C 实现智能指针

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• C++11版本开始支持智能指针的原理其实就是引用计数，C并不原生支持，但可以通过一些方法模拟出来，下面是参考 libre 的实现，可以使用 Valgrind 调试内存的使用状态，不废话，用代码解释代码：

#include <stdio.h>
#include <ctype.h>
#include <stdint.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>

typedef void (mem_destroy_h)(void *data);

/** Defines a reference-counting memory object */
struct mem {
    uint32_t nrefs;     /**< Number of references  */
    mem_destroy_h *dh;  /**< Destroy handler       */
};

/**
 * Allocate a new reference-counted memory object
 *
 * @param size Size of memory object
 * @param dh   Optional destructor, called when destroyed
 *
 * @return Pointer to allocated object
 */
void *mem_alloc(size_t size, mem_destroy_h *dh)
{
    struct mem *m;

    m = malloc(sizeof(*m) + size);
    if (!m)
        return NULL;

    m->nrefs = 1;
    m->dh    = dh;

    return (void *)(m + 1);
}

/**
 * Reference a reference-counted memory object
 *
 * @param data Memory object
 *
 * @return Memory object (same as data)
 */
void *mem_ref(void *data)
{
    struct mem *m;

    if (!data)
        return NULL;

    m = ((struct mem *)data) - 1;

    ++m->nrefs;

    return data;
}

/**
 * Dereference a reference-counted memory object. When the reference count
 * is zero, the destroy handler will be called (if present) and the memory
 * will be freed
 *
 * @param data Memory object
 *
 * @return Always NULL
 */
void *mem_deref(void *data)
{
    struct mem *m;

    if (!data)
        return NULL;

    m = ((struct mem *)data) - 1;

    if (--m->nrefs > 0)
        return NULL;

    if (m->dh)
        m->dh(data);

    /* NOTE: check if the destructor called mem_ref() */
    if (m->nrefs > 0)
        return NULL;

    free(m);

    return NULL;
}

int main(int argc, const char * argv[])
{
    void * data_foo = mem_alloc(512, NULL);
    data_foo = mem_deref(data_foo);
    return 0;
}

参考文档：

1. https://github.com/creytiv/re/