使用字符串在Qt中创建QVariantMap

Question

Apologies for asking something this trivial but I just can't seem to get it right so I guess I've completely misunderstood everything I thought I knew about memory management.

I have a function that parses a network reply for some data and it looks like this:

// Call from another function
    QVariantMap *mappedResult = handleReply(reply);
...
// Function
QVariantMap *handleReply(QNetworkReply *reply) {
    QVariantMap *result = new QVariantMap;
    QVariant testvalue = new QVariant("testvalue");
    result->insert("testkey", testvalue);
    if (reply->error() > 0) {
        qDebug() << "Error number = " + reply->errorString();
        QVariant variant = QVariant.fromValue(reply->errorString());
        result->insert("error", variant);
    } else {
        QJsonDocument jsonResponse = QJsonDocument::fromJson(jsonString.toUtf8());
        QJsonObject jsonResponseObject = jsonResponse.object();
        *result = jsonResponseObject.toVariantMap();
    }
    return result;
}

If there is no error, the result is parsed fine by the built in toVariantMap function. When there is an error however, I would like to create an entry in the result that is sent back from the function. As you can see in the function, I'm trying to create QVariants from Strings in two different ways.

The first approach gives an error like this:

C:\Qt\5.2.0\mingw48_32\include\QtCore\qvariant.h:466: error: 'QVariant::QVariant(void*)' is private inline QVariant(void *) Q_DECL_EQ_DELETE;
        ^

The second approach like this:

[PATH] error: expected primary-expression before '.' token QVariant variant = QVariant.fromValue(reply->errorString());

I've also tried setting the values like this:

result->insert("testkey", "testvalue");
result->insert("error", reply->errorString());

The last approach doesn't give compile errors but the result variable in the return statement cannot be inspected in the debugger and as soon as the return statement is executed the application crashes with memory problems indicating I'm not using "new" properly.

"Error accessing memory address 0x...".

If someone with at least a little knowledge could help me sort out how to perform this simple task, I would be very greatful. How can I return a QVariantMap from my function with custom strings as key/value pairs?

Cheers.

Answer 1

I would write your function in the following way (with fixes):

QVariantMap *handleReply(QNetworkReply *reply) {
    QVariantMap *result = new QVariantMap;
    QVariant testvalue("testvalue"); // <- fixed here
    result->insert("testkey", testvalue);
    if (reply->error() > 0) {
        qDebug() << "Error number = " + reply->errorString();
        QVariant variant(reply->errorString()); // <- fixed here
        result->insert("error", variant);
    } else {
        QJsonDocument jsonResponse = QJsonDocument::fromJson(jsonString.toUtf8());
        QJsonObject jsonResponseObject = jsonResponse.object();
        *result = jsonResponseObject.toVariantMap();
    }
    return result;
}

However it is still unclear, why do you need to work with a pointer to the variant map instread of passing it by value?