c++ - 使用 UTF-8 或 Latin1 编码将 QString 转换为 QByteArray

Question

我想将 QString 转换为 utf8 或 latin1 QByteArray，但今天我将所有内容都转换为 utf8。

我正在用高于 0x7f 的 latin1 较高段中的一些字符对此进行测试，其中德语 ü 就是一个很好的例子。

如果我这样做：

QString name("\u00fc"); // U+00FC = ü
QByteArray utf8;
utf8.append(name);
qDebug() << "utf8" << name << utf8.toHex();

QByteArray latin1;
latin1.append(name.toLatin1());
qDebug() << "Latin1" << name << latin1.toHex();

QTextCodec *codec = QTextCodec::codecForName("ISO 8859-1");
QByteArray encodedString = codec->fromUnicode(name);
qDebug() << "ISO 8859-1" << name << encodedString.toHex();

我得到以下输出。

utf8 "ü" "c3bc" 
Latin1 "ü" "c3bc" 
ISO 8859-1 "ü" "c3bc"

正如你所看到的，我到处都得到了 unicode 0xc3bc，我希望在第 2 步和第 3 步得到 Latin1 0xfc。

我的猜测是我应该得到这样的东西：

utf8 "ü" "c3bc" 
Latin1 "ü" "fc" 
ISO 8859-1 "ü" "fc"

这里发生了什么？

/谢谢

一些字符表的链接：

此代码是在基于 Ubuntu 10.04 的系统上构建和执行的。

$> uname -a
Linux frog 2.6.32-28-generic-pae #55-Ubuntu SMP Mon Jan 10 22:34:08 UTC 2011 i686 GNU/Linux
$> env | grep LANG
LANG=en_US.utf8

如果我尝试使用

utf8.append(name.toUtf8());

我得到这个输出

utf8 "ü" "c383c2bc" 
Latin1 "ü" "c3bc" 
ISO 8859-1 "ü" "c3bc"

所以 latin1 是 unicode 而 utf8 是双重编码的......

这必须取决于某些系统设置？

如果我运行它（无法构建 .name()）

qDebug() << "system name:"      << QLocale::system().name();
qDebug() << "codecForCStrings:" << QTextCodec::codecForCStrings();
qDebug() << "codecForLocale:"   << QTextCodec::codecForLocale()->name();

然后我得到这个：

system name: "en_US" 
codecForCStrings: 0x0 
codecForLocale: "System"

解决方案

如果我指定它是 UTF-8 我正在使用以便不同的类知道这一点，那么它就可以工作。

QTextCodec::setCodecForLocale(QTextCodec::codecForName("UTF-8"));
QTextCodec::setCodecForCStrings(QTextCodec::codecForName("UTF-8"));

qDebug() << "system name:"      << QLocale::system().name();
qDebug() << "codecForCStrings:" << QTextCodec::codecForCStrings()->name();
qDebug() << "codecForLocale:"   << QTextCodec::codecForLocale()->name();

QString name("\u00fc"); 
QByteArray utf8;
utf8.append(name);
qDebug() << "utf8" << name << utf8.toHex();

QByteArray latin1;
latin1.append(name.toLatin1());
qDebug() << "Latin1" << name << latin1.toHex();

QTextCodec *codec = QTextCodec::codecForName("latin1");
QByteArray encodedString = codec->fromUnicode(name);
qDebug() << "ISO 8859-1" << name << encodedString.toHex();

然后我得到这个输出：

system name: "en_US" 
codecForCStrings: "UTF-8" 
codecForLocale: "UTF-8" 
utf8 "ü" "c3bc" 
Latin1 "ü" "fc" 
ISO 8859-1 "ü" "fc"

看起来应该如此。

score 8 · Accepted Answer

要知道的事情：

执行字符页面

在 C++ 标准中有一种叫做执行字符集的东西，它描述了编译器生成的二进制文件中字符串和字符文字的输出。您可以在http://gcc.gnu.org网站上的 C 预处理器手册的1 概述部分的1.1 字符集小节中阅读它。

问题：字符串文字
会产生什么结果？"\u00fc"

答：
这取决于执行字符集是什么。对于 gcc（这是您正在使用的），默认情况下为 UTF-8，除非您使用-fexec-charset选项指定不同的内容。您可以在http://gcc.gnu.org网站上的GCC 手册中的3 GCC 命令选项部分的3.11 选项控制预处理器小节中阅读有关此选项和其他选项控制预处理阶段的信息。现在，当我们知道执行字符集是 UTF-8 时，我们知道它将被转换为Unicode 代码点的 UTF-8 编码，即两个字节的序列。"\u00fc"U+00FC0xc3 0xbc

QString::QString ( const char * str )并QByteArray & QByteArray::append ( const QString & str )依赖于全局状态

