objective-c has "c" linkage. Objective-c++ has c++ linkage. C and C++ are different languages that share some common syntax.
You need to declare the function as extern "C" in your .mm context
for example
declarations.h:
#ifdef __cplusplus
extern "C" {
#endif
// all function declarations here are common to C and C++. They will all have C linkage
// i.e. no name mangling and no overloading
double maxRadiusFinder(double* algoPoints1,int count,int numberC);
#ifdef __cplusplus
}
#endif