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