4.QT4中的connect的实现

信号槽通过connect进行连接，connect的源码在qobject.cpp中

QT4中的connect的声明如下

static QMetaObject::Connection connect(const QObject *sender, const char *signal, const QObject *receiver, const char *member, Qt::ConnectionType = Qt::AutoConnection);

使用时，一般是这样的

connect(sender, SIGNAL(destroyed()), this, SLOT(objectDestroyed()));

其中的SIGNAL和SLOT定义如下，在qobjectdefs.h中

# define SLOT(a)     "1"#a
# define SIGNAL(a)   "2"#a

#a 得到参数的字符串形式，对于槽函数的宏，前面加 "1"，对于信号的宏，前面加 "2"，则上面的connect代码就变成如下形式

connect(sender, "2destroyed()", this, "1objectDestroyed()");

connect具体实现如下

QMetaObject::Connection QObject::connect(const QObject *sender, const char *signal,
                                     const QObject *receiver, const char *method,
                                     Qt::ConnectionType type)
{
    if (sender == nullptr || receiver == nullptr || signal == nullptr || method == nullptr) {
        qWarning("QObject::connect: Cannot connect %s::%s to %s::%s",
                 sender ? sender->metaObject()->className() : "(nullptr)",
                 (signal && *signal) ? signal+1 : "(nullptr)",
                 receiver ? receiver->metaObject()->className() : "(nullptr)",
                 (method && *method) ? method+1 : "(nullptr)");
        return QMetaObject::Connection(0);
    }//检查sender、receiver、signal、method是否为空，为空，返回个空连接
    
    QByteArray tmp_signal_name;

    if (!check_signal_macro(sender, signal, "connect", "bind"))//检查signal字符串中是否有2，如果没有，空连接返回
        return QMetaObject::Connection(0);
        
    const QMetaObject *smeta = sender->metaObject();
    const char *signal_arg = signal;
    ++signal; //跳过2，指向具体的字符串
    QArgumentTypeArray signalTypes;
    Q_ASSERT(QMetaObjectPrivate::get(smeta)->revision >= 7);
    QByteArray signalName = QMetaObjectPrivate::decodeMethodSignature(signal, signalTypes);
    int signal_index = QMetaObjectPrivate::indexOfSignalRelative(
            &smeta, signalName, signalTypes.size(), signalTypes.cons