How to redirect stdout to stderr

Question

I am working with a C++ QT application that also uses JavaScript. In C++ I use the qDebug function and capture all data with qInstallMessageHandler.

This captures everything that is directed to stderr. In JavaScript I am using console.info which writes data to stdout.

What I want to do is redirect stdout to stderr so that all the messages written by console.info find they're way into the same message handler.

void qDebugMsgHandler(QtMsgType type, const QMessageLogContext& context, const QString& strMsg) {
    QString strOutput;

    if ( context.file ) {
        strOutput += QString(context.file);

        if ( context.function ) {
            if ( context.line > 0 ) {
                strOutput += QString(" L%1").arg(context.line, 8, 10, QChar('0')) + QString(":");
           }
            strOutput += QString(context.function);
        }
    }
    if ( strMsg.length() > 0 ) {
        if ( strOutput.length() > 0 ) {
            strOutput += ": ";
        }
        strOutput += strMsg;
    }
    switch( type ) {
    case QtDebugMsg:
        fprintf(stderr, "   Debug:%s\n", strOutput.toLatin1().data());
        break;
    case QtInfoMsg:
        fprintf(stderr, "    Info:%s\n", strOutput.toLatin1().data());
        break;
    case QtWarningMsg:
        fprintf(stderr, " Warning:%s\n", strOutput.toLatin1().data());
        break;
    case QtCriticalMsg:
        fprintf(stderr, "Critical:%s\n", strOutput.toLatin1().data());
        break;
    case QtFatalMsg:
        fprintf(stderr, "   Fatal:%s\n", strOutput.toLatin1().data());
        break;
    }
    fflush(stderr);
}

Answer 1

You can use ios::rdbuf .

Example:

#include <iostream>

int main() {
    std::cout << "to stdout\n";
    std::cout.rdbuf(std::cerr.rdbuf());
    std::cout << "to stderr\n";
}

Answer 2

For my purpose, I ended up created an invokable routine in C++ which could be called from the JavaScript, C++ prototype:

Q_INVOKABLE void log(QString strMsg, QString strFile, long ulngLine);

C++ implementation:

void clsScriptHelper::log(QString strMsg, QString strFile, long ulngLine) {
    QJsonObject json;
    json["file"] = strFile;
    json["line"] = QString("%1").arg(ulngLine);
    json["msg"] = strMsg;
    json["type"] = QString("%1").arg(QtDebugMsg);
    qDebug() << json;
}

In the JavaScript I expose my C++ layer through the object reference "api", then call the log routine:

api.log("Testing", "SomeFile.js", 99);

The message handler now looks like this:

void qDebugMsgHandler(QtMsgType type, const QMessageLogContext& context, const QString& strMsg) {
    static const QString scstrQJSONObject("QJsonObject(");

    QString strFile, strFunction, strInfo, strOutput;
    long lngLine = 0;

    switch( type ) {
    case QtDebugMsg:
        strOutput = "   Debug:";
        break;
    case QtInfoMsg:
        strOutput = "    Info:";
        break;
    case QtWarningMsg:
        strOutput = " Warning:";
        break;
    case QtCriticalMsg:
        strOutput = "Critical:";
        break;
    case QtFatalMsg:
        strOutput = "   Fatal:";
        break;
    }
    if ( strMsg.startsWith(scstrQJSONObject) ) {
//Message contains a JSON object, extract the details
        int intLength = strMsg.length() - (scstrQJSONObject.length() + 1);
        QString strJSON = strMsg.mid(scstrQJSONObject.length(), intLength);
        QJsonDocument objDoc = QJsonDocument::fromJson(strJSON.toUtf8());

        if ( objDoc.isNull() ) {
            return;
        }
        QJsonObject objJSON = objDoc.object();
        strFile = objJSON.take("file").toString();
        lngLine = static_cast<long>(objJSON.take("line").toDouble());
        strInfo = objJSON.take("msg").toString();
        type = static_cast<QtMsgType>(objJSON.take("type").toInt());
    } else {
        strFile = QString(context.file);

        if ( context.function ) {
            strFunction = QString(context.function);
        }
        if ( context.line > 0 ) {
            lngLine = context.line;
        }
        strInfo = strMsg;
    }
    if ( strFile.length() > 0 ) {
        strOutput += strFile;
    }
    if ( lngLine > 0 ) {
        strOutput += QString(" L%1").arg(lngLine, 8, 10, QChar('0')) + QString(":");
    }
    if ( strFunction.length() > 0 ) {
        strOutput += strFunction;
    }
    if ( strInfo.length() > 0 ) {
        if ( strOutput.length() > 0 ) {
            strOutput += ": ";
        }
        strOutput += strInfo;
    }
    std::cout << strOutput.toLatin1().data() << std::endl << std::flush;
}