79 lines
2.4 KiB
C++
79 lines
2.4 KiB
C++
//
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// Created by light on 20-1-6.
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//
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#include <iostream>
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#include "boost/hana.hpp"
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#include "structData.h"
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template<typename UnnamedType>
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struct container {
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// Let's put the test in private.
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private:
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// We use std::declval to 'recreate' an object of 'UnnamedType'.
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// We use std::declval to also 'recreate' an object of type 'Param'.
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// We can use both of these recreated objects to test the validity!
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template<typename Param>
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constexpr auto testValidity(int /* unused */)
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-> decltype(std::declval<UnnamedType>()(std::declval<Param>()),boost::hana::true_c) {
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// If substitution didn't fail, we can return a true_type.
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return boost::hana::true_c;
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}
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template<typename Param>
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constexpr std::false_type testValidity(...) {
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// Our sink-hole returns a false_type.
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return boost::hana::false_c;
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}
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public:
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// A public operator() that accept the argument we wish to test onto the UnnamedType.
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// Notice that the return type is automatic!
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template<typename Param>
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constexpr auto operator()(const Param &p) {
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// The argument is forwarded to one of the two overloads.
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// The SFINAE on the 'true_type' will come into play to dispatch.
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// Once again, we use the int for the precedence.
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return testValidity<Param>(int());
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}
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};
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template<typename UnnamedType>
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constexpr auto is_valid(const UnnamedType &t) {
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// We used auto for the return type: it will be deduced here.
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return container<UnnamedType>();
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}
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// Check if a type has a serialize method.
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auto hasSerialize = is_valid([](auto &&x) -> decltype(x.serialize()) {});
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// Notice how I simply swapped the return type on the right?
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template<class T>
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auto serialize(T &obj)
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-> typename std::enable_if<decltype(hasSerialize(obj))::value, std::string>::type {
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return obj.serialize();
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}
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template<class T>
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auto serialize(T &obj)
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-> typename std::enable_if<!decltype(hasSerialize(obj))::value, std::string>::type {
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return to_string(obj);
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}
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int main() {
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A a;
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B b;
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C c;
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auto test = is_valid([](const auto &t) -> decltype(t.serialize()) {});
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std::cout << test(a) << std::endl;
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std::cout << test(b) << std::endl;
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std::cout << test(c) << std::endl;
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// The following lines work like a charm!
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std::cout << serialize(a) << std::endl;
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std::cout << serialize(b) << std::endl;
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std::cout << serialize(c) << std::endl;
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}
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