如何定义返回嵌套类对象的模板类的成员函数 [英] how to define a member function of template class which return a nested class object

问题描述

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 以及嵌套类 Event ，我正在编写 Poller 的成员函数，该函数返回 Event 对象，但编译器报告
错误C2061语法错误：标识符'Event'IPC poller.cpp 8
，如何我该怎么办？谢谢！

I define a class template Poller and also a nested class Event, I am writing a member function of Poller which return a Event object,but the compiler reports " Error C2061 syntax error: identifier 'Event' IPC poller.cpp 8 ", how should I do? thank you!

推荐答案

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();
}

---

我注意到了一些令人担忧的问题：

---

I noticed a few issues of concern:

• 1） std :: shared_ptr< IPC> ipc; 我相信应该是 std :: shared_ptr< IPC_TYPE> ipc;


• 2）已经由 user：Hiroki -回答在 Poller< IPC_TYPE> :: Event 之前，需要使用typename 来声明类型名，以便编译器知道如何识别您的预期用途。请参阅他的答案，以获取更详细的描述和为什么需要 typename 的完整解释。


• 3）由于您声明了在超类主体之外的函数中， MSVS 2017 CE 给出了有关具有默认值的编译器错误。 （请参见下文）。


• 4）不确定是否要创建临时文件...然后通过其构造函数创建并返回实例，或者 template参数是您正在调用的 functor 或函数指针。


• 5）您在 Event std :: shared_ptr< IPC_TYPE> 成员c>，但没有看到为 IPC_TYPE 类型创建的任何动态内存。因此，我添加了一个用户定义的默认构造函数，该构造函数对此进行了设置，以便查看对象的构造函数，析构函数，运算符，成员函数等是否被正确调用，创建和销毁。

• 1) std::shared_ptr<IPC> ipc; I believe should be std::shared_ptr<IPC_TYPE> ipc;
• 2) Already been answered by user:Hiroki -- typename needs to be used before Poller<IPC_TYPE>::Event to declare a typename so that the compiler knows how to recognize your intended use. Refer to his answer for a more detailed description and a fuller explanation of why you need typename.
• 3) Since you are declaring the function outside of the super class's body, MSVS 2017 CE gives a compiler error about having a default value. (See Below).
• 4) Not sure if you are creating a temporary... Then creating and return an instance by its constructor or if the template argument will be some kind of functor or function pointer that you are invoking.
• 5) You have a std::shared_ptr<IPC_TYPE> member within Event but did not see any dynamic memory being created for type IPC_TYPE. So I added a user defined default constructor that set's this in order to see the object's constructors, destructors, operators, member functions, etc. being called, created and destroyed properly.

（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 ==========

有两种方法可以解决上述编译器错误：

There are 2 ways of fixing the above compiler error:

• A）删除超类之外定义中的默认值。


• B）在内部类中编写函数的主体。如果决定选择这种编写函数体的方法；

下面是您上一堂课的工作示例：

Here is a working example of your class above:

#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;
}

---

---

-Output-

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

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