c++ - 自动释放由外部 api 分配的对象

Question

我有一个 Visual Studio 2013 C++11 程序，我需要使用一个在数组中分配多个字符缓冲区的 API:

int main(int argc, char* argv[])
{
    char* names[10] = {};
    unsigned int count = _countof(names);
    some_external_api_to_get_values(count, names, &count);
    // do something with the names array...
    for (unsigned int i = 0; i < count; ++i)
        release_objects(names[i]);
    return 0;
}

我希望对象在销毁时自动释放。所以我添加了这个类，但是我遗漏了一些允许编译器在这个类和它期望的 char** 类型之间进行转换的东西。

template<typename T>
class release_objects_on_destruction : private boost::noncopyable
{
public:
    explicit release_objects_on_destruction(T* p) : p_(p) { };
    release_objects_on_destruction() : p_(nullptr) { };
    ~release_objects_on_destruction() { if (p_) release_objects(p_); };
    T** operator&() { return &p_; };
    operator T*() const { return p_; };    
    operator const T*() const { return p_; };
private:
    T* p_;
};

int main( int argc, char* argv[])
{
    release_objects_on_destruction<char> names[10];
    unsigned int count = _countof(names);
    // error C2664: 'int some_external_api_to_get_values(unsigned int,char**,unsigned int*)' : cannot convert argument 2 from 'release_objects_on_destruction<char> [10]' to 'char**'
    some_external_api_to_get_values(count, names, &count);
    return 0;
}

我缺少什么转换运算符？

Answer 1

它不起作用，因为您的 API 处于错误的级别。您需要将 external_api 调用和 release_objects 调用封装到同一个对象中。这就是您当前的包装器失败的原因。

class api_wrapper {
    static const object_count = 10;
    std::array<char*, object_count> objects;
    unsigned count = object_count;
public:
    api_wrapper() {
        some_external_api_to_get_values(count, objects.data(), &count);
    }
    // example accessors
    char** data() { return objects.data(); }
    unsigned count() { return count; }

    api_wrapper(api_wrapper&& other)
        : objects(other.objects), count(other.count)
    {
        other.count = 0;
    }
    api_wrapper(const api_wrapper&) = delete;
    ~api_wrapper() {
        for (unsigned int i = 0; i < count; ++i)
            release_objects(objects[i]);
    }
};