如何定义模板 class 的成员函数，其中 return 嵌套 class 对象

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 class 模板 Poller 和一个嵌套的 class Event，我正在编写 Poller 的成员函数 return 一个 Event 对象，但编译器报告 " Error C2061 语法错误：标识符'Event' 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

因此我们必须键入 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();
}

我注意到一些值得关注的问题：

1) std::shared_ptr<IPC> ipc; 我认为应该是 std::shared_ptr<IPC_TYPE> ipc;
2) 已被 user:Hiroki 回答——需要在 Poller<IPC_TYPE>::Event 之前使用 typename 来声明类型名，以便编译器知道如何识别您的预期用途。请参阅他的回答以获得更详细的描述和更完整的解释为什么你需要 typename.
3) 由于您是在超级 class 的主体之外声明函数，因此 MSVS 2017 CE 会给出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和 return 一个实例，或者 template argument 是否是某种 functor 或function pointer 您正在调用。
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 创建的任何动态内存。所以我添加了一个用户定义的默认构造函数，它设置了 this 以便看到对象的构造函数、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) 删除定义中超出超级 class.
B) 在内部 class 中写入函数体。如果您决定选择这种编写函数体的方法；它实际上首先会消除您问题的全部需求和目的，因为您将在内部 class.

这是上面 class 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;
}

-Output-

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

如何定义模板 class 的成员函数，其中 return 嵌套 class 对象

how to define a member function of template class which return a nested class object

c++

templates

inner-classes

visual-c++

c++11