c++ - 将类模板转换为 SFINAE 和构造函数委托(delegate)，现在出现编译器错误

标签 c++ initialization c++17 sfinae ambiguous

最初我有这个类模板声明:

Register.h -- 原创

#pragma once

#include <assert.h>
#include <bitset>
#include <cstdint>
#include <limits>
#include <type_traits>

using  u8 = std::uint8_t;
using u16 = std::uint16_t;
using u32 = std::uint32_t;
using u64 = std::uint64_t;

template <typename T>
struct Register {
    T data;
    T value;
    bitset<sizeof(T) * CHAR_BIT> bits;

    Register() : data(), value() {}

    template <typename P>
    explicit Register(const P val) : data(static_cast<T>(val)), value(data), bits(data) {}

    template <typename P>
    Register(const P val, const unsigned char idx) : data(static_cast<T>((val >> std::size(bits) * idx) & numeric_limits<make_unsigned_t<T>>::max())), value(data), bits(data) {
        assert(idx == '\0' || idx < sizeof(P) / sizeof(T));
    }

    template <typename P>
    Register(const Register<P>& reg) : data(static_cast<T>(reg.data)), value(data), bits(data) {}
};

using Reg8  = Register<u8>;
using Reg16 = Register<u16>;
using Reg32 = Register<u32>;
using Reg64 = Register<u64>;

我能够这样构建这些:

void someFunc() {
    // by const value
    Reg64 r64{ 0x0123456789ABCDEF };
    Reg32 r32{ 0x89ABCDEF };
    Reg16 r16{ 0xCDEF };
    Reg8  r8{ 0xEf };

    // unsigned type
    u8  byte  = 8;
    u16 word  = 16;
    u32 dword = 32;
    u64 qword = 64;

    Reg8 r8a{byte};
    Reg16 r16a{word};
    Reg32 r32a{dword};
    Reg64 r64a{qword};

    // Or from any other Register<T> type.
}

现在我将我的类模板的构造函数转换为 SFINAE，它们现在看起来像这样:

#pragma once

#include <assert.h>
#include <bitset>
#include <cstdint>
#include <limits>
#include <type_traits>

using u8  = std::uint8_t;
using u16 = std::uint16_t;
using u32 = std::uint32_t;
using u64 = std::uint64_t;

template<typename T>
struct Register {
    T data;
    T value;
    std::bitset<sizeof(T)* CHAR_BIT> bits;

    Register() : data{ 0 }, value{ 0 }, bits{ 0 } {}

    template<typename P, std::enable_if_t<(sizeof(P) > sizeof(T))>* = nullptr>
    Register(const P val, const u8 idx) :
        data{ static_cast<T>((val >> std::size(bits) * idx) &
              std::numeric_limits<std::make_unsigned_t<T>>::max()) },
        value{ data },
        bits{ data }
    {

        constexpr u16 sizeT = sizeof(T);
        constexpr u16 sizeP = sizeof(P);
        assert((idx >= 0) && (idx <= ((sizeP / sizeT) - 1)) );
    }


    template<typename P, std::enable_if_t<(sizeof(P) < sizeof(T))>* = nullptr>
    Register(const P val, const u8 idx) :
        data{ /*static_cast<T>((val >> std::size(bits) * idx) &
              std::numeric_limits<std::make_unsigned_t<T>>::max()) },*/
            static_cast<T>(val)
            },
        value{ data },
        bits{ data }
    {
        constexpr u16 sizeT = sizeof(T);
        constexpr u16 sizeP = sizeof(P);
        assert((idx >= 0) && (idx <= ((sizeT / sizeP) - 1)) );
    }


    template<typename P, std::enable_if_t<(sizeof(P) == sizeof(T))>* = nullptr>
    Register(const P val, const u8 idx = 0) :
        data{ static_cast<T>( val ) }, value{ data }, bits{ data }
    {}

    template<typename P>
    Register(const Register<P>& reg, const u8 idx = 0) : Register(reg.data, idx) {}

};

using Reg8  = Register<u8>;
using Reg16 = Register<u16>;
using Reg32 = Register<u32>;
using Reg64 = Register<u64>;

现在，当我尝试像以前那样构造它们时，我没有收到编译器错误:注意*，这些编译器错误不是来自上面的虚拟函数 someFunc，这是完整列表从我的 testing 函数中重新生成的错误，这些函数以前编译和运行时没有错误。如果需要，我可以在这些错误下方发布这些功能；只需提出请求，我就会添加它们。

1>------ Build started: Project: TestRegister, Configuration: Debug x64 ------
1>main.cpp
1>c:\***\main.cpp(54): error C2440: 'initializing': cannot convert from 'initializer list' to 'vpc::Register<vpc::u16>'
1>c:\***\main.cpp(54): note: No constructor could take the source type, or constructor overload resolution was ambiguous
1>c:\***\main.cpp(55): error C2440: 'initializing': cannot convert from 'initializer list' to 'vpc::Register<vpc::u8>'
1>c:\***\main.cpp(55): note: No constructor could take the source type, or constructor overload resolution was ambiguous
1>c:\***\main.cpp(195): error C2665: 'vpc::Register<vpc::u8>::Register': none of the 3 overloads could convert all the argument types
1>c:\***\register.h(73): note: could be 'vpc::Register<vpc::u8>::Register(vpc::Register<vpc::u8> &&)'
1>c:\***\register.h(73): note: or       'vpc::Register<vpc::u8>::Register(const vpc::Register<vpc::u8> &)'
1>c:\***\register.h(34): note: or       'vpc::Register<vpc::u8>::Register(void)'
1>c:\***\main.cpp(195): note: while trying to match the argument list '(u16)'
1>c:\***\main.cpp(196): error C2665: 'vpc::Register<vpc::u8>::Register': none of the 3 overloads could convert all the argument types
1>c:\***\register.h(73): note: could be 'vpc::Register<vpc::u8>::Register(vpc::Register<vpc::u8> &&)'
1>c:\***\register.h(73): note: or       'vpc::Register<vpc::u8>::Register(const vpc::Register<vpc::u8> &)'
1>c:\***\register.h(34): note: or       'vpc::Register<vpc::u8>::Register(void)'
1>c:\***\main.cpp(196): note: while trying to match the argument list '(u32)'
1>c:\***\main.cpp(197): error C2665: 'vpc::Register<vpc::u8>::Register': none of the 3 overloads could convert all the argument types
1>c:\***\register.h(73): note: could be 'vpc::Register<vpc::u8>::Register(vpc::Register<vpc::u8> &&)'
1>c:\***\register.h(73): note: or       'vpc::Register<vpc::u8>::Register(const vpc::Register<vpc::u8> &)'
1>c:\***\register.h(34): note: or       'vpc::Register<vpc::u8>::Register(void)'
1>c:\***\main.cpp(197): note: while trying to match the argument list '(u64)'
1>c:\***\main.cpp(238): error C2665: 'vpc::Register<vpc::u16>::Register': none of the 3 overloads could convert all the argument types
1>c:\***\register.h(73): note: could be 'vpc::Register<vpc::u16>::Register(vpc::Register<vpc::u16> &&)'
1>c:\***\register.h(73): note: or       'vpc::Register<vpc::u16>::Register(const vpc::Register<vpc::u16> &)'
1>c:\***\register.h(34): note: or       'vpc::Register<vpc::u16>::Register(void)'
1>c:\***\main.cpp(238): note: while trying to match the argument list '(u8)'
1>c:\***\main.cpp(239): error C2665: 'vpc::Register<vpc::u16>::Register': none of the 3 overloads could convert all the argument types
1>c:\***\register.h(73): note: could be 'vpc::Register<vpc::u16>::Register(vpc::Register<vpc::u16> &&)'
1>c:\***\register.h(73): note: or       'vpc::Register<vpc::u16>::Register(const vpc::Register<vpc::u16> &)'
1>c:\***\register.h(34): note: or       'vpc::Register<vpc::u16>::Register(void)'
1>c:\***\main.cpp(239): note: while trying to match the argument list '(u8)'
1>c:\***\main.cpp(241): error C2665: 'vpc::Register<vpc::u16>::Register': none of the 3 overloads could convert all the argument types
1>c:\***\register.h(73): note: could be 'vpc::Register<vpc::u16>::Register(vpc::Register<vpc::u16> &&)'
1>c:\***\register.h(73): note: or       'vpc::Register<vpc::u16>::Register(const vpc::Register<vpc::u16> &)'
1>c:\***\register.h(34): note: or       'vpc::Register<vpc::u16>::Register(void)'
1>c:\***\main.cpp(241): note: while trying to match the argument list '(u32)'
1>c:\***\main.cpp(242): error C2665: 'vpc::Register<vpc::u16>::Register': none of the 3 overloads could convert all the argument types
1>c:\***\register.h(73): note: could be 'vpc::Register<vpc::u16>::Register(vpc::Register<vpc::u16> &&)'
1>c:\***\register.h(73): note: or       'vpc::Register<vpc::u16>::Register(const vpc::Register<vpc::u16> &)'
1>c:\***\register.h(34): note: or       'vpc::Register<vpc::u16>::Register(void)'
1>c:\***\main.cpp(242): note: while trying to match the argument list '(u64)'
1>c:\***\main.cpp(267): error C2665: 'vpc::Register<vpc::u32>::Register': none of the 3 overloads could convert all the argument types
1>c:\***\register.h(73): note: could be 'vpc::Register<vpc::u32>::Register(vpc::Register<vpc::u32> &&)'
1>c:\***\register.h(73): note: or       'vpc::Register<vpc::u32>::Register(const vpc::Register<vpc::u32> &)'
1>c:\***\register.h(34): note: or       'vpc::Register<vpc::u32>::Register(void)'
1>c:\***\main.cpp(267): note: while trying to match the argument list '(u8)'
1>c:\***\main.cpp(268): error C2665: 'vpc::Register<vpc::u32>::Register': none of the 3 overloads could convert all the argument types
1>c:\***\register.h(73): note: could be 'vpc::Register<vpc::u32>::Register(vpc::Register<vpc::u32> &&)'
1>c:\***\register.h(73): note: or       'vpc::Register<vpc::u32>::Register(const vpc::Register<vpc::u32> &)'
1>c:\***\register.h(34): note: or       'vpc::Register<vpc::u32>::Register(void)'
1>c:\***\main.cpp(268): note: while trying to match the argument list '(u8)'
1>c:\***\main.cpp(269): error C2665: 'vpc::Register<vpc::u32>::Register': none of the 3 overloads could convert all the argument types
1>c:\***\register.h(73): note: could be 'vpc::Register<vpc::u32>::Register(vpc::Register<vpc::u32> &&)'
1>c:\***\register.h(73): note: or       'vpc::Register<vpc::u32>::Register(const vpc::Register<vpc::u32> &)'
1>c:\***\register.h(34): note: or       'vpc::Register<vpc::u32>::Register(void)'
1>c:\***\main.cpp(269): note: while trying to match the argument list '(u16)'
1>c:\***\main.cpp(271): error C2665: 'vpc::Register<vpc::u32>::Register': none of the 3 overloads could convert all the argument types
1>c:\***\register.h(73): note: could be 'vpc::Register<vpc::u32>::Register(vpc::Register<vpc::u32> &&)'
1>c:\***\register.h(73): note: or       'vpc::Register<vpc::u32>::Register(const vpc::Register<vpc::u32> &)'
1>c:\***\register.h(34): note: or       'vpc::Register<vpc::u32>::Register(void)'
1>c:\***\main.cpp(271): note: while trying to match the argument list '(u64)'
1>c:\***\main.cpp(285): error C2665: 'vpc::Register<vpc::u64>::Register': none of the 3 overloads could convert all the argument types
1>c:\***\register.h(73): note: could be 'vpc::Register<vpc::u64>::Register(vpc::Register<vpc::u64> &&)'
1>c:\***\register.h(73): note: or       'vpc::Register<vpc::u64>::Register(const vpc::Register<vpc::u64> &)'
1>c:\***\register.h(34): note: or       'vpc::Register<vpc::u64>::Register(void)'
1>c:\***\main.cpp(285): note: while trying to match the argument list '(u8)'
1>c:\***\main.cpp(286): error C2665: 'vpc::Register<vpc::u64>::Register': none of the 3 overloads could convert all the argument types
1>c:\***\register.h(73): note: could be 'vpc::Register<vpc::u64>::Register(vpc::Register<vpc::u64> &&)'
1>c:\***\register.h(73): note: or       'vpc::Register<vpc::u64>::Register(const vpc::Register<vpc::u64> &)'
1>c:\***\register.h(34): note: or       'vpc::Register<vpc::u64>::Register(void)'
1>c:\***\main.cpp(286): note: while trying to match the argument list '(u8)'
1>c:\***\main.cpp(287): error C2665: 'vpc::Register<vpc::u64>::Register': none of the 3 overloads could convert all the argument types
1>c:\***\register.h(73): note: could be 'vpc::Register<vpc::u64>::Register(vpc::Register<vpc::u64> &&)'
1>c:\***\register.h(73): note: or       'vpc::Register<vpc::u64>::Register(const vpc::Register<vpc::u64> &)'
1>c:\***\register.h(34): note: or       'vpc::Register<vpc::u64>::Register(void)'
1>c:\***\main.cpp(287): note: while trying to match the argument list '(u16)'
1>c:\***\main.cpp(288): error C2665: 'vpc::Register<vpc::u64>::Register': none of the 3 overloads could convert all the argument types
1>c:\***\register.h(73): note: could be 'vpc::Register<vpc::u64>::Register(vpc::Register<vpc::u64> &&)'
1>c:\***\register.h(73): note: or       'vpc::Register<vpc::u64>::Register(const vpc::Register<vpc::u64> &)'
1>c:\***\register.h(34): note: or       'vpc::Register<vpc::u64>::Register(void)'
1>c:\users\skilz99\source\repos\testregister\testregister\main.cpp(288): note: while trying to match the argument list '(u32)'
1>Done building project "TestRegister.vcxproj" -- FAILED.
========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========

所以现在我遇到了一些重复的类型错误，首先它无法从初始化列表转换，然后我收到了不明确的构造函数调用...

最佳答案

您从两个构造函数模板中删除了默认参数。 

template<typename P, std::enable_if_t<(sizeof(P) > sizeof(T))>* = nullptr>
    Register(const P val, const u8 idx) : // ...

template<typename P, std::enable_if_t<(sizeof(P) < sizeof(T))>* = nullptr>
    Register(const P val, const u8 idx) : // ...

应该是:

template<typename P, std::enable_if_t<(sizeof(P) > sizeof(T))>* = nullptr>
    Register(const P val, const u8 idx = 0) : // ...

template<typename P, std::enable_if_t<(sizeof(P) < sizeof(T))>* = nullptr>
    Register(const P val, const u8 idx = 0) : // ...

因此编译器无法转换为 Register<T>来自P与不同sizeof ，因为它只看到一个参数而没有考虑其他两个构造函数模板。

关于c++ - 将类模板转换为 SFINAE 和构造函数委托(delegate)，现在出现编译器错误，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/56195130/

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