c++ - 区分值模板类和左值模板类的正确特化是什么？

Question

我需要进行模板特化，以区分具有(仅)值参数的模板类，如下所示:

template<auto ... __vz>
struct values{};

和一个带有(仅)引用参数的模板类，如下所示:

template<auto& ... __lvz>
struct lvalues{};

但是通过以下特化我没有成功:

template<typename>
struct is_values{
    static constexpr bool value = false;
};
template<template<auto ...>class __m, auto ... __vz>
struct is_values<__m<__vz ...> >{
    static constexpr bool value = true;
};

template<typename>
struct is_lvalues{
    static constexpr bool value = false;
};
template<template<auto& ...>class __m, auto& ... __vz>
struct is_lvalues<__m<__vz ...> >{
    static constexpr bool value = true;
};

当它们进行如下测试时:

const int a = 1;
const int b = 2;
const int c = 3;

int main()
{
    std::cout << is_values<lvalues<a,b,c> >::value <<std::endl; // wrong result
    std::cout << is_lvalues<values<1,2,3> >::value <<std::endl; // causes error
}

其中一个给出了错误的结果，一个给出了以下错误:

test.cpp: In instantiation of ‘struct is_lvalues<values<1, 2, 3> >’:
test.cpp:51:41:   required from here
test.cpp:32:34: error: initializing ‘int&’ with ‘int’ in converted constant expression does not bind directly
   32 | struct is_lvalues<__m<__vz ...> >{
      |                                  ^
test.cpp:32:34: error: could not convert ‘1’ from ‘int’ to ‘int&’
test.cpp:32:34: error: initializing ‘int&’ with ‘int’ in converted constant expression does not bind directly
test.cpp:32:34: error: could not convert ‘2’ from ‘int’ to ‘int&’
test.cpp:32:34: error: initializing ‘int&’ with ‘int’ in converted constant expression does not bind directly
test.cpp:32:34: error: could not convert ‘3’ from ‘int’ to ‘int&’
test.cpp:33:24: error: initializing ‘int&’ with ‘int’ in converted constant expression does not bind directly
   33 |  static constexpr bool value = true;
      |                        ^~~~~
test.cpp:33:24: error: could not convert ‘1’ from ‘int’ to ‘int&’
test.cpp:33:24: error: initializing ‘int&’ with ‘int’ in converted constant expression does not bind directly
test.cpp:33:24: error: could not convert ‘2’ from ‘int’ to ‘int&’
test.cpp:33:24: error: initializing ‘int&’ with ‘int’ in converted constant expression does not bind directly
test.cpp:33:24: error: could not convert ‘3’ from ‘int’ to ‘int&’
test.cpp: In function ‘int main()’:
test.cpp:51:43: error: ‘value’ is not a member of ‘is_lvalues<values<1, 2, 3> >’
   51 |  std::cout << is_lvalues<values<1,2,3> >::value <<std::endl; // causes error
      |                                           ^~~~~
make: *** [src/subdir.mk:20: src/test.o] Error 1
"make all" terminated with exit code 2. Build might be incomplete.

12:14:37 Build Failed. 14 errors, 0 warnings. (took 677ms)

到目前为止，我只在以下专业方面取​​得了成功:

template<auto ... __vz>
struct is_values<values<__vz ...> >{
    static constexpr bool value = true;
};
template<auto& ... __vz>
struct is_lvalues<lvalues<__vz ...> >{
    static constexpr bool value = true;
};

但这些不是我们所希望的，因为它们只检测到 values 产生的类型。和 lvalues不是任何其他具有不同名称的类似模板。
那么什么是正确的专业呢？

Answer 1

如果您正在为类型解决此问题，您可以使用
std::is_lvalue_reference 为单一类型执行此操作。由于您似乎使用的是 C++17，因此您可以使用折叠表达式将其应用于参数包:

template <typename... Ts>
using are_types_lvalues = std::integral_constant<bool, (std::is_lvalue_reference_v<Ts> && ...)>;

template <typename... Ts>
using are_types_values = std::integral_constant<bool, !(std::is_reference_v<Ts> || ...)>;

如果您可以测试类型，则可以使用 decltype获取非类型参数的类型，而不用担心强制参数绑定(bind)到 auto或 auto& ...

template<typename>
struct are_values : std::false_type {};
template<template<auto ...>class __m, auto ... __vz>
struct are_values<__m<__vz ...>> : std::integral_constant<bool, !(std::is_reference_v<decltype(__vz)> || ...)> {};

template<typename>
struct are_lvalues : std::false_type {};
template<template<auto ...>class __m, auto ... __vz>
struct are_lvalues<__m<__vz ...>> : std::integral_constant<bool, (std::is_lvalue_reference_v<decltype(__vz)> && ...)> {};

用法:

template<auto... __vz>
struct values{};

template<auto& ... __lvz>
struct lvalues{};

const int a = 1;
const int b = 2;
const int c = 3;

static_assert(are_values<values<1,2,3>>::value, "");
static_assert(!are_values<lvalues<a,b,c>>::value, "");
static_assert(!are_lvalues<values<1,2,3>>::value, "");
static_assert(are_lvalues<lvalues<a,b,c>>::value, "");

演示:
https://godbolt.org/z/KTh9Wj