QString 的构造函数char *调用QString QString::fromAscii ( const char * str, int size = -1 )使用编解码器集void QTextCodec::setCodecForCStrings ( QTextCodec * codec )（如果已设置任何编解码器）或执行与（如果未设置编解码器）相同的事情QString QString::fromLatin1 ( const char * str, int size = -1 )。

问题：
QString 的构造函数将使用什么编解码器来解码0xc3 0xbc它得到的两个字节序列（）？

答：
默认情况下没有设置编解码器，QTextCodec::setCodecForCStrings()这就是为什么使用 Latin1 来解码字节序列的原因。因为0xc3和0xbc在拉丁语 1 中都是有效的，分别代表 Ã 和 ¼（这应该已经很熟悉了，因为它是直接取自你之前问题的答案），我们得到带有这两个字符的 QString。

qDebug()不是 8 位干净的

您不应该使用QDebugclass 来输出ASCII之外的任何内容。你不能保证你得到什么。

测试程序：

#include <QtCore>

void dbg(char const * rawInput, QString s) {

    QString codepoints;
    foreach(QChar chr, s) {
        codepoints.append(QString::number(chr.unicode(), 16)).append(" ");
    }

    qDebug() << "Input: " << rawInput
             << ", "
             << "Unicode codepoints: " << codepoints;
}

int main(int argc, char *argv[])
{
    QCoreApplication app(argc, argv);

    qDebug() << "system name:"
             << QLocale::system().name();

    for (int i = 1; i <= 5; ++i) {

        switch(i) {

        case 1:
            qDebug() << "\nWithout codecForCStrings (default is Latin1)\n";
            break;
        case 2:
            qDebug() << "\nWith codecForCStrings set to UTF-8\n";
            QTextCodec::setCodecForCStrings(QTextCodec::codecForName("UTF-8"));
            break;
        case 3:
            qDebug() << "\nWithout codecForCStrings (default is Latin1), with codecForLocale set to UTF-8\n";
            QTextCodec::setCodecForCStrings(0);
            QTextCodec::setCodecForLocale(QTextCodec::codecForName("UTF-8"));
            break;
        case 4:
            qDebug() << "\nWithout codecForCStrings (default is Latin1), with codecForLocale set to Latin1\n";
            QTextCodec::setCodecForCStrings(0);
            QTextCodec::setCodecForLocale(QTextCodec::codecForName("Latin1"));
            break;
        }

        qDebug() << "codecForCStrings:" << (QTextCodec::codecForCStrings()
                                           ? QTextCodec::codecForCStrings()->name()
                                           : "NOT SET");
        qDebug() << "codecForLocale:"   << (QTextCodec::codecForLocale()
                                           ? QTextCodec::codecForLocale()->name()
                                           : "NOT SET");

        qDebug() << "\n1. Using QString::QString(char const *)";
        dbg("\\u00fc", QString("\u00fc"));
        dbg("\\xc3\\xbc", QString("\xc3\xbc"));
        dbg("LATIN SMALL LETTER U WITH DIAERESIS", QString("ü"));

        qDebug() << "\n2. Using QString::fromUtf8(char const *)";
        dbg("\\u00fc", QString::fromUtf8("\u00fc"));
        dbg("\\xc3\\xbc", QString::fromUtf8("\xc3\xbc"));
        dbg("LATIN SMALL LETTER U WITH DIAERESIS", QString::fromUtf8("ü"));

        qDebug() << "\n3. Using QString::fromLocal8Bit(char const *)";
        dbg("\\u00fc", QString::fromLocal8Bit("\u00fc"));
        dbg("\\xc3\\xbc", QString::fromLocal8Bit("\xc3\xbc"));
        dbg("LATIN SMALL LETTER U WITH DIAERESIS", QString::fromLocal8Bit("ü"));
    }

    return app.exec();
}

Windows XP 上 mingw 4.4.0 的输出：

system name: "pl_PL"

Without codecForCStrings (default is Latin1)

codecForCStrings: "NOT SET"
codecForLocale: "System"

1. Using QString::QString(char const *)
Input:  \u00fc ,  Unicode codepoints:  "c3 bc "
Input:  \xc3\xbc ,  Unicode codepoints:  "c3 bc "
Input:  LATIN SMALL LETTER U WITH DIAERESIS ,  Unicode codepoints:  "fc "

2. Using QString::fromUtf8(char const *)
Input:  \u00fc ,  Unicode codepoints:  "fc "
Input:  \xc3\xbc ,  Unicode codepoints:  "fc "
Input:  LATIN SMALL LETTER U WITH DIAERESIS ,  Unicode codepoints:  "fffd "

3. Using QString::fromLocal8Bit(char const *)
Input:  \u00fc ,  Unicode codepoints:  "102 13d "
Input:  \xc3\xbc ,  Unicode codepoints:  "102 13d "
Input:  LATIN SMALL LETTER U WITH DIAERESIS ,  Unicode codepoints:  "fc "

With codecForCStrings set to UTF-8

codecForCStrings: "UTF-8"
codecForLocale: "System"

1. Using QString::QString(char const *)
Input:  \u00fc ,  Unicode codepoints:  "fc "
Input:  \xc3\xbc ,  Unicode codepoints:  "fc "
Input:  LATIN SMALL LETTER U WITH DIAERESIS ,  Unicode codepoints:  "fffd "

2. Using QString::fromUtf8(char const *)
Input:  \u00fc ,  Unicode codepoints:  "fc "
Input:  \xc3\xbc ,  Unicode codepoints:  "fc "
Input:  LATIN SMALL LETTER U WITH DIAERESIS ,  Unicode codepoints:  "fffd "

3. Using QString::fromLocal8Bit(char const *)
Input:  \u00fc ,  Unicode codepoints:  "102 13d "
Input:  \xc3\xbc ,  Unicode codepoints:  "102 13d "
Input:  LATIN SMALL LETTER U WITH DIAERESIS ,  Unicode codepoints:  "fc "

Without codecForCStrings (default is Latin1), with codecForLocale set to UTF-8

codecForCStrings: "NOT SET"
codecForLocale: "UTF-8"

1. Using QString::QString(char const *)
Input:  \u00fc ,  Unicode codepoints:  "c3 bc "
Input:  \xc3\xbc ,  Unicode codepoints:  "c3 bc "
Input:  LATIN SMALL LETTER U WITH DIAERESIS ,  Unicode codepoints:  "fc "

2. Using QString::fromUtf8(char const *)
Input:  \u00fc ,  Unicode codepoints:  "fc "
Input:  \xc3\xbc ,  Unicode codepoints:  "fc "
Input:  LATIN SMALL LETTER U WITH DIAERESIS ,  Unicode codepoints:  "fffd "

3. Using QString::fromLocal8Bit(char const *)
Input:  \u00fc ,  Unicode codepoints:  "fc "
Input:  \xc3\xbc ,  Unicode codepoints:  "fc "
Input:  LATIN SMALL LETTER U WITH DIAERESIS ,  Unicode codepoints:  "fffd "

Without codecForCStrings (default is Latin1), with codecForLocale set to Latin1

codecForCStrings: "NOT SET"
codecForLocale: "ISO-8859-1"

1. Using QString::QString(char const *)
Input:  \u00fc ,  Unicode codepoints:  "c3 bc "
Input:  \xc3\xbc ,  Unicode codepoints:  "c3 bc "
Input:  LATIN SMALL LETTER U WITH DIAERESIS ,  Unicode codepoints:  "fc "

2. Using QString::fromUtf8(char const *)
Input:  \u00fc ,  Unicode codepoints:  "fc "
Input:  \xc3\xbc ,  Unicode codepoints:  "fc "
Input:  LATIN SMALL LETTER U WITH DIAERESIS ,  Unicode codepoints:  "fffd "

3. Using QString::fromLocal8Bit(char const *)
Input:  \u00fc ,  Unicode codepoints:  "c3 bc "
Input:  \xc3\xbc ,  Unicode codepoints:  "c3 bc "
Input:  LATIN SMALL LETTER U WITH DIAERESIS ,  Unicode codepoints:  "fc "
codecForCStrings: "NOT SET"
codecForLocale: "ISO-8859-1"

1. Using QString::QString(char const *)
Input:  \u00fc ,  Unicode codepoints:  "c3 bc "
Input:  \xc3\xbc ,  Unicode codepoints:  "c3 bc "
Input:  LATIN SMALL LETTER U WITH DIAERESIS ,  Unicode codepoints:  "fc "

2. Using QString::fromUtf8(char const *)
Input:  \u00fc ,  Unicode codepoints:  "fc "
Input:  \xc3\xbc ,  Unicode codepoints:  "fc "
Input:  LATIN SMALL LETTER U WITH DIAERESIS ,  Unicode codepoints:  "fffd "

3. Using QString::fromLocal8Bit(char const *)
Input:  \u00fc ,  Unicode codepoints:  "c3 bc "
Input:  \xc3\xbc ,  Unicode codepoints:  "c3 bc "
Input:  LATIN SMALL LETTER U WITH DIAERESIS ,  Unicode codepoints:  "fc "

我要感谢#qt freenode.org IRC 频道的 thiago、cbreak、peppe和heinz展示并帮助我理解这里涉及的问题。

c++ - 使用 UTF-8 或 Latin1 编码将 QString 转换为 QByteArray

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