环境: VS 2019, v16.4.3,使用C++17和最新开关
以下代码标准正确吗?或者我做错了什么?它在最新的gcc/clang编译器下可以编译通过,但在MSVC上失败了。(请参见以下错误消息)
template<typename T>
struct mixin {};
struct thing : mixin<thing>
{
constexpr explicit thing(int value) : value_(value) {}
constexpr int value() const { return value_; }
private:
int value_ = 0;
};
template<typename T>
constexpr auto do_mixin_thing(mixin<T> const& m)
{
return static_cast<T const&>(m).value() * 10;
}
int main()
{
constexpr thing t1{ 10 };
// this works
static_assert(t1.value() == 10);
// this fails
static_assert(do_mixin_thing(t1) == 100);
}
以下是输出结果:
error C2131: expression did not evaluate to a constant
message : failure was caused by attempting to access a member on an object of dynamic type 'mixin<thing>' in which the member is not defined
message : see usage of 'thing::value_'
错误是指在
main()
中的第二个static_assert
,两条消息指的是thing
内部的value()
成员函数。似乎
do_mixin_thing()
中的static_cast
引起了问题。我尝试通过constexpr T const& self() const { return static_cast<T const&>(*this); }
向mixin
添加强制转换,但错误仍然存在。
mixin
模板类实际上使用了 CRTP。一个 mixin 看起来像这样:template<typename Base> struct mixin : Base {}; /* ... */ struct foo : mixin<bar> {};
参见 https://dev59.com/xWMl5IYBdhLWcg3wAC2h - alter_igel