如何将 class 的静态方法作为 std::unique_ptr 的删除器传递？

Question

我一直在尝试将遵循 class 的静态方法传递为 unique_ptr 的 deleter，但我无法找出正确的语法。如果我刚开始使用 unique_ptr.

，如果下面的语法非常错误，请原谅

#include <iostream>
#include <memory>


class Singleton {

public:

    static void deleter(Singleton* p) {
        delete p;
        std::cout << "In deleter\n" << std::endl;
    }

    static std::unique_ptr<Singleton, decltype(&Singleton::deleter)>& getInstance(void);

    void hello (void) {
       std::cout << "Hello!\n" << std::endl;
    }

    Singleton(const Singleton&) = delete;
    Singleton(Singleton&&) = delete;
    Singleton& operator=(const Singleton&) = delete;
    Singleton& operator=(Singleton&&) = delete;

private:
    static std::unique_ptr<Singleton, decltype(&Singleton::deleter)> s_instance_;

    Singleton() { std::cout << "Constructed\n" << std::endl; }

    ~Singleton() { std::cout << "Destroyed\n" << std::endl; }

};

std::unique_ptr<Singleton, decltype(&Singleton::deleter)>&
Singleton::getInstance (void)
{
    if (s_instance_ == nullptr) {
        s_instance_ = std::unique_ptr<Singleton, decltype(&Singleton::deleter)>(new Singleton(), &Singleton::deleter);
    }

    return s_instance_;
}

std::unique_ptr<Singleton, decltype(&Singleton::deleter)> Singleton::s_instance_{nullptr};

int main ()
{
    bool some_condition = true;
    std::unique_ptr<Singleton, decltype(&Singleton::deleter)> &ins = Singleton::getInstance();

    ins->hello();

    if (some_condition) {
       ins.reset();
    }

    std::cout << "End of main\n" << std::endl;
    return 0;
}

g++ (--version == g++ (Ubuntu 5.4.0-6ubuntu1~16.04.12) 5.4.0 20160609)

出现以下错误

In file included from /usr/include/c++/5/memory:81:0,
                 from uptr.cpp:2:
/usr/include/c++/5/bits/unique_ptr.h: In instantiation of ‘constexpr std::unique_ptr<_Tp, _Dp>::unique_ptr() [with _Tp = Singleton; _Dp = void (*)(Singleton*)]’:
/usr/include/c++/5/bits/unique_ptr.h:200:61:   required from ‘constexpr std::unique_ptr<_Tp, _Dp>::unique_ptr(std::nullptr_t) [with _Tp = Singleton; _Dp = void (*)(Singleton*); std::nullptr_t = std::nullptr_t]’
uptr.cpp:44:89:   required from here
/usr/include/c++/5/bits/unique_ptr.h:159:9: error: static assertion failed: constructed with null function pointer deleter
       { static_assert(!is_pointer<deleter_type>::value,
         ^

还有，有什么办法可以缩短 std::unique_ptr<Singleton, decltype(&Singleton::deleter)> 吗？我尝试在 class 定义之前使用以下方法，但编译器因类型不完整而出错：

using SingletonUPtr = std::unique_ptr<Singleton, decltype(&Singleton::deleter)>;

Answer 1

错误在第 44 行，这似乎是 Singleton::s_instance_ 变量的定义。您不在那里提供删除功能。您应该将其包含在定义中，就像您之前在 getInstance.

中所做的那样

std::unique_ptr<Singleton, decltype(&Singleton::deleter)> Singleton::s_instance_{nullptr, &Singleton::deleter};

Answer 2

不需要同时存储类型和值。该类型可以进行调度。

在c++11中你可以只写这个玩具：

template<class F, F f>
struct call_t {
  constexpr operator F() const{ return f; }
};

神奇的效果。（当您使用 () 调用它时，C++ 会查找到函数指针的转换）。这是 std::integral_constant 的一部分的实现，但 c++11 中缺少一些功能。在这种情况下，我将其用作调用类型 F 的常量评估值 f 的类型。它还有其他（类似）用途。

试试这个：

void foo() { std::cout << "hello world\n"; }
int main() {
  using foo_t = call_t< void(*)(), foo >;
  foo_t foo_v = {};
  foo_v(); // hello world
  foo_t{}(); // hello world
  std::cout << sizeof(foo_t) << "\n"; // 1, object is stateless
  std::cout << sizeof(&foo) << "\n"; // 4 or 8, pointer has state
}

剩下的看起来像：

class Singleton {

public:

    static void deleter(Singleton* p) {
        delete p;
        std::cout << "In deleter\n" << std::endl;
    }
    using upSingleton = std::unique_ptr<Singleton, call_t<decltype(&Singleton::deleter), &Singleton::deleter> >;

    static upSingleton& getInstance(void);

    void hello() {
       std::cout << "Hello!\n" << std::endl;
    }

    Singleton(const Singleton&) = delete;
    Singleton(Singleton&&) = delete;
    Singleton& operator=(const Singleton&) = delete;
    Singleton& operator=(Singleton&&) = delete;

private:
    static upSingleton s_instance_;

    Singleton() { std::cout << "Constructed\n" << std::endl; }

    ~Singleton() { std::cout << "Destroyed\n" << std::endl; }
};

Singleton::upSingleton&
Singleton::getInstance()
{
    if (s_instance_ == nullptr) {
        s_instance_ = upSingleton(new Singleton());
    }

    return s_instance_;
}

Singleton::upSingleton Singleton::s_instance_{nullptr};

int main()
{
    bool some_condition = true;
    Singleton::upSingleton &ins = Singleton::getInstance();

    ins->hello();

    if (some_condition) {
       ins.reset();
    }

    std::cout << "End of main\n" << std::endl;
    return 0;
}

Live example.

如何将 class 的静态方法作为 std::unique_ptr 的删除器传递？

How to pass static method of the class as deleter for std::unique_ptr?

c++

c++11

singleton

unique-ptr