C++多重繼承的順序

多重繼承的順序

當一個類別繼承自多個類別時，它的多個父類別的constructor是會依照繼承順序被呼叫的。

以下是TensorRT/samples/common/logging.h裡LogStreamConsumer類別的定義：

class LogStreamConsumer : protected LogStreamConsumerBase, public std::ostream
{
public:
    //! \brief Creates a LogStreamConsumer which logs messages with level severity.
    //!  Reportable severity determines if the messages are severe enough to be logged.
    //此處initializer list的順序：先LogStreamConsumerBase後std::ostream
    LogStreamConsumer(Severity reportableSeverity, Severity severity)
        //LogStreamConsumerBase(stream, prefix, shouldLog)
        : LogStreamConsumerBase(severityOstream(severity), severityPrefix(severity), severity <= reportableSeverity)
        //mBuffer繼承自LogStreamConsumerBase
        //注意這裡是先呼叫LogStreamConsumerBase建構子，
        //使得mBuffer指向一個有效的位置後，
        //才使用mBuffer來初始化ostream
        , std::ostream(&mBuffer) // links the stream buffer with the stream
        , mShouldLog(severity <= reportableSeverity)
        , mSeverity(severity)
    {
    }
    //...
}

由於在繼承順序中LogStreamConsumerBase先於std::ostream：

class LogStreamConsumer : protected LogStreamConsumerBase, public std::ostream

，所以會先呼叫LogStreamConsumerBase的建構子：

LogStreamConsumerBase(severityOstream(severity), severityPrefix(severity), severity <= reportableSeverity)

以下是LogStreamConsumerBase建構子的定義：

LogStreamConsumerBase(std::ostream& stream, const std::string& prefix, bool shouldLog)
        : mBuffer(stream, prefix, shouldLog)
{
}

在這一步會初始化LogStreamConsumer的成員變數mBuffer。

在LogStreamConsumerBase建構子返回後，接著才調用std::ostream的建構子：

std::ostream(&mBuffer)

可以看到這一步需要mBuffer這個成員變數，這也是為什麼在代碼注釋中強調LogStreamConsumerBase及std::ostream的繼承順序不能被替換的原因。

參考連結

C++ multiple inheritance order