“类型 - 开关”构造在C ++ 11中 [英] "type-switch" construct in C++11
本文介绍了“类型 - 开关”构造在C ++ 11中的处理方法,对大家解决问题具有一定的参考价值,需要的朋友们下面随着小编来一起学习吧!
问题描述
所有的时间,我发现自己做这样的事情:
All the time, I find myself doing something like this:
Animal *animal = ...
if (Cat *cat = dynamic_cast<Cat *>(animal)) {
...
}
else if (Dog *dog = dynamic_cast<Dog *>(animal)) {
...
}
else { assert(false); }
一旦我看到C ++ 11中的闭包,我想知道, / p>
Once I see closures in C++11, I wonder, is something like this possible?
Animal *animal = ...
typecase(animal,
[](Cat *cat) {
...
},
[](Dog *dog) {
...
});
实现类型转换应该很容易,但我会遇到一个问题,函数的参数,所以它不能知道什么去尝试dynamic_cast,因为很难推断lambdas的参数。
Implementing typecase should have been easy, but I keep running into a problem where it can't figure out the function's argument, so it can't know what to try to dynamic_cast to, because it's hard to deduce lambdas' parameters. Took a few days of searching google and SO, but finally figured it out, so I'll share my answer below.
推荐答案
// Begin ecatmur's code
template<typename T> struct remove_class { };
template<typename C, typename R, typename... A>
struct remove_class<R(C::*)(A...)> { using type = R(A...); };
template<typename C, typename R, typename... A>
struct remove_class<R(C::*)(A...) const> { using type = R(A...); };
template<typename C, typename R, typename... A>
struct remove_class<R(C::*)(A...) volatile> { using type = R(A...); };
template<typename C, typename R, typename... A>
struct remove_class<R(C::*)(A...) const volatile> { using type = R(A...); };
template<typename T>
struct get_signature_impl { using type = typename remove_class<
decltype(&std::remove_reference<T>::type::operator())>::type; };
template<typename R, typename... A>
struct get_signature_impl<R(A...)> { using type = R(A...); };
template<typename R, typename... A>
struct get_signature_impl<R(&)(A...)> { using type = R(A...); };
template<typename R, typename... A>
struct get_signature_impl<R(*)(A...)> { using type = R(A...); };
template<typename T> using get_signature = typename get_signature_impl<T>::type;
// End ecatmur's code
// Begin typecase code
template<typename Base, typename FirstSubclass, typename... RestOfSubclasses>
void typecase(
Base *base,
FirstSubclass &&first,
RestOfSubclasses &&... rest) {
using Signature = get_signature<FirstSubclass>;
using Function = std::function<Signature>;
if (typecaseHelper(base, (Function)first)) {
return;
}
else {
typecase(base, rest...);
}
}
template<typename Base>
void typecase(Base *) {
assert(false);
}
template<typename Base, typename T>
bool typecaseHelper(Base *base, std::function<void(T *)> func) {
if (T *first = dynamic_cast<T *>(base)) {
func(first);
return true;
}
else {
return false;
}
}
// End typecase code
示例用法如下:
class MyBaseClass {
public:
virtual ~MyBaseClass() { }
};
class MyDerivedA : public MyBaseClass { };
class MyDerivedB : public MyBaseClass { };
int main() {
MyBaseClass *basePtr = new MyDerivedB();
typecase(basePtr,
[](MyDerivedA *a) {
std::cout << "is type A!" << std::endl;
},
[](MyDerivedB *b) {
std::cout << "is type B!" << std::endl;
});
return 0;
}
如果有任何改进,请告诉我!
If anyone has any improvements, please tell me!
这篇关于“类型 - 开关”构造在C ++ 11中的文章就介绍到这了,希望我们推荐的答案对大家有所帮助,也希望大家多多支持IT屋!
查看全文