Question

出于教育目的，我正在尝试重新创建ANSI-C中的封装原理。我本质上所做的就是在 .c 文件中建立一些结构：

struct _private
{
    unsigned char SizeInBytes;
    unsigned char* matrix;
    struct Stack* S;
    unsigned char ByteX;
};

代表我想看不见的变量。然后在struct（类）内部的 .h 文件中，创建了一个不透明的指针：

struct Maze
{
    void* _private;
};

我稍后在构造函数中分配如下：

void* Maze_ctor(void* self, va_list *ap)
{
    struct Maze* this = self;

    this->DimX = va_arg(*ap, unsigned char);
    this->DimY = va_arg(*ap, unsigned char);

    this->_private = &(struct _private)          // passing address of struct to void*
    { 
        .SizeInBytes = this->DimX*this->DimY >> 1,
        .S = new(Stack),
        .ByteX = this->DimX % 8 > 0 ? this->DimX / 8 + 1 : this->DimX / 8
    };
    // 
    private.matrix = (unsigned char*)malloc(private.ByteX*this->DimY);
    S = new(Stack);     // this in my new() and it works similar to C++ new

    for (int i = 0; i < private.ByteX*this->DimY; i++)
        *(private.matrix + i) = 0;
}

这时一切正常，但随后我试图调用Next（）方法：

int Next(void* self, ...)
{
    struct Maze* this = self;

    struct _private *r = this->_private;

    short t;

    toBinary(this);          // after this point the struct private breaks
}

toBinary（）的原型是：

void toBinary(const void* self)
{
    // somehow char local is defined and equals to 204??
    struct Maze *this = self;
    struct _private *r = this->_private;

    unsigned char local;     // right after this point SizeInBytes equals to 204!
...
}

问题是：如何解决此问题。 禁止使用C ++！ 对于感兴趣的人：这是new（）

void* new(const void* _class,...)
{
    const struct Class* class = _class; // we need to convert pointer from void* to class* safely
    void *p = calloc(1, class->size);   // allocation of memory for class .using size param

    assert(p);                          // if Null -> throw an error
    *(const struct Class**)p = class;   // safe assignment of class pointer to (value) of p, to have memory and built in funcs
    if (class->ctor)                    // if has constructor with some dynal in it, execute with varargs on its input
    {
        va_list ap;
        va_start(ap, _class);           // 
        p = class->ctor(p, &ap);        // pass arguments as a list of pointers.
        va_end(ap);
    }
    return p;                           //returns a pointer to class pointer (weird but worx)
}

Answer 1

如注释中所指出，问题在于您创建了一个本地对象并将其分配给指针this。在该函数之外，this的值无效。

您编码，

void* Maze_ctor(void* self, va_list *ap)
{
//....
// this creates a temporary object and will be destroyed after Maz_ctor returns.

this->_private = &(struct _private)          // passing address of struct to void*
{ 
    .SizeInBytes = this->DimX*this->DimY >> 1,
    .S = new(Stack),
    .ByteX = this->DimX % 8 > 0 ? this->DimX / 8 + 1 : this->DimX / 8
};
// ---
}

Answer 2

感谢@Sami Kuhmonen指出动态分配，并感谢@CS Pei进行错误分析。我为解决此问题所做的事情是：

struct Maze
{
    char _private[32]; // allocate the memory size of struct(32)
}


// assign values to void ptr
private.SizeInBytes = this->DimX*this->DimY >> 1;
private.S = new(Stack);
private.ByteX = this->DimX % 8 > 0 ? this->DimX / 8 + 1 : this->DimX / 8;
private.matrix = (unsigned char*)malloc(private.ByteX*this->DimY);

现在可以按预期运行，但速度稍慢

在函数中调用时，如何解决匿名创建的结构的奇怪行为？

2 个答案: