UChar* sourceEnd, 
    char** targetStart, char* targetEnd, bool strict)
{
    ConversionResult result = conversionOK;
    const UChar* source = *sourceStart;
    char* target = *targetStart;
    while (source < sourceEnd) {
        UChar32 ch;
        unsigned short bytesToWrite = 0;
        const UChar32 byteMask = 0xBF;
        const UChar32 byteMark = 0x80; 
        const UChar* oldSource = source; // In case we have to back up because of
        //target overflow.
        ch = static_cast<unsigned short>(*source++);
        // If we have a surrogate pair, convert to UChar32 first.
        if (ch >= 0xD800 && ch <= 0xDBFF) {
            // If the 16 bits following the high surrogate are in the source buffer...
            if (source < sourceEnd) {
                UChar32 ch2 = static_cast<unsigned short>(*source);
                // If it's a low surrogate, convert to UChar32.
                if (ch2 >= 0xDC00 && ch2 <= 0xDFFF) {
                    ch = ((ch - 0xD800) << 10) + (ch2 - 0xDC00) + 0x0010000;
                    ++source;
                } else if (strict) { // it's an unpaired high surrogate
                    --source; // return to the illegal value itself
                    result = sourceIllegal;
                    break;
                }
            } else { // We don't have the 16 bits following the high surrogate.
                --source; // return to the high surrogate
                result = sourceExhausted;
                break;
            }
        } else if (strict) {
            // UTF-16 surrogate values are illegal in UTF-32
            if (ch >= 0xDC00 && ch <= 0xDFFF) {
                --source; // return to the illegal value itself
                result = sourceIllegal;
                break;
            }
        }
        // Figure out how many bytes the result will require
        if (ch < (UChar32)0x80) {
            bytesToWrite = 1;
        } else if (ch < (UChar32)0x800) {
            bytesToWrite = 2;
        } else if (ch < (UChar32)0x10000) {
            bytesToWrite = 3;
        } else if (ch < (UChar32)0x110000) {
            bytesToWrite = 4;
        } else {
            bytesToWrite = 3;
            ch = 0xFFFD;
        }

        target += bytesToWrite;
        if (target > targetEnd) {
            source = oldSource; // Back up source pointer!
            target -= bytesToWrite;
            result = targetExhausted;
            break;
        }
        switch (bytesToWrite) { // note: everything falls through.
            case 4: *--target = (char)((ch | byteMark) & byteMask); ch >>= 6;
            case 3: *--target = (char)((ch | byteMark) & byteMask); ch >>= 6;
            case 2: *--target = (char)((ch | byteMark) & byteMask); ch >>= 6;
            case 1: *--target =  (char)(ch | firstByteMark[bytesToWrite]);
        }
        target += bytesToWrite;
    }
    *sourceStart = source;
    *targetStart = target;
    return result;
}

so the most common type used in webcore is "UChar",
the most common type we use to print is "char*"
the most common way for the conversion is "UChar->utf8->char*".
definition

    ConversionResult convertUTF8ToUTF16(
                    const char** sourceStart, const char* sourceEnd, 
                    UChar** targetStart, UChar* targetEnd, bool strict = true);

    ConversionResult convertUTF16ToUTF8(
                    const UChar** sourceStart, const UChar* sourceEnd, 
                    char** targetStart, char* targetEnd, bool strict = true);

example:

    ConversionResult result = convertUTF16ToUTF8(&characters, characters + length, 
        &buffer, buffer + bufferVector.size(), strict);

for 2.3 

#include "PlatformString.h"
#include "CString.h"

to include the head file