Failure to Match Template Using Function Pointer as Argument - c++

Question

I am attempting to write a function which will operate on a std::map of any key and class pointer given to it, and create a new std::map with an index based on the return value of a function in the class. Essentially, a template function to re-index a map based on a function in its contained class. However, I am having compiler errors when trying to invoke the function.
template<class AnyType, class ValueType, class FunctionType>
AssocArray<FunctionType,ValueType> reindex( const AssocArray<AnyType, ValueType>& original, FunctionType (*getterFunction)() ) {
AssocArray<FunctionType, ValueType> ret;
FunctionType index;
for(typename AssocArray<AnyType,ValueType>::const_iterator it = original.begin(); it!=original.end(); it++) {
index = ((*it->second).*getterFunction)();
ret[index] = it->second;
}
return ret;
}
Invoked by:
floatIndexed = reindex( intIndexed, &test::getB );
Where getB is of float type.
This results in the compiler error:
src/World.cpp:78:50: error: no matching function for call to ‘reindex(std::map<int, onathacar::test*>&, float (onathacar::test::*)())’
src/World.cpp:78:50: note: candidate is:
./include/Types.h:123:36: note: template<class AnyType, class ValueType, class FunctionType> std::map<PreservedType, ValueType> onathacar::reindex(const std::map<LookupType, ValueType>&, FunctionType (*)())
I have tried different variations of this, including using "FunctionType (ValueType::*getterFunction)()" and changing "AssocArray" to "AssocArray". The only one which worked added a fourth template argument:
template<class AnyType, class ValueType, class FunctionType, class SomeType>
AssocArray<FunctionType,ValueType> reindex( const AssocArray<AnyType, ValueType>& original, FunctionType (SomeType::*getterFunction)() ) {
However, this seems like it would potentially allow functions that are not actually members of ValueType to be called, and so I would prefer some other option. I am not even sure what is wrong, as it appears the templates match, at least with the addition of "ValueType::". Why does the call not match the template, and is there a way to fix it without a fourth templated type?
For more context, Header Containing Implementation and Calling Function.

Answer 1

You have two problems. The first is that reindex implies that the value types are values, but you use them as pointers:
AssocArray<float, test*> floatIndexed;
floatIndexed = reindex( intIndexed, &test::getB );
The second is that reindex's second parameter needs to be declared as a member function, not a free function. So reindex should look like this:
template<class AnyType, class ValueType, class FunctionType>
AssocArray<FunctionType,ValueType *> reindex( const AssocArray<AnyType, ValueType *>& original, FunctionType (ValueType:: *getterFunction)() ) {
AssocArray<FunctionType, ValueType*> ret;
FunctionType index;
for(typename AssocArray<AnyType,ValueType*>::const_iterator it = original.begin(); it!=original.end(); it++) {
index = ((*it->second)->*getterFunction)();
ret[index] = it->second;
}
return ret;
}

Answer 2

It seems that you are trying to use your rindex() function with members but your function is declared to use non-members. This won't work. The reason this won't work is that the you need an object to access either function or data members of a class.

Answer 3

template<class A, class B>
class X{
public:
};
class Y{
public:
int func() { return 42; }
};
template<class A, class B, class C>
X<C,B> reindex( const X<A, B>& original, C (B::*getterFunction)() ) {
X<C, B> x2;
cout << "hello" << endl;
return x2;
}
int main() {
X x1;
reindex(x1,&Y::func);
return 0;
}
This works and gives perfectly valid results.
The problem with your call AssocArray intIndexed, is the fact that you are trying to pass &test::getB as the getter function, which assumes valueType = test, where as your actual value type, is test*.