C++核心准则​NR.5:不要使用两阶段初始化

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 ·

2020-11-06 23:28

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 then
Picture 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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