错误：函数返回一个函数

Question

虽然至少有one similar question，但我仍然问我的，因为那个还没有解决，而且看起来更复杂。我正在尝试简化我的。

我有一个使用 .h 的 .cpp 文件，如下所示，编译这些棚子错误如下。任何想法都值得赞赏。请注意，简化代码是为了尽量减少仅显示有问题的部分。

FC_boost_prove.h：

#ifndef FC_H
#define FC_H

#include <vector>
#include "iostream"

#include "boost/signal.hpp"
#include "boost/bind.hpp"
#include <boost/random.hpp>

typedef boost::signal0<void()> PreUpdateSignal;
typedef PreUpdateSignal::slot_function_type PreUpdateSlot;
typedef boost::signal0<void()> PostUpdateSignal;
typedef PostUpdateSignal::slot_function_type PostUpdateSlot;

class FC {
public:
    FC(uint width, uint height) {
        std::cout << "In constructor." << std::endl;
    }
    ~FC() {
        //Do ...
    }
    void connectPreUpdate(PreUpdateSlot s) {
        preUpdateSignal_.connect(s);
    }
    void connectPostUpdate(PostUpdateSlot s) {
        postUpdateSignal_.connect(s);
    }

protected:
    PreUpdateSignal preUpdateSignal_;
    PostUpdateSignal postUpdateSignal_;
};
#endif

FC_boost_prove.cpp：

#include <iostream>
#include <string>
#include "FC_boost_prove.h"

int main() {
    std::cout << "test." << std::endl;
}

编译错误：

$ g++ FC_boost_prove.cpp 
In file included from /usr/include/boost/signals/signal_template.hpp:22,
                 from /usr/include/boost/signals/signal0.hpp:24,
                 from /usr/include/boost/signal.hpp:19,
                 from FC_boost_prove.h:7,
                 from FC_boost_prove.cpp:3:
/usr/include/boost/last_value.hpp: In instantiation of ‘boost::last_value<void()>’:
/usr/include/boost/signals/signal_template.hpp:178:   instantiated from ‘boost::signal0<void(), boost::last_value<void()>, int, std::less<int>, boost::function0<void()> >’
FC_boost_prove.h:12:   instantiated from here
/usr/include/boost/last_value.hpp:22: error: function returning a function
In file included from /usr/include/boost/signals/signal0.hpp:24,
                 from /usr/include/boost/signal.hpp:19,
                 from FC_boost_prove.h:7,
                 from FC_boost_prove.cpp:3:
/usr/include/boost/signals/signal_template.hpp: In instantiation of ‘boost::signal0<void(), boost::last_value<void()>, int, std::less<int>, boost::function0<void()> >’:
FC_boost_prove.h:12:   instantiated from here
/usr/include/boost/signals/signal_template.hpp:330: error: function returning a function
/usr/include/boost/signals/signal_template.hpp:370: error: function returning a function
In file included from /usr/include/boost/function/detail/maybe_include.hpp:13,
                 from /usr/include/boost/function/function0.hpp:11,
                 from /usr/include/boost/signals/signal_template.hpp:38,
                 from /usr/include/boost/signals/signal0.hpp:24,
                 from /usr/include/boost/signal.hpp:19,
                 from FC_boost_prove.h:7,
                 from FC_boost_prove.cpp:3:
/usr/include/boost/function/function_template.hpp: In instantiation of ‘boost::function0<void()>’:
FC_boost_prove.h:24:   instantiated from here
/usr/include/boost/function/function_template.hpp:1006: error: function returning a function
/usr/include/boost/function/function_template.hpp: In instantiation of ‘boost::detail::function::basic_vtable0<void()>’:
/usr/include/boost/function/function_template.hpp:856:   instantiated from ‘void boost::function0<R>::clear() [with R = void()]’
/usr/include/boost/function/function_template.hpp:752:   instantiated from ‘boost::function0<R>::~function0() [with R = void()]’
/usr/include/boost/signals/slot.hpp:105:   instantiated from here
/usr/include/boost/function/function_template.hpp:486: error: function returning a function
/usr/include/boost/function/function_template.hpp:643: error: function returning a function

环境：Ubuntu 10.10、g++ (Ubuntu/Linaro 4.4.4-14ubuntu5) 4.4.5

Answer 1

为什么要指定boost::signal0<>？ signalN 模板适用于无法正确解析函数签名的有缺陷的编译器。

按照现代编译器的建议，使用信号并指定函数签名：

typedef boost::signal<void()> PreUpdateSignal;
typedef boost::signal<void()> PostUpdateSignal;

或使用信号N 并显式指定返回类型（以及每个参数类型），这需要有缺陷的编译器：

typedef boost::signal0<void> PreUpdateSignal;
typedef boost::signal0<void> PostUpdateSignal;