Question

我正在尝试创建一个回调注册类，允许针对不同类型的字符串标识符注册回调。每个回调都有签名void function( T val )，其中T是更改类型。

我创建了以下基本注册器类，它将字符串映射到函数。

#include <string>
#include <map>
#include <functional>
#include <cstdint>
#include <iostream>

using namespace std;

template< typename ValueType >
class BasicConfigCallbackRegistrar 
{
public:
    typedef ValueType value_type;
    typedef BasicConfigCallbackRegistrar< value_type > type;
    typedef function< void( const value_type ) > signature_type;
    typedef map< string, signature_type > callback_map_type;

    /// @brief constructor
    BasicConfigCallbackRegistrar() : callbackMap_()
    {
    }

    /// @brief register a create callback 
    /// @param nodePath the path identifying the node in config database
    /// @param callback the callback to register
    type& RegisterCallback( string nodePath, signature_type callback )
    {
        callbackMap_.insert( make_pair( move(nodePath), callback ) );
        return *this;
    }

    void MakeCallback( const string& nodePath, value_type val )
    {
        // no checking assumes item is in map,
        // do not do this in production code
        auto iter = callbackMap_.find( nodePath );
        iter->second( val );
    }

private:
    callback_map_type callbackMap_;       ///< the callback map
};

然后我使用可变参数模板为我想支持的每种类型创建派生类。

template< typename... Types >
class ConfigCallbackRegistrar : public BasicConfigCallbackRegistrar<Types>... 
{
public:
    /// @brief constructor
    ConfigCallbackRegistrar() : BasicConfigCallbackRegistrar<Types>()...
    {}
};

然后将其命名为：

typedef ConfigCallbackRegistrar< uint32_t, string > CallbackRegistrar;

当我尝试按如下方式使用此类时：

struct UintFtor
{
    void operator()( uint32_t val )
    {
        cout << val << "\n"; 
    }
};

struct StringFtor
{
    void operator ()( string val )
    {
        cout << val << "\n"; 
    }
};

int main()
{
    CallbackRegistrar registrar{};

    registrar.RegisterCallback( "SomeNode", UintFtor() );
    registrar.RegisterCallback( "SomeNode", StringFtor() );

    return 0;
}

不幸的是，当我尝试编译时，我得到以下歧义错误：

variadic-wrap.cpp: In function ‘int main()’:
variadic-wrap.cpp:87: error: request for member ‘RegisterCallback’ is ambiguous
variadic-wrap.cpp:27: error: candidates are: BasicConfigCallbackRegistrar<ValueType>&  BasicConfigCallbackRegistrar<ValueType>::RegisterCallback(std::string, std::function<void(ValueType)>) [with ValueType = std::basic_string<char, std::char_traits<char>, std::allocator<char> >]
variadic-wrap.cpp:27: error:                 BasicConfigCallbackRegistrar<ValueType>& BasicConfigCallbackRegistrar<ValueType>::RegisterCallback(std::string, std::function<void(ValueType)>) [with ValueType = unsigned int]
variadic-wrap.cpp:88: error: request for member ‘RegisterCallback’ is ambiguous
variadic-wrap.cpp:27: error: candidates are: BasicConfigCallbackRegistrar<ValueType>& BasicConfigCallbackRegistrar<ValueType>::RegisterCallback(std::string, std::function<void(ValueType)>) [with ValueType = std::basic_string<char, std::char_traits<char>, std::allocator<char> >]
variadic-wrap.cpp:27: error:                 BasicConfigCallbackRegistrar<ValueType>& BasicConfigCallbackRegistrar<ValueType>::RegisterCallback(std::string, std::function<void(ValueType)>) [with ValueType = unsigned int]

调用带有MakeCallback类型参数的value_type也会产生相同的歧义错误。

如果没有registrar显式转换为基类类型，我该如何解决这个问题？

Answer 1

我有一个解决方案。所有需要的是一个额外的间接水平。在Derived类中，我添加了一个模板函数，该函数调度到正确的基数，如下所示：

template< typename... Types >
class ConfigCallbackRegistrar : public BasicConfigCallbackRegistrar<Types>...
{
    typedef ConfigCallbackRegistrar<Types...> type;
public:
    /// @brief constructor
    ConfigCallbackRegistrar() : BasicConfigCallbackRegistrar<Types>()...
    {}

    template<typename T> 
    type& Register( string nodePath, 
            typename BasicConfigCallbackRegistrar<T>::signature_type callback )
    {
        BasicConfigCallbackRegistrar<T>::RegisterCallback( std::move(nodePath), 
                std::move(callback) );
        return *this;
    } 
};

调用代码变为：

int main()
{
    CallbackRegistrar registrar{};

    registrar.Register<uint32_t>( "SomeNode", UintFtor() );
    registrar.Register<string>( "SomeNode", StringFtor() );

    return 0;
}

可变参数模板和mixin的模糊错误

1 个答案: