将抽象方法 class 作为 std::function 传递

Question

我有一些这样的代码（这只是一个片段，不是完全有效的代码）：

class AbstractClass {
public:
    AbstractClass() {}
    virtual ~AbstractClass() {}
    virtual void doA() { std::cout << "doA1\n"; };
    virtual void doB() = 0;
};

class ImplClass : public AbstractClass {
public:
    ImplClass() {}
    virtual ~ImplClass() {}
    virtual void doA() override { std::cout << "doA2\n"; };
    virtual void doB() override { std::cout << "doB2\n"; };
};

现在我想像这样在 std::function 中存储对 AbstractClass 成员方法的调用：

int main() {
    AbstractClass* aClass = new ImplClass();
    std::function<void()> func = std::bind(&AbstractClass::doA, *aClass)

    delete aClass;
    return 0;
}

然而，根据编译器的说法，由于 AbstractClass 中存在纯虚方法，这不起作用。

编辑：完整错误供参考

In file included from source_file.cpp:2:0:
/usr/include/c++/5/functional: In instantiation of ‘struct std::_Bind_helper<false, void (AbstractClass::*)(), AbstractClass&>’:
/usr/include/c++/5/functional:1462:5:   required by substitution of ‘template<class _Func, class ... _BoundArgs> typename std::_Bind_helper<std::__is_socketlike<_Func>::value, _Func, _BoundArgs ...>::type std::bind(_Func&&, _BoundArgs&& ...) [with _Func = void (AbstractClass::*)(); _BoundArgs = {AbstractClass&}]’
source_file.cpp:26:72:   required from here
/usr/include/c++/5/functional:1445:71: error: invalid abstract parameter type ‘AbstractClass’
       typedef _Bind<__func_type(typename decay<_BoundArgs>::type...)> type;
                                                                       ^
source_file.cpp:5:7: note:   because the following virtual functions are pure within ‘AbstractClass’:
 class AbstractClass {
       ^
source_file.cpp:10:18: note:    virtual void AbstractClass::doB()
     virtual void doB() = 0;
                  ^
source_file.cpp: In function ‘int main()’:
source_file.cpp:26:72: error: no matching function for call to ‘bind(void (AbstractClass::*)(), AbstractClass&)’
     std::function<void()> func = std::bind(&AbstractClass::doA, *aClass)
                                                                        ^
In file included from source_file.cpp:2:0:
/usr/include/c++/5/functional:1462:5: note: candidate: template<class _Func, class ... _BoundArgs> typename std::_Bind_helper<std::__is_socketlike<_Func>::value, _Func, _BoundArgs ...>::type std::bind(_Func&&, _BoundArgs&& ...)
     bind(_Func&& __f, _BoundArgs&&... __args)
     ^
/usr/include/c++/5/functional:1462:5: note:   substitution of deduced template arguments resulted in errors seen above
/usr/include/c++/5/functional:1490:5: note: candidate: template<class _Result, class _Func, class ... _BoundArgs> typename std::_Bindres_helper<_Result, _Func, _BoundArgs>::type std::bind(_Func&&, _BoundArgs&& ...)
     bind(_Func&& __f, _BoundArgs&&... __args)
     ^
/usr/include/c++/5/functional:1490:5: note:   template argument deduction/substitution failed:
source_file.cpp:26:72: note:   couldn't deduce template parameter ‘_Result’
     std::function<void()> func = std::bind(&AbstractClass::doA, *aClass)
                                                                        ^

我的问题有什么解决办法吗？目前我还没有找到。

Answer 1

您的代码按值绑定了 aClass 指向的对象。你想通过引用绑定它：

std::function<void()> func = std::bind(&AbstractClass::doA, std::ref(*aClass));

或者直接绑定aClass，指针也可以用于隐式this:

std::function<void()> func = std::bind(&AbstractClass::doA, aClass);

[Live example]

Answer 2

目前最好的建议是使用 lambda 而不是 std::bind，自 lambda 出现以来，这在标准中被视为不合时宜。

Lambdas：

更明确
效率更高
不那么令人困惑
不需要像 std::ref 和 std::cref

示例：

#include <functional>
#include <iostream>

class AbstractClass {
public:
    AbstractClass() {}
    virtual ~AbstractClass() {}
    virtual void doA() { std::cout << "doA1\n"; };
    virtual void doB() = 0;
};

class ImplClass : public AbstractClass {
public:
    ImplClass() {}
    virtual ~ImplClass() {}
    virtual void doA() override { std::cout << "doA2\n"; };
    virtual void doB() override { std::cout << "doB2\n"; };
};

int main() {
    AbstractClass* aClass = new ImplClass();

    // aClass is a pointer, and a copy of that pointer will be
    // captured. 
    std::function<void()> func = [aClass]{ aClass->doA(); };

    func();

    delete aClass;
    return 0;
}

将抽象方法 class 作为 std::function 传递

Passing the method of an abstract class as std::function

c++

lambda

std