NR.5: Don't use two-phase initialization

NR.5：不要使用两阶段初始化

Reason(原因)

Splitting initialization into two leads to weaker invariants, more complicated code (having to deal with semi-constructed objects), and errors (when we didn't deal correctly with semi-constructed objects consistently).

将初始化分为两部分会导致不变性较弱，代码更复杂(必须处理半结构化对象)和错误(当我们不能始终如一地正确处理半结构化对象时)。

Example, bad(反面示例)

// Old conventional style: many problems

class Picture{    int mx;    int my;    char * data;public:    // main problem: constructor does not fully construct    Picture(int x, int y)    {        mx = x;         // also bad: assignment in constructor body                        // rather than in member initializer        my = y;        data = nullptr; // also bad: constant initialization in constructor                        // rather than in member initializer    }

    ~Picture()    {        Cleanup();    }

    // ...

    // bad: two-phase initialization    bool Init()    {        // invariant checks        if (mx <= 0 || my <= 0) {            return false;        }        if (data) {            return false;        }        data = (char*) malloc(mx*my*sizeof(int));   // also bad: owning raw * and malloc        return data != nullptr;    }

    // also bad: no reason to make cleanup a separate function    void Cleanup()    {        if (data) free(data);        data = nullptr;    }};

Picture picture(100, 0); // not ready-to-use picture here// this will fail..if (!picture.Init()) {    puts("Error, invalid picture");}// now have a invalid picture object instance.

Example, good(范例)

class Picture{    int mx;    int my;    vector data;    static int check_size(int size)    {        // invariant check        Expects(size > 0);        return size;    }public:    // even better would be a class for a 2D Size as one single parameter    Picture(int x, int y)        : mx(check_size(x))        , my(check_size(y))        // now we know x and y have a valid size        , data(mx * my * sizeof(int)) // will throw std::bad_alloc on error    {        // picture is ready-to-use    }    // compiler generated dtor does the job. (also see C.21)    // ...};Picture picture1(100, 100);// picture is ready-to-use here...// not a valid size for y,// default contract violation behavior will call std::terminate thenPicture picture2(100, 0);// not reach here...

Alternative(代替选项)

• Always establish a class invariant in a constructor.

  始终在构造函数中建立类不变式。

• Don't define an object before it is needed.

  不要在需要之前定义对象。

原文链接

https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#nr5-dont-use-two-phase-initialization

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