C++ template to read value from member variable or member function

Question

I am writing code generator and using flatbuffers for generating classes. The rest of the code generator will work with these classes in C++.

I have not been able to figure out how to keep the API consistent for reading data for two different types of classes that flatbuffer may generate. I am using the object api (testRecordT) in the example for whenever an object needs to be written to (and can be read back as well) and flatbuffer overlay for when the data can only be read from.

I have not been able to get any template or free functions to work to give me a consistent api that would work in both the cases.

Below is a snippet of what I am trying to get to work.

struct testRecordT {
  int32_t field1;
  std::string field2;
};

struct testRecord {
    int32_t field1() const {
        return 0;
        // flatbuffer generated - return GetField<int32_t>(VT_FIELD1, 0);
    }
    const flatbuffers::String *field2() const {
        return nullptr;
       // flatbuffer generated - return GetPointer<const flatbuffers::String *>(VT_FIELD3);
    }

};

void Test() {
   testRