使用std :: function时如何解决此<unresolved overloaded function type>错误？

Question

在下面的(工作)代码示例中,模板化的register_enum()函数用于迭代枚举并调用用户提供的回调以将枚举值转换为c-string.所有枚举都在一个类中定义,枚举到字符串转换是使用静态to_cstring(enum)函数完成的.当一个类(如下面的着色器类)具有多个枚举和相应的重载to_cstring(enum)函数时,编译器无法确定哪个是正确的to_cstring()函数传递给register_enum().我认为代码解释得比我能好......

#include <functional>
#include <iostream>

// Actual code uses Lua, but for simplification
// I'll hide it in this example.
typedef void lua_State;

class widget
{
    public:
        enum class TYPE
        {
            BEGIN = 0,
            WINDOW = BEGIN,
            BUTTON,
            SCROLL,
            PROGRESS,
            END
        };

        static const char *to_cstring( const TYPE value )
        {
            switch ( value )
            {
                case TYPE::WINDOW: return "window";
                case TYPE::BUTTON: return "button";
                case TYPE::SCROLL: return "scroll";
                case TYPE::PROGRESS: return "progress";
                default: break;
            }
            return nullptr;
        }
};

class shader
{
    public:
        enum class FUNC
        {
            BEGIN = 0,
            TRANSLATE = BEGIN,
            ROTATE,
            SCALE,
            COLOR,
            COORD,
            END
        };

        enum class WAVEFORM
        {
            BEGIN = 0,
            SINE = BEGIN,
            SQUARE,
            TRIANGLE,
            LINEAR,
            NOISE,
            END
        };

        static const char *to_cstring( const FUNC value )
        {
            switch ( value )
            {
                case FUNC::TRANSLATE: return "translate";
                case FUNC::ROTATE: return "rotate";
                case FUNC::SCALE: return "scale";
                case FUNC::COLOR: return "color";
                case FUNC::COORD: return "coord";
                default: break;
            }
            return nullptr;
        }

        static const char *to_cstring( const WAVEFORM value )
        {
            switch ( value )
            {
                case WAVEFORM::SINE: return "sine";
                case WAVEFORM::SQUARE: return "square";
                case WAVEFORM::TRIANGLE: return "triangle";
                case WAVEFORM::LINEAR: return "linear";
                case WAVEFORM::NOISE: return "noise";
                default: break;
            }
            return nullptr;
        }
};

// Increment an enum value.
// My compiler (g++ 4.6) doesn't support type_traits for enumerations, so
// here I just static_cast the enum value to int... Something to be fixed
// later...
template < class E >
E &enum_increment( E &value )
{
    return value = ( value == E::END ) ? E::BEGIN : E( static_cast<int>( value ) + 1 );
}

widget::TYPE &operator++( widget::TYPE &e )
{
    return enum_increment< widget::TYPE >( e );
}

shader::FUNC &operator++( shader::FUNC &e )
{
    return enum_increment< shader::FUNC >( e );
}

shader::WAVEFORM &operator++( shader::WAVEFORM &e )
{
    return enum_increment< shader::WAVEFORM >( e );
}


// Register the enumeration with Lua
template< class E >
void register_enum( lua_State *L, const char *table_name, std::function< const char*( E ) > to_cstring )
{
    (void)L; // Not used in this example.
    // Actual code creates a table in Lua and sets table[ to_cstring( i ) ] = i
    for ( auto i = E::BEGIN; i < E::END; ++i )
    {
        // For now, assume to_cstring can't return nullptr...
        const char *key = to_cstring( i );
        const int value = static_cast<int>(i);
        std::cout << table_name << "." << key << " = " << value << std::endl;
    }
}

int main( int argc, char **argv )
{
    (void)argc; (void)argv;

    lua_State *L = nullptr;

    // Only one to_cstring function in widget class so this works...
    register_enum< widget::TYPE >( L, "widgets", widget::to_cstring );

    // ... but these don't know which to_cstring to use.
    register_enum< shader::FUNC >( L, "functions", shader::to_cstring );
    //register_enum< shader::WAVEFORM >( L, "waveforms", shader::to_cstring );

    return 0;
}

编译器输出:

$ g++ -std=c++0x -Wall -Wextra -pedantic test.cpp -o test && ./test
test.cpp: In function ‘int main(int, char**)’:
test.cpp:140:69: error: no matching function for call to ‘register_enum(lua_State*&, const char [10], <unresolved overloaded function type>)’
test.cpp:140:69: note: candidate is:
test.cpp:117:7: note: template<class E> void register_enum(lua_State*, const char*, std::function<const char*(E)>)

如何将正确的to_cstring函数传递给register_enum()？我意识到我可以重命名个别的to_cstring()函数,但我想尽可能避免这种情况.也许我的设计很臭,你可以推荐一个更好的方法.

我的问题类似于使用模板调用重载函数(未解析的重载函数类型编译器错误)和如何获取重载成员函数的地址？但到目前为止,我无法将这些信息应用于我的具体问题.

Answer 1

该错误告诉您可能会使用两个潜在的重载,编译器无法为您做出决定.另一方面,您可以使用强制转换来确定要使用哪一个:

typedef const char *(*func_ptr)( shader::FUNC );
register_enum< shader::FUNC >( L, "functions", (func_ptr)shader::to_cstring );

或者没有typedef(更难读取单行):

register_enum< shader::FUNC >( L, "functions", 
             (const char *(*)( shader::FUNC ))shader::to_cstring );

^*请注意,在功能签名中,顶级const会被删除.

接下来的问题是为什么编译器本身没有找到适当的重载？这个问题存在,在调用register_enum您通过枚举类型,以及确定的类型std::function是std::function< const char* ( shader::FUNC ) >的,但std::function有一个模板构造函数,并试图推断出参数的类型的构造函数之前,编译器必须知道你想要使用哪个超载.