C++使用初始化列表的方式来初始化字段的方法分享！

几个月之前，接触Android recovery源代码的时候，看ScreenRecoveryUI类的时候，那时候C++基础还不是特别好，一直不明白以下的初始化方式：

下面这个是Recovery的一个构造函数，代码位于：screen_ui.cpp，它的类的实现在screen_ui.h。

如下这个ScreenRecoveryUI类，这个类是继承于RecoveryUI类的：

这个文件在screen_ui.h

  class ScreenRecoveryUI : public RecoveryUI {   public:    ScreenRecoveryUI();    void Init();    void SetLocale(const char* locale);    // overall recovery state ("background image")    void SetBackground(Icon icon);    // progress indicator    void SetProgressType(ProgressType type);    void ShowProgress(float portion, float seconds);    void SetProgress(float fraction);    void SetStage(int current, int max);    // text log    void ShowText(bool visible);    bool IsTextVisible();    bool WasTextEverVisible();    // printing messages    void Print(const char* fmt, ...) __printflike(2, 3);    void ShowFile(const char* filename);    // menu display    void StartMenu(const char* const * headers, const char* const * items,            int initial_selection);    int SelectMenu(int sel);    void EndMenu();    void KeyLongPress(int);    void Redraw();    enum UIElement {      HEADER, MENU, MENU_SEL_BG, MENU_SEL_BG_ACTIVE, MENU_SEL_FG, LOG, TEXT_FILL, INFO    };    void SetColor(UIElement e);   private:    Icon currentIcon;    int installingFrame;    const char* locale;    bool rtl_locale;    pthread_mutex_t updateMutex;    GRSurface* backgroundIcon[5];    GRSurface* backgroundText[5];    GRSurface** installation;    GRSurface* progressBarEmpty;    GRSurface* progressBarFill;    GRSurface* stageMarkerEmpty;    GRSurface* stageMarkerFill;    ProgressType progressBarType;    float progressScopeStart, progressScopeSize, progress;    double progressScopeTime, progressScopeDuration;    // true when both graphics pages are the same (except for the progress bar).    bool pagesIdentical;    size_t text_cols_, text_rows_;    // Log text overlay, displayed when a magic key is pressed.    char** text_;    size_t text_col_, text_row_, text_top_;    bool show_text;    bool show_text_ever;  // has show_text ever been true?    char** menu_;    const char* const* menu_headers_;    bool show_menu;    int menu_items, menu_sel;    // An alternate text screen, swapped with 'text_' when we're viewing a log file.    char** file_viewer_text_;    pthread_t progress_thread_;    int animation_fps;    int installing_frames;    int iconX, iconY;    int stage, max_stage;    void draw_background_locked(Icon icon);    void draw_progress_locked();    void draw_screen_locked();    void update_screen_locked();    void update_progress_locked();    static void* ProgressThreadStartRoutine(void* data);    void ProgressThreadLoop();    void ShowFile(FILE*);    void PutChar(char);    void ClearText();    void DrawHorizontalRule(int* y);    void DrawTextLine(int* y, const char* line, bool bold);    void DrawTextLines(int* y, const char* const* lines);    void LoadBitmap(const char* filename, GRSurface** surface);    void LoadBitmapArray(const char* filename, int* frames, GRSurface*** surface);    void LoadLocalizedBitmap(const char* filename, GRSurface** surface);  };

下面是这个类的构造函数的实现，其中构造函数就采用了初始化列表的方式来初始化字段，以下构造函数的实现在screen_ui.cpp文件中可以找到。

  ScreenRecoveryUI::ScreenRecoveryUI() :    currentIcon(NONE),    installingFrame(0),    locale(nullptr),    rtl_locale(false),    progressBarType(EMPTY),    progressScopeStart(0),    progressScopeSize(0),    progress(0),    pagesIdentical(false),    text_cols_(0),    text_rows_(0),    text_(nullptr),    text_col_(0),    text_row_(0),    text_top_(0),    show_text(false),    show_text_ever(false),    menu_(nullptr),    show_menu(false),    menu_items(0),    menu_sel(0),    file_viewer_text_(nullptr),    animation_fps(20),    installing_frames(-1),    stage(-1),    max_stage(-1) {    for (int i = 0; i < 5; i++) {      backgroundIcon[i] = nullptr;    }    pthread_mutex_init(&updateMutex, nullptr);  }

可以来看看RecoveryUI类:

在ui.h中：

  class RecoveryUI {   public:    RecoveryUI();    virtual ~RecoveryUI() { }    // Initialize the object; called before anything else.    virtual void Init();    // Show a stage indicator. Call immediately after Init().    virtual void SetStage(int current, int max) = 0;    // After calling Init(), you can tell the UI what locale it is operating in.    virtual void SetLocale(const char* locale) = 0;    // Set the overall recovery state ("background image").    enum Icon { NONE, INSTALLING_UPDATE, ERASING, NO_COMMAND, ERROR };    virtual void SetBackground(Icon icon) = 0;    // --- progress indicator ---    enum ProgressType { EMPTY, INDETERMINATE, DETERMINATE };    virtual void SetProgressType(ProgressType determinate) = 0;    // Show a progress bar and define the scope of the next operation:    //  portion - fraction of the progress bar the next operation will use    //  seconds - expected time interval (progress bar moves at this minimum rate)    virtual void ShowProgress(float portion, float seconds) = 0;    // Set progress bar position (0.0 - 1.0 within the scope defined    // by the last call to ShowProgress).    virtual void SetProgress(float fraction) = 0;    // --- text log ---    virtual void ShowText(bool visible) = 0;    virtual bool IsTextVisible() = 0;    virtual bool WasTextEverVisible() = 0;    // Write a message to the on-screen log (shown if the user has    // toggled on the text display).    virtual void Print(const char* fmt, ...) __printflike(2, 3) = 0;    virtual void ShowFile(const char* filename) = 0;    // --- key handling ---    // Wait for a key and return it. May return -1 after timeout.    virtual int WaitKey();    virtual bool IsKeyPressed(int key);    virtual bool IsLongPress();    // Returns true if you have the volume up/down and power trio typical    // of phones and tablets, false otherwise.    virtual bool HasThreeButtons();    // Erase any queued-up keys.    virtual void FlushKeys();    // Called on each key press, even while operations are in progress.    // Return value indicates whether an immediate operation should be    // triggered (toggling the display, rebooting the device), or if    // the key should be enqueued for use by the main thread.    enum KeyAction { ENQUEUE, TOGGLE, REBOOT, IGNORE };    virtual KeyAction CheckKey(int key, bool is_long_press);    // Called when a key is held down long enough to have been a    // long-press (but before the key is released). This means that    // if the key is eventually registered (released without any other    // keys being pressed in the meantime), CheckKey will be called with    // 'is_long_press' true.    virtual void KeyLongPress(int key);    // Normally in recovery there's a key sequence that triggers    // immediate reboot of the device, regardless of what recovery is    // doing (with the default CheckKey implementation, it's pressing    // the power button 7 times in row). Call this to enable or    // disable that feature. It is enabled by default.    virtual void SetEnableReboot(bool enabled);    // --- menu display ---    // Display some header text followed by a menu of items, which appears    // at the top of the screen (in place of any scrolling ui_print()    // output, if necessary).    virtual void StartMenu(const char* const * headers, const char* const * items,                int initial_selection) = 0;    // Set the menu highlight to the given index, wrapping if necessary.    // Returns the actual item selected.    virtual int SelectMenu(int sel) = 0;    // End menu mode, resetting the text overlay so that ui_print()    // statements will be displayed.    virtual void EndMenu() = 0;  protected:    void EnqueueKey(int key_code);  private:    // Key event input queue    pthread_mutex_t key_queue_mutex;    pthread_cond_t key_queue_cond;    int key_queue[256], key_queue_len;    char key_pressed[KEY_MAX + 1];   // under key_queue_mutex    int key_last_down;         // under key_queue_mutex    bool key_long_press;        // under key_queue_mutex    int key_down_count;        // under key_queue_mutex    bool enable_reboot;        // under key_queue_mutex    int rel_sum;    int consecutive_power_keys;    int last_key;    bool has_power_key;    bool has_up_key;    bool has_down_key;    struct key_timer_t {      RecoveryUI* ui;      int key_code;      int count;    };    pthread_t input_thread_;    void OnKeyDetected(int key_code);    static int InputCallback(int fd, uint32_t epevents, void* data);    int OnInputEvent(int fd, uint32_t epevents);    void ProcessKey(int key_code, int updown);    bool IsUsbConnected();    static void* time_key_helper(void* cookie);    void time_key(int key_code, int count);  };  ui.cpp中，也是采用字段初始化的方式来实现构造函数：  RecoveryUI::RecoveryUI()      : key_queue_len(0),       key_last_down(-1),       key_long_press(false),       key_down_count(0),       enable_reboot(true),       consecutive_power_keys(0),       last_key(-1),       has_power_key(false),       has_up_key(false),       has_down_key(false) {    pthread_mutex_init(&key_queue_mutex, nullptr);    pthread_cond_init(&key_queue_cond, nullptr);    memset(key_pressed, 0, sizeof(key_pressed));  }

现在看明白了。

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