使用指向C库中结构的智能指针，它通过typedef的实现（即不完整的类型）隐藏了实现]]

Question

我正在尝试使用一个C库，该库基本上只将typedefs公开给内部使用的结构。问题是我想使用智能指针来管理作为库接口的原始指针的生存期，但是由于出现incomplete_type错误，因此无法创建智能指针。注意，我已经问过一个先前的问题previous question，试图解决相同的问题，但是这个问题对我的实际问题却表现不佳：

我无法弄清楚如何使用智能指针指向C库中所需的类型。取而代之的是，我一直在使用指向基础原始指针的智能指针（请参见下文），但这并不是我想要的，而且可能也不理想。

这里是一些代码：

#include "librdf.h" // the library I need to use


int main() {
    // call necessary to use the librdf library. 
    librdf_world *world = librdf_new_world();
    librdf_world_open(world);

    /* 
     * Error: In instantiation of function template specialization
     * 'std::make_unique<librdf_world_s, librdf_world_s *&>'
     * allocation of incomplete type 'librdf_world_s'
     */
    std::unique_ptr<librdf_world> w1 = std::make_unique<librdf_world>(world);

    /* // errors: 
     * 1) Template argument for template type parameter must be a type // (on librdf_free_world left)
     *
     * 2) No matching function for call to 'make_unique' // (on std::make_unique<librdf_world, librdf_free_world>, right)
     *
     */
    std::unique_ptr<librdf_world, librdf_free_world> w2 = std::make_unique<librdf_world, librdf_free_world>(world);

    /* Error: 
     * In instantiation of template class 'std::unique_ptr<librdf_world_s, void (librdf_world_s *)>'
     * data member instantiated with function type 'void (librdf_world_s *)' // (on w3)
     *
     * No matching function for call to 'make_unique' candidate function template not viable:
     * cannot convert argument of incomplete type 'librdf_world *'
     * (aka 'librdf_world_s *') to 'void (&&)(librdf_world_s *)' for 1st argument // (on std::make_unique<librdf_world, decltype(librdf_free_world)>)
     */
    std::unique_ptr<librdf_world, decltype(librdf_free_world)> w3 = std::make_unique<librdf_world, decltype(librdf_free_world)>(world);


    /* No error: 
     * This version actually works and has been my strategy for a while now. Note,
     * I've been using shared_ptr because I need other objects to have a reference
     * to the `world` to create other objects from the library. An example call to the library would be:
     *     librdf_new_node(world, ... other arguments ... );
     * However using a smart pointer to a raw pointer doesn't solve the problem of automatically
     * managing the lifetime of the underlying raw pointer (according to valgrind). My attempt at
     * overcoming this issue is to wrap the smart pointer in a class and have the shared_ptr as
     * the single private variable. Then, in the destructor, when the shared_ptr::use_count gets to 1
     * I call the C destructor.
     *  i.e.
     *      ~LibrdfWorld(){ // wrapper class name
     *          if (world_.use_count == 1){ // world_ is private instance of shared pointer to librdf_world*
     *              librdf_free_world(*world_.getWorld()); // call the c library destructor when ref count gets to 1
     *          }
     *
     */
    std::shared_ptr<librdf_world*> w4 = std::make_shared<librdf_world*>(world);
}

那么从c ++使用此库的最佳方法是什么？我是否使用原始指针并手动管理生命周期？我是否使用智能指针指向原始指针？还是我没有想到的另一种方式。 

我正在尝试使用一个C库，该库基本上只将typedef公开给内部使用的结构。问题是我想使用智能指针来管理原始指针的生命周期，这些原始指针的生命周期...

Answer 1

std::make_shared和std::make_unique用于分配然后构造对象，并将其作为shared_或unique_指针返回。它希望接收将传递给您的类的构造函数的参数，并且需要完整的类型以便可以调用此构造函数。