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https://github.com/XRPLF/rippled.git
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95 lines
2.2 KiB
C++
95 lines
2.2 KiB
C++
/*
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This code normally fails to compile under Visual Studio 2012
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The fix is marked with _MSC_VER
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*/
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#include <iostream>
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#include <typeinfo>
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template <class T> struct has_lowest_layer_type {
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typedef char yes;
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typedef struct {char dummy[2];} no;
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template <class C> static yes f(typename C::lowest_layer_type*);
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template <class C> static no f(...);
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#ifdef _MSC_VER
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static const bool value = sizeof(f<T>(0)) == 1;
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#else
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static const bool value = sizeof(has_lowest_layer_type<T>::f<T>(0)) == 1;
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#endif
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};
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template <bool Enable>
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struct EnableIf : std::false_type { };
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template <>
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struct EnableIf <true> : std::true_type { };
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struct tcp { };
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template <class Protocol>
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struct basic_socket
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{
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typedef basic_socket <Protocol> lowest_layer_type;
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};
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template <class Protocol>
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struct basic_stream_socket : basic_socket <Protocol>
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{
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typedef basic_socket <Protocol> next_layer_type;
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};
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struct A
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{
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typedef basic_socket <tcp> lowest_layer_type;
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};
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struct B
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{
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};
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template <typename T>
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void show (std::true_type)
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{
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std::cout << typeid(T).name() << " has lowest_layer_type" << std::endl;
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}
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template <typename T>
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void show (std::false_type)
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{
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std::cout << typeid(T).name() << " does not have lowest_layer_type" << std::endl;
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}
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template <typename T>
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void show ()
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{
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show <T> (EnableIf <has_lowest_layer_type <T>::value> ());
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}
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int main ()
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{
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show <A> ();
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show <B> ();
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show <basic_socket <tcp> > ();
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show <basic_stream_socket <tcp> > ();
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return 0;
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}
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/*
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1>..\..\Subtrees\beast\notes\1\test.cpp(16): error C2783: 'has_lowest_layer_type<T>::no has_lowest_layer_type<T>::f(...)' : could not deduce template argument for 'C'
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1> with
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1> [
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1> T=A
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1> ]
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1> ..\..\Subtrees\beast\notes\1\test.cpp(15) : see declaration of 'has_lowest_layer_type<T>::f'
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1> with
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1> [
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1> T=A
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1> ]
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1> ..\..\Subtrees\beast\notes\1\test.cpp(63) : see reference to class template instantiation 'has_lowest_layer_type<T>' being compiled
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1> with
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1> [
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1> T=A
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1> ]
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1> ..\..\Subtrees\beast\notes\1\test.cpp(68) : see reference to function template instantiation 'void show<A>(void)' being compiled
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*/
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