[英]Using decltype in partial template specialization
With the following template and specialization: 使用以下模板和专业化:
template<typename T, typename = void>
struct A {
void operator()() {
std::cout << "primary" << std::endl;
}
};
template<typename T>
struct A<T, decltype(T().f())> {
void operator()() {
std::cout << "specialization" << std::endl;
}
};
used like this: 像这样使用:
struct X {
bool f() { return false; }
};
int main()
{
A<X>()();
return 0;
}
the primary template is resolved, when one would expect partial specialisation to be selected. 当人们期望选择部分特化时,解析主模板。 However, when changed to:
但是,当改为:
template<typename T>
struct A<T, typename std::enable_if<std::is_object<decltype(T().f())>::value>::type> {
void operator()() {
std::cout << "specialization" << std::endl;
}
};
the specialization is chosen. 选择专业化。 Why isn't specialization chosen in the original example?
为什么在原始示例中没有选择专业化?
You have instantiated: 你已经实例化了:
A<X>
Which leverages a default template parameter to become: 利用默认模板参数成为:
A<X,void>
Your specialization, however, is: 但是,你的专长是:
template<typename T>
struct A<T, decltype(T().f())>
Which, for template parameter X
becomes: 其中,对于模板参数
X
变为:
struct A<X, decltype(X().f())>
Which is: 这是:
struct A<X, bool>
So, your specialization is not the correct choice, because A<X,bool>
does not specialize A<X,void>
. 所以,你的专业化不是正确的选择,因为
A<X,bool>
不专门化A<X,void>
。
If you want your specialization to work for all well-formed instances of T().f()
, you can use C++17's std::void_t
如果你希望你的专业化适用于所有格式良好的
T().f()
实例,你可以使用C ++ 17的std::void_t
std::void_t
will evaluate to void
for any well-formed template parameters passed to it. 对于传递给它的任何格式良好的模板参数,
std::void_t
将评估为void
。
template<typename T>
struct A<T, std::void_t<decltype(T().f())>> {
void operator()() {
std::cout << "specialization" << std::endl;
}
};
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