c++ - '': cannot convert from ' initializer list' to 'std::shared_ptr' on creating shared_ptr with custom deleter

Question

我为 SDL2 库方法创建了包装仿函数，以返回带有自定义删除器的智能指针。它似乎适用于 unqiue_ptr(类 Image)，但对于在构建期间返回 shared_ptr(类 Window)的类给出以下错误:

'<function-style-cast>': cannot convert from 'initializer list' to 'std::shared_ptr<SDL_Window>'

SDL_CreateWindow这里返回原始 SDL_Window*和 IMG_Load返回原始 SDL_Surface* .

我试过移动Deleter公开并删除 Window 类的复制限制，但它仍然失败并出现相同的错误。另外，如果我只返回 nullptr从 Window 的函数转换中，它构建得很好。所以问题似乎与 shared_ptr 本身的创建有关。令我感到困惑的是为什么它与 unique_ptr 一起工作得很好但不是 shared_ptr .

#pragma once
#include <memory>
#include <SDL.h>
#include "Uncopyable.h"

// fails during build with error: '<function-style-cast>': 
// cannot convert from 'initializer list' to 'std::shared_ptr<SDL_Window>'
class Window:private Uncopyable {
private:


public:
    class Deleter {
        void operator()(SDL_Window *window) {
            SDL_DestroyWindow(window);
        }
    };

    static const int SCREEN_WIDTH = 800;
    static const int SCREEN_HEIGHT = 600;
    std::shared_ptr<SDL_Window> operator()() const {
        return std::shared_ptr<SDL_Window>(
            SDL_CreateWindow("SDL Tutorial",
                SDL_WINDOWPOS_UNDEFINED,
                SDL_WINDOWPOS_UNDEFINED,
                SCREEN_WIDTH,
                SCREEN_HEIGHT,
                SDL_WINDOW_SHOWN),
            Deleter());
    }
};

#pragma once
#include <memory>
#include <string>
#include <SDL.h>
#include <SDL_image.h>
#include "Uncopyable.h"

// builds fine
class Image: private Uncopyable {
public:
    class Deleter{
        void operator()(SDL_Surface *image) {
            SDL_FreeSurface(image);
        }
    };

    std::unique_ptr<SDL_Surface, Deleter> operator()(const std::string &path) const {
        return std::unique_ptr<SDL_Surface, Deleter>(
            IMG_Load(path.c_str()),
            Deleter());
    }
};

预期结果:Window 类应该像 Image 类一样正确构建

实际结果:Window 类失败并出现上述错误，而 Image 类构建良好

更新:通过将 shared_ptr 创建逻辑移动到简单函数来进一步缩小范围，我发现删除自定义 Deleter()删除构建错误。所以它似乎是罪魁祸首。但我需要 Deleter，而且，为什么同样的结构使用 unique_ptr 可以很好地处理 Image。

Answer 1

我已经把你的例子精简了一点:

#include <memory>

// Stub these out since we don't have them available and they don't really matter
// for the substance of the question.
struct SDL_Window {};
void SDL_DestroyWindow( SDL_Window* win ) { delete win; }
SDL_Window* SDL_CreateWindow() { return new SDL_Window{}; }

// fails during build with error: '<function-style-cast>': 
// cannot convert from 'initializer list' to 'std::shared_ptr<SDL_Window>'
class Window {
public:
    class Deleter {
        void operator()(SDL_Window *window) {
            SDL_DestroyWindow(window);
        }
    };

    std::shared_ptr<SDL_Window> operator()() const {
        return std::shared_ptr<SDL_Window>(
            SDL_CreateWindow(),
            Deleter());
    }
};

int main()
{
    auto win = Window();
    auto sp = win();
}

现在问题更明显了:

/usr/local/include/c++/8.2.0/bits/shared_ptr_base.h:642:11: error: 
  'void Window::Deleter::operator()(SDL_Window*)' is private within this context
        __d(__p); // Call _Deleter on __p.
        ~~~^~~~~

main.cpp:16:14: note: declared private here
         void operator()(SDL_Window *window) {

在 Coliru 上实时查看它的失败情况.

如果您将 public 添加到您的删除器类或使其成为一个结构，它将起作用。但是，您也可以跳过该类并直接传入删除函数，如果这就是它需要做的所有工作(或者如果它更复杂一点，则使用 lambda):

    std::shared_ptr<SDL_Window> operator()() const {
        return std::shared_ptr<SDL_Window>(
            SDL_CreateWindow(),
            SDL_DestroyWindow);
    }

    // Or with a lambda if it's more complicated (here also using a factory func)
    static std::shared_ptr<SDL_Window> Create() {
        return std::shared_ptr<SDL_Window>(
            SDL_CreateWindow(),
            [] (auto win) { 
                UnregisterMyWindow( win );
                SDL_DestroyWindow( win ); 
             });
    }

在 Coliru 上实时查看它的工作情况.

此外，像那样使用 Window::operator() 是否明智也值得怀疑。我建议您创建一个非成员或静态成员工厂函数来代替创建窗口。