qt初入门0：结构体中QString用memset导致崩溃分析及QLatin1String简单查看源码

const QArrayData QArrayData::shared_null[2] = {{ -1, 0, 0, 0, sizeof(QArrayData) }, }
inline QString::QString() noexcept : d(Data::sharedNull()) {}
inline QString::~QString() { if (!d->ref.deref()) Data::deallocate(d); }
class Q_CORE_EXPORT QString
{
public:
    typedef QStringData Data; //基类是 QArrayData
    inline QString() noexcept;
private:
    Data *d; //基类是 QArrayData 控制引用计数
public:
    typedef Data * DataPtr;
    inline DataPtr &data_ptr() { return d; }
};
QString::QString(const QChar *unicode, int size)
{
   if (!unicode) {
        d = Data::sharedNull();
    } else {
        if (size < 0) {
            size = 0;
            while (!unicode[size].isNull())
                ++size;
        }
        if (!size) {
            d = Data::allocate(0);
        } else {
            d = Data::allocate(size + 1);
            Q_CHECK_PTR(d);
            d->size = size;
            memcpy(d->data(), unicode, size * sizeof(QChar));
            d->data()[size] = '\0';
        }
    }
}

struct Q_CORE_EXPORT QArrayData
{
    QtPrivate::RefCount ref; //基类实现原子变量的递增递减
    int size;
    uint alloc : 31;
    uint capacityReserved : 1;
    qptrdiff offset; // in bytes from beginning of header
    void *data() {
        Q_ASSERT(size == 0  || offset < 0 || size_t(offset) >= sizeof(QArrayData));
        return reinterpret_cast<char *>(this) + offset;
    }
    static const QArrayData shared_null[2];
    static QArrayData *sharedNull() noexcept { return const_cast<QArrayData*>(shared_null); }
};
//真正的申请内存
QArrayData *QArrayData::allocate(size_t objectSize, size_t alignment,
        size_t capacity, AllocationOptions options) noexcept
{
    Q_ASSERT(alignment >= Q_ALIGNOF(QArrayData)            && !(alignment & (alignment - 1)));
    if (!(options & RawData) && !capacity) {
        return const_cast<QArrayData *>(&qt_array_empty);
    }
    size_t headerSize = sizeof(QArrayData);
    if (!(options & RawData))
        headerSize += (alignment - Q_ALIGNOF(QArrayData));
    if (headerSize > size_t(MaxAllocSize))
        return nullptr;
    size_t allocSize = calculateBlockSize(capacity, objectSize, headerSize, options);
    QArrayData *header = static_cast<QArrayData *>(::malloc(allocSize));
    if (header) {
        quintptr data = (quintptr(header) + sizeof(QArrayData) + alignment - 1)
                & ~(alignment - 1);
        header->ref.atomic.storeRelaxed(1);
        header->size = 0;
        header->alloc = capacity;
        header->capacityReserved = bool(options & CapacityReserved);
        header->offset = data - quintptr(header);
    }
    return header;
}
//真正的释放内存 根据引用计数控制
void QArrayData::deallocate(QArrayData *data, size_t objectSize, size_t alignment) noexcept
{
    // Alignment is a power of two
    Q_ASSERT(alignment >= Q_ALIGNOF(QArrayData)  && !(alignment & (alignment - 1)));
    Q_UNUSED(objectSize) Q_UNUSED(alignment)
    Q_ASSERT_X(data == 0 || !data->ref.isStatic(), "QArrayData::deallocate",
               "Static data cannot be deleted");
    ::free(data);
}

class QLatin1String
{
public:
    Q_DECL_CONSTEXPR inline QLatin1String() noexcept : m_size(0), m_data(nullptr) {}
    Q_DECL_CONSTEXPR inline explicit QLatin1String(const char *s) noexcept : m_size(s ? int(strlen(s)) : 0), m_data(s) {}
    Q_DECL_CONSTEXPR explicit QLatin1String(const char *f, const char *l)
        : QLatin1String(f, int(l - f)) {}
    Q_DECL_CONSTEXPR inline explicit QLatin1String(const char *s, int sz) noexcept : m_size(sz), m_data(s) {}
    inline explicit QLatin1String(const QByteArray &s) noexcept : m_size(int(qstrnlen(s.constData(), s.size()))), m_data(s.constData()) {}
    //。。。其他都是基本的函数接口
    Q_DECL_CONSTEXPR const char *latin1() const noexcept { return m_data; }
    Q_DECL_CONSTEXPR int size() const noexcept { return m_size; }
    Q_DECL_CONSTEXPR const char *data() const noexcept { return m_data; }
    Q_DECL_CONSTEXPR bool isNull() const noexcept { return !data(); }
    Q_DECL_CONSTEXPR bool isEmpty() const noexcept { return !size(); }
private:
    int m_size;
    const char *m_data;
};