使用 class 模板和 std::void_t 检查默认构造函数

Question

下面是试图在编译时检查是否存在默认构造函数的代码片段。使用

编译

clang version 11.0.0
Target: x86_64-apple-darwin19.6.0
Thread model: posix
InstalledDir: /usr/local/opt/llvm/bin

使用选项 clang++ --std=c++17 -o test_default_ctor test_default_ctor.cpp

#include <type_traits>

template<typename T, typename = void>
struct has_default_ctor_1 : std::false_type {}; 

template<typename T>
struct has_default_ctor_1<T, std::void_t<decltype(T())>> : std::true_type {}; 

template<typename T, typename = void>
struct has_default_ctor_2 : std::false_type {}; 

template<typename T>
struct has_default_ctor_2<T, std::void_t<decltype(T{})>> : std::true_type {}; 

struct Test1 {
    Test1() = default;
};

struct Test2 {
    Test2() {}
};

struct Test3 {
    Test3() = delete;
};

int main() {
    static_assert(has_default_ctor_1<Test1>::value, "Test has default ctor");
    static_assert(has_default_ctor_2<Test1>::value, "Test has default ctor");
    static_assert(has_default_ctor_1<Test2>::value, "Test has default ctor");
    static_assert(has_default_ctor_2<Test2>::value, "Test has default ctor");
    static_assert(not has_default_ctor_1<Test3>::value, "Test has default ctor");
    static_assert(not has_default_ctor_2<Test3>::value, "Test has default ctor");
}

编译的输出将是

test_default_ctor.cpp:33:5: error: static_assert failed due to requirement '!has_default_ctor_2<Test3, void>::value' "Test has default ctor"
    static_assert(not has_default_ctor_2<Test3>::value, "Test has default ctor");
    ^             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1 error generated.

问题是为什么在模板特化中使用 () 与 {} 进行构造函数调用使其在一种情况下有效而在另一种情况下无效？

Answer 1

template<typename T>
struct has_default_ctor_2<T, std::void_t<decltype(T{})>> : std::true_type {};

当您检查 T{} 是否格式正确时，您还允许可以通过聚合初始化的方式初始化的类型。 Test3 是 C++20 之前的集合 class，因为它没有 用户提供的 构造函数。来自[dcl.fct.def.default]/5 [摘录，强调我的]：

[...] A function is user-provided if it is user-declared and not explicitly defaulted or deleted on its first declaration.

有关不同标准版本聚合的详细信息，请参见例如：

The fickle aggregate

Aggregates in C++20

As of C++20, particularly due to the implementation of P1008R1 (Prohibit aggregates with user-declared constructors) most of the frequently surprising aggregate behaviour covered above has been addressed, specifically by no longer allowing aggregates to have user-declared constructors, a stricter requirement for a class to be an aggregate than just prohibiting user-provided constructors.

使用 class 模板和 std::void_t 检查默认构造函数

Checking for a default Constructor using class templates with std::void_t

c++

template-meta-programming

c++11

c++17

Aggregates in C++20