i创建了用于SDL2库方法的包装函数,以返回使用自定义删除器的智能指针。对于unqiue_ptr(类图像(,它似乎可以正常工作,但是在构建过程中返回共享_ptr(类窗口(的类以下错误:
'<function-style-cast>': cannot convert from 'initializer list' to 'std::shared_ptr<SDL_Window>'
SDL_CreateWindow
这里返回RAW SDL_Window*
,IMG_Load
返回RAW SDL_Surface*
。
我已经尝试将Deleter
移至公共场所并删除窗口类的复制限制,但仍会出现同样的错误。另外,如果我只是从Window的功能铸件中返回nullptr
,则可以构建正常。因此,问题似乎是在共享_ptr本身的创建中。让我感到困惑的是,为什么它可以与unique_ptr
一起工作,但不能使用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:窗口类应不会像图像类
那样构建窗口类别实际结果:窗口类失败,上面给出的错误,而图像类则构建良好
更新:通过将共享_ptr创建逻辑移动到简单函数,进一步缩小了缩小,我发现删除自定义Deleter()
删除构建错误。因此,这似乎是罪魁祸首。但是我需要deleter,也需要,为什么使用unique_ptr可以与image相同的效果。
我略微缩小了您的示例:
#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();
}
现在问题更为明显:
/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) {
看到它在 coliru 上进行失败。
如果将public
添加到Deleter类或使其成为结构,则它将起作用。但是,如果这是需要做的所有工作,您也可以跳过该类,然后直接通过删除功能(或者如果lambda更为复杂,请使用(:
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 );
});
}
看到它在 coliru 。
上实时工作另外,使用这样的Window::operator()
是可疑的智慧。我建议您制作非成员或静态成员出厂功能以制作Windows。