Pass generic template class as functor parameter - c++

Question

I want to improve the following code snippet:
THNNetIPInfoIter last = std::unique(fist, end, HNInfoIPComparator());
where currently HNInfoIPComparator() is implemented as following:
// equal comparator
class HNInfoIPComparator
{
public:
bool operator()(const THNNetIPInfo &a, const THNNetIPInfo &b);
bool operator()(const SDK::TIPAddressDescription &a, const SDK::TIPAddressDescription &b);
bool operator()(const THNNetIPInfo &a, const SDK::TIPAddressDescription &b);
bool operator()(const SDK::TIPAddressDescription &a, const THNNetIPInfo &b);
};
The reason for this comparator definition is that it might be used with another STL algorithms, like std::set_difference and should handle case when ranges has different types.
The problem is that I have to write huge amount of very similar comparators and it is easy to be entangled with which comparator to use.
I want to write the following snippet:
template<typename SDKClass, typename IDLClass>
class equal {
public:
bool operator()(const IDLClass &a, const IDLClass &b) {
if (strcmp(a.ipaddr.in(), b.ipaddr.in())) {
return false;
}
return true;
}
bool operator()(const SDKClass &a, const SDKClass &b) {
if (strcmp(a.ip_address().c_str(), b.ip_address().c_str())) {
return false;
}
return true;
}
bool operator()(const IDLClass &a, const SDKClass &b) {
if (strcmp(a.ipaddr.in(), b.ip_address().c_str())) {
return false;
}
return true;
}
bool operator()(const SDKClass &a, const IDLClass &b) {
if (strcmp(a.ip_address().c_str(), b.ipaddr.in())) {
return false;
}
return true;
}
};
So HNInfoIPComparator() would be generated depending on types passed as its arguments inside std::unique function.
Therefore I want to pass to std::unique templated functor (class). Is it possible to do that and how?
Also I want to handle case when functor contains some internal data, which are used for comparisons
Most important code samples:
// Automatically generated structure from IDL specification
// Basically simple structure
struct THNNetIPInfo
{
typedef THNNetIPInfo_var _var_type;
typedef THNNetIPInfo_out _out_type;
static void _tao_any_destructor (void *);
::TAO::String_Manager ipaddr;
::TAO::String_Manager netmask;
};
// THNNetIPInfoIter - class external iterator
// which was written manually
typedef Util::CorbaSeqIter<THNNetIPInfoList, THNNetIPInfo> THNNetIPInfoIter;
// THNNetIPInfoList - also automatically generated class
// from IDL specification, list of THNNetIPInfo elements
THNNetIPInfoList list(...);
THNNetIPInfoIter first(&list, 0);
THNNetIPInfoIter end(&list, list.length());

Answer 1

Instead of writing a class template with four comparison operators, write a plain class with a templated comparison operator that adapts the inputs into the keys you want to compare:
class HNInfoIPComparator {
static const char* adapt(const THNNetIPInfo& t) {
return t.ipaddr.in();
}
static const char* adapt(const SDK::TIPAddressDescription& t) {
return t.ip_address().c_str();
}
public:
template <typename T, typename U>
bool operator()(const T& t, const U& u) const {
return !strcmp(adapt(t), adapt(u));
}
};
You can extend the comparator easily by adding overloads of adapt for additional types, e.g., std::string or const char*.