c++ - 使用 std::enable_if<> 的模板特化

Question

编译并运行以下代码:

#include <cinttypes>
#include <cstdlib>
#include <iostream>
#include <limits>
#include <sstream>
#include <stdexcept>

class UnsignedBox {
public:
    typedef std::uint64_t box_type;

    template<typename UNSIGNED_TYPE, 
        typename std::enable_if<
        std::numeric_limits<UNSIGNED_TYPE>::is_signed==false &&
        (sizeof(UNSIGNED_TYPE) >= sizeof(UnsignedBox::box_type)), int>::type = 0 
    >
    UNSIGNED_TYPE toUnsigned()const {
        //We've established we're not returning a smaller type so we can just 
       //return our value.
        return value;
    }

    template<typename UNSIGNED_TYPE, 
       typename std::enable_if<std::numeric_limits<UNSIGNED_TYPE>::is_signed==false &&
       (sizeof(UNSIGNED_TYPE) < sizeof(UnsignedBox::box_type)), int>::type = 0
    >
    UNSIGNED_TYPE toUnsigned()const {
        //We are returning  a smaller type so we need a range check.
        if(value>static_cast<box_type>(std::numeric_limits<UNSIGNED_TYPE>::max())){
            std::ostringstream msg;
            msg<<value<<'>'<<
               static_cast<box_type>(std::numeric_limits<UNSIGNED_TYPE>::max());
            throw std::logic_error(msg.str());
        }
        return value;
    }

    UnsignedBox(const box_type ivalue): value(ivalue){}
private:
    box_type value;

};

int main(int argc, char*argv[]) {

    UnsignedBox box(
        static_cast<UnsignedBox::box_type>(    
           std::numeric_limits<std::uint32_t>::max())+10
        );

    std::uint64_t v(box.toUnsigned<std::uint64_t>());
    std::cout<<v<<std::endl;

    try {
        std::uint32_t v(box.toUnsigned<std::uint32_t>());
    }catch(const std::logic_error err){
        std::cout<<err.what()<<std::endl;
    }

    return EXIT_SUCCESS;
}

预期输出(所有支持平台):

4294967305
4294967305>4294967295

到目前为止一切顺利。

但我真正想做的(为了代码清晰):

声明如下:

template<typename UNSIGNED_TYPE>
UNSIGNED_TYPE toUnsigned()const;

然后提供专门的实现，例如:

template<typename UNSIGNED_TYPE, 
        typename std::enable_if<
        std::numeric_limits<UNSIGNED_TYPE>::is_signed==false &&
        (sizeof(UNSIGNED_TYPE) >= sizeof(UnsignedBox::box_type)), int>::type = 0 
    >
    UNSIGNED_TYPE UnsignedBox::toUnsigned()const {
        //We've established we're not returning a smaller type so we can just 
       //return our value.
        return value;
    }


    template<typename UNSIGNED_TYPE, 
       typename std::enable_if<std::numeric_limits<UNSIGNED_TYPE>::is_signed==false &&
       (sizeof(UNSIGNED_TYPE) < sizeof(UnsignedBox::box_type)), int>::type = 0
    >
    UNSIGNED_TYPE UnsignedBox::toUnsigned()const {
        //We are returning  a smaller type so we need a range check.
        if(value>static_cast<box_type>(std::numeric_limits<UNSIGNED_TYPE>::max())){
            std::ostringstream msg;
            msg<<value<<'>'<<
               static_cast<box_type>(std::numeric_limits<UNSIGNED_TYPE>::max());
            throw std::logic_error(msg.str());
        }
        return value;
    }

但是我得到这个错误:

xxx.cpp:nn:20: error: prototype for 'UNSIGNED_TYPE UnsignedBox::toUnsigned() const' does not match any in class 'UnsignedBox'
      UNSIGNED_TYPE UnsignedBox::toUnsigned()const {
                    ^ xxx.cpp:nn:23: error: candidate is: template<class UNSIGNED_TYPE> UNSIGNED_TYPE UnsignedBox::toUnsigned() const
         UNSIGNED_TYPE toUnsigned()const;
                       ^

这很奇怪，因为如果你问我

UNSIGNED_TYPE UnsignedBox::toUnsigned() const

非常适合

UNSIGNED_TYPE toUnsigned()const;

我做错了什么？

PS:这不是实际问题，但我的问题是类似的，我想根据编译时检查的基本类型的属性来特殊化一些模板。

Answer 1

你不能用一个签名声明一个函数:

template<typename UNSIGNED_TYPE>
UNSIGNED_TYPE toUnsigned() const;

然后用不同的签名定义它:

template<typename UNSIGNED_TYPE, 
    typename std::enable_if<
    std::numeric_limits<UNSIGNED_TYPE>::is_signed==false &&
    (sizeof(UNSIGNED_TYPE) >= sizeof(UnsignedBox::box_type)), int>::type = 0 
>
UNSIGNED_TYPE UnsignedBox::toUnsigned() const;

第一个接受一个模板参数，第二个接受两个——尽管默认有一个。两者必须完全匹配。所以你需要两个声明:

template <typename UNSIGNED_TYPE,
          typename = typename std::enable_if<
                        std::is_unsigned<UNSIGNED_TYPE>::value &&
                        sizeof(UNSIGNED_TYPE) >= sizeof(UnsignedBox::box_type)
                        >::type>
UNSIGNED_TYPE toUnsigned() const;

template <typename UNSIGNED_TYPE,
          typename = typename std::enable_if<
                        std::is_unsigned<UNSIGNED_TYPE>::value &&
                        sizeof(UNSIGNED_TYPE) < sizeof(UnsignedBox::box_type)
                        >::type>
UNSIGNED_TYPE toUnsigned() const;

然后是两个定义。这本身也不起作用，因为我们有效地重新定义了默认模板参数，因此您需要对返回类型进行 SFINAE，例如:

template <typename UNSIGNED_TYPE>
typename std::enable_if<
    std::is_unsigned<UNSIGNED_TYPE>::value &&
    sizeof(UNSIGNED_TYPE) >= sizeof(UnsignedBox::box_type),
    UNSIGNED_TYPE>::type
toUnsigned() const;


template <typename UNSIGNED_TYPE>
typename std::enable_if<
    std::is_unsigned<UNSIGNED_TYPE>::value &&
    sizeof(UNSIGNED_TYPE) < sizeof(UnsignedBox::box_type),
    UNSIGNED_TYPE>::type
toUnsigned() const;

尽管将一个toUnsigned()转发到基于sizeof的其他两个成员函数可能更简单:

template <typename UNSIGNED_TYPE,
          typename = typename std::enable_if<std::is_unsigned<UNSIGNED_TYPE>::value>::type>
UNSIGNED_TYPE toUnsigned() const {
    return toUnsigned<UNSIGNED_TYPE>(
        std::integral_constant<bool, 
            (sizeof(UNSIGNED_TYPE) >= sizeof(UnsignedBox::box_type))>{});
}

template <typename UNSIGNED_TYPE>
UNSIGNED_TYPE toUnsigned(std::true_type /* bigger */);

template <typename UNSIGNED_TYPE>
UNSIGNED_TYPE toUnsigned(std::false_type /* smaller */);