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@ -40,89 +40,89 @@
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#define DEFINE_HAS_MEMBER_TRAITS(PREFIX, MEMBER_NAME) __NL__ \
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__NL__ \
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/* This defines a templated class PREFIX##_HasMember_##MEMBER_NAME. __NL__ \
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* The double hashes just glue together the value of macro __NL__ \
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* parameters like PREFIX or MEMBER_NAME and other __NL__ \
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* string tokens to form new identifiers (here the struct name). __NL__ \
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*/ __NL__ \
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/* This defines a templated class PREFIX##_HasMember_##MEMBER_NAME.*/__NL__ \
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/* The double hashes just glue together the value of macro */ __NL__ \
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/* parameters like PREFIX or MEMBER_NAME and other */ __NL__ \
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/* string tokens to form new identifiers (here the struct name). */ __NL__ \
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/*/ */ __NL__ \
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template<typename T> __NL__ \
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struct CAT3(PREFIX, _HasMember_, MEMBER_NAME) { __NL__ \
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__NL__ \
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/* This code defines an inner class, Fallback, with one __NL__ \
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* member – named MEMBER_NAME (remember that MEMBER_NAME is a __NL__ \
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* macro parameter, so it will be replaced with the name of the __NL__ \
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* actual member. Using proper terminology, this class is __NL__ \
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* essentially a Mixin. __NL__ \
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*/ __NL__ \
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/* This code defines an inner class, Fallback, with one */ __NL__ \
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/* member – named MEMBER_NAME (remember that MEMBER_NAME is a */ __NL__ \
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/* macro parameter, so it will be replaced with the name of the*/ __NL__ \
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/* actual member. Using proper terminology, this class is */ __NL__ \
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/* essentially a Mixin. */ __NL__ \
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__NL__ \
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struct Fallback { __NL__ \
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int MEMBER_NAME; __NL__ \
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}; __NL__ \
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__NL__ \
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/* Next, we introduce a new class: Derived. It inherits both __NL__ \
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* from the class T we’re templated on, and the previous Fallback __NL__ \
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* class. Note that the Derived class inherits the MEMBER_NAME __NL__ \
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* member from class Fallback and possibly another MEMBER_NAME __NL__ \
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* from T. Keep this (possible) ambiguity in mind. __NL__ \
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*/ __NL__ \
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/* Next, we introduce a new class: Derived. It inherits both */ __NL__ \
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/* from the class T we’re templated on, & the previous Fallback*/ __NL__ \
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/* class. Note that the Derived class inherits the MEMBER_NAME */ __NL__ \
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/* member from class Fallback and possibly another MEMBER_NAME */ __NL__ \
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/* from T. Keep this (possible) ambiguity in mind. */ __NL__ \
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__NL__ \
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struct Derived : T, Fallback { }; __NL__ \
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__NL__ \
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/* The ChT class is templated on a typename C, and an object __NL__ \
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* of that type. As a side note, only compile time constant __NL__ \
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* fixed types are possible here. We will use this class to __NL__ \
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* generate the ambiguity mentioned above. __NL__ \
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*/ __NL__ \
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/* The ChT class is templated on a typename C, and an object */ __NL__ \
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/* of that type. As a side note, only compile time constant */ __NL__ \
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/* fixed types are possible here. We will use this class to */ __NL__ \
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/* generate the ambiguity mentioned above. */ __NL__ \
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__NL__ \
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template<typename C, C> __NL__ \
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struct ChT; __NL__ \
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__NL__ \
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/* This is where the fun starts. These two lines form a __NL__ \
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* declaration of a templated function f. That function __NL__ \
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* receives a pointer to the ChT class, instantiated with a __NL__ \
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* pointer-to-member type whose parameter is the __NL__ \
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* address-of-member MEMBER_NAME in class C. Note that __NL__ \
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* if we attempt to instantiate this with __NL__ \
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* the Derived class, one of two things will happen: either we __NL__ \
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* will have a substitution failure due to the aforementioned __NL__ \
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* ambiguity (if T also has a member by that name), or it will __NL__ \
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* be successful (if there is no member called MEMBER_NAME, in T). __NL__ \
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* This function returns a reference to a char array of size 1. __NL__ \
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* __NL__ \
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* How returning arrays by reference works becomes a little __NL__ \
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* clearer by means of a more simple example. The definition __NL__ \
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* of a function that returns a reference to a char-array of __NL__ \
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* size 1 e.g. reads __NL__ \
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* __NL__ \
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* char (&f())[1]; __NL__ \
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*/ __NL__ \
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/* This is where the fun starts. These two lines form a */ __NL__ \
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/* declaration of a templated function f. That function */ __NL__ \
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/* receives a pointer to the ChT class, instantiated with a */ __NL__ \
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/* pointer-to-member type whose parameter is the */ __NL__ \
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/* address-of-member MEMBER_NAME in class C. Note that */ __NL__ \
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/* if we attempt to instantiate this with */ __NL__ \
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/* the Derived class, one of two things will happen: either we */ __NL__ \
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/* will have a substitution failure due to the aforementioned */ __NL__ \
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/* ambiguity (if T also has a member by that name), or it will */ __NL__ \
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/* be successful (if there is no member called MEMBER_NAME, in T).*/ __NL__ \
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/* This function returns a reference to a char array of size 1.*/ __NL__ \
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/* */ __NL__ \
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/* How returning arrays by reference works becomes a little */ __NL__ \
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/* clearer by means of a more simple example. The definition */ __NL__ \
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/* of a function that returns a reference to a char-array of */ __NL__ \
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/* size 1 e.g. reads */ __NL__ \
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/* */ __NL__ \
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/* char (&f())[1]; */ __NL__ \
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__NL__ \
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template<typename C> __NL__ \
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static char (&f(ChT<int Fallback::*, &C::MEMBER_NAME>*))[1]; __NL__ \
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__NL__ \
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/* This is what gets instantiated if the previous template __NL__ \
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* can’t be instantiated and SFINAE kicks in (since variadic __NL__ \
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* functions have the lowest possible priority when selecting __NL__ \
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* which overloaded function to call). This variadic function __NL__ \
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* returns a reference to a char array of size 2. Note that __NL__ \
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* instantiation of this function means that the previous __NL__ \
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* one failed, implying that class T has a member __NL__ \
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* named MEMBER_NAME. __NL__ \
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*/ __NL__ \
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__NL__ \
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/* This is what gets instantiated if the previous template */ __NL__ \
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/* can’t be instantiated and SFINAE kicks in (since variadic */ __NL__ \
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/* functions have the lowest possible priority when selecting */ __NL__ \
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/* which overloaded function to call). This variadic function */ __NL__ \
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/* returns a reference to a char array of size 2. Note that */ __NL__ \
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/* instantiation of this function means that the previous */ __NL__ \
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/* one failed, implying that class T has a member */ __NL__ \
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/* named MEMBER_NAME. */ __NL__ \
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__NL__ \
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template<typename C> __NL__ \
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static char (&f(...))[2]; __NL__ \
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__NL__ \
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/* As you could guess, we will try to check which f function __NL__ \
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* can be instantiated. We will do this by checking the sizeof __NL__ \
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* of the return value of that f. If the first signature did __NL__ \
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* not fit (could not be instantiated), then according to __NL__ \
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* SFINAE the second one will be the chosen one __NL__ \
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* (since it can’t fail) and we will take the sizeof of a __NL__ \
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* char array of size 2, which is 2. Therefore, the value __NL__ \
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* will be evaluated to true – meaning that class T really __NL__ \
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* has a member by the name MEMBER_NAME. Otherwise, the first __NL__ \
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* function can be successfully instantiated, and we will measure __NL__ \
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* the sizeof of a char array of size 1, which is 1. In that case __NL__ \
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* the value is evaluated to false – which is good for us, since __NL__ \
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* that means there was no ambiguity when using __NL__ \
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* Derived::MEMBER_NAME, and that means there was no other __NL__ \
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* MEMBER_NAME but the one in Fallback. __NL__ \
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*/ __NL__ \
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/* As you could guess, we will try to check which f function */ __NL__ \
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/* can be instantiated. We will do this by checking the sizeof */ __NL__ \
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/* of the return value of that f. If the first signature did */ __NL__ \
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/* not fit (could not be instantiated), then according to */ __NL__ \
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/* SFINAE the second one will be the chosen one */ __NL__ \
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/* (since it can’t fail) and we will take the sizeof of a */ __NL__ \
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/* char array of size 2, which is 2. Therefore, the value */ __NL__ \
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/* will be evaluated to true – meaning that class T really */ __NL__ \
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/* has a member by the name MEMBER_NAME. Otherwise, the first */ __NL__ \
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/* function can be successfully instantiated, and we will */ __NL__ \
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/* measure the sizeof of a char array of size 1, which is 1. */ __NL__ \
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/* In that case, the value is evaluated to false – which is */ __NL__ \
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/* good for us sincethat means there was no ambiguity when */ __NL__ \
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/* using Derived::MEMBER_NAME, and that means there was no */ __NL__ \
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/* other MEMBER_NAME but the one in Fallback. */ __NL__ \
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__NL__ \
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static bool const value = sizeof(f<Derived>(0)) == 2; __NL__ \
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};
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Jesse Vincent
4 years ago
committed by
Gergely Nagy