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

NR.5：不要使用兩階段初始化

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).

將初始化分為兩部分會(huì)導(dǎo)致不變性較弱，代碼更復(fù)雜（必須處理半結(jié)構(gòu)化對(duì)象）和錯(cuò)誤（當(dāng)我們不能始終如一地正確處理半結(jié)構(gòu)化對(duì)象時(shí)）。

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 then
Picture picture2(100, 0);
// not reach here...

Alternative（代替選項(xiàng)）

• Always establish a class invariant in a constructor.

  始終在構(gòu)造函數(shù)中建立類不變式。
  

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

  不要在需要之前定義對(duì)象。
  

  
  

原文鏈接

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

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