如何定义返回嵌套类对象的模板类的成员函数

Question

template<typename IPC_TYPE>
class Poller
{
private:

public:
    struct Event
    {
        std::shared_ptr<IPC> ipc;
        enum Status
        {
            NONE = 0, POLLIN = 1, POLLHUP = 2, MessageArrival = 3
        }status;
    };

    //block wait
    Event wait(size_t max_wait_time = 50);
};

 template<typename IPC_TYPE>
    Poller<IPC_TYPE>::Event Poller<IPC_TYPE>::wait(size_t max_wait_time = 50)
    {
        Event e;
        return Event();
    }

我定义了一个类模板Poller 和一个嵌套类Event，我正在编写一个Poller 的成员函数，它返回一个Event 对象，但是编译器报告" 错误 C2061 语法错误：标识符“事件”IPC poller.cpp 8 "，我该怎么办？谢谢！

Answer 1

编译器不知道Poller<IPC_TYPE>::Event是Poller<IPC_TYPE>的成员变量还是嵌套类型。

因此我们必须输入typename 来消除这种歧义，如下所示：

DEMO is here.

template<typename IPC_TYPE>
typename Poller<IPC_TYPE>::Event Poller<IPC_TYPE>::wait(size_t max_wait_time)
{
    Event e;
    return Event();
}

Answer 2

查看您当前的代码：

template<typename IPC_TYPE>
class Poller {    
public:
    struct Event {
        std::shared_ptr<IPC> ipc;
        enum Status
        {
            NONE = 0, POLLIN = 1, POLLHUP = 2, MessageArrival = 3
        } status;
    };

    //block wait
    Event wait(size_t max_wait_time = 50);
};

template<typename IPC_TYPE>
Poller<IPC_TYPE>::Event Poller<IPC_TYPE>::wait(size_t max_wait_time = 50) {
    Event e;
    return Event();
}

---

我注意到几个问题：

• 1) std::shared_ptr<IPC> ipc; 我相信应该是std::shared_ptr<IPC_TYPE> ipc;
• 2) 已经被user:Hiroki 回答——typename 需要在Poller<IPC_TYPE>::Event 之前使用来声明类型名，以便编译器知道如何识别您的预期用途。请参阅他的回答，以获得更详细的描述和更完整的说明，了解您为什么需要 typename。
• 3) 由于您在超类的主体之外声明函数，MSVS 2017 CE 给出了有关具有默认值的编译器错误。 （见下文）。
• 4) 不确定您是否正在创建一个临时... 然后通过其构造函数创建并返回一个实例，或者template argument 是否将是您正在调用的某种functor 或function pointer。
• 5) 您在Event 中有一个std::shared_ptr<IPC_TYPE> 成员，但没有看到为类型IPC_TYPE 创建任何动态内存。所以我添加了一个用户定义的默认构造函数来设置这个，以便查看对象的构造函数、析构函数、运算符、成员函数等被正确调用、创建和销毁。

---

(3) - 编译器错误：

1>------ Build started: Project: StackQA, Configuration: Debug Win32 ------
1>main.cpp
1>c:\users\...\main.cpp(41): error C5037: 'Poller<IPC_TYPE>::wait': an out-of-line definition of a member of a class template cannot have default arguments
1>Done building project "StackQA.vcxproj" -- FAILED.
========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========

有两种方法可以修复上述编译器错误：

• A) 删除超类之外的定义中的默认值。
• B) 在内部类中编写函数体。如果您决定选择这种编写函数体的方法；它实际上会首先消除您问题的全部需求和目的，因为您将在内部类中定义它。

---

这是你上面的类的一个工作示例：

#include <iostream>
#include <exception>
#include <memory>

// Classes A & B are just basic classes with ctor & dtor displaying a message
class A {
public:
    A() { std::cout << "A CTOR called\n"; }
    ~A() { std::cout << "A DTOR called\n"; }
};

class B {
public:
    B() { std::cout << "B CTOR called\n"; }
    ~B() { std::cout << "B DTOR called\n"; }
};

// Classes C & D are functors where their operator invokes a message to be displayed
class C {
public:
    void operator()() { std::cout << "Functor C called\n"; }
};

class D {
public:
    void operator()() { std::cout << "Functor D called\n"; }
};

template <typename IPC_TYPE>
class Poller {
public:
    struct Event {
        std::shared_ptr<IPC_TYPE> ipc; // Made correction here from IPC to IPC_TYPE
        enum Status {
            NONE = 0,
            POLLIN = 1,
            POLLHUP = 2,
            MESSAGE_ARRIVAL = 3, // Changed to All Caps... (personal preference)
        } status;

            // Added this constructor to actually make a shared_ptr of IPC_TYPE
        Event() {
            ipc = std::make_shared<IPC_TYPE>();
        }    
    };

    // Defined the function body within the inner class which also prevents your compiler error.
    Event wait( size_t max_wait_time = 50 ) {
        // Not sure of your intentions here, but for demonstration purposes
        // I've just commented out the temporary and just returned the ctor
        // Event e;
        return Event();
    }
};

// To define it outside of class remove the body from the inner class above,
// uncomment this section, and don't forget to use `typename`.
// Also make sure that your parameter does not have a default value here.
/*template<typename IPC_TYPE>
typename Poller<IPC_TYPE>::Event Poller<IPC_TYPE>::wait( size_t wait_time ) {
    // Not sure of your intentions here, but for demonstration purposes
    // I've just commented out the temporary and just returned the ctor
    //Event e;
    return Event();
}
*/    

int main() {
    try {
        Poller<A> p1;
        p1.wait( 10 );

        Poller<B> p2;
        p2.wait( 12 );

        Poller<C> p3;       
        Poller<C>::Event e1 = p3.wait( 7 );
        e1.ipc->operator()();

        Poller<D> p4;
        Poller<D>::Event e2 = p4.wait( 9 );
        e2.ipc->operator()();

    } catch( std::runtime_error& e ) {
        std::cerr << e.what() << std::endl;
        return EXIT_FAILURE;
    }
    return EXIT_SUCCESS;
}

---

-输出-

A CTOR called
A DTOR called
B CTOR called
B DTOR called
Functor C called
Functor D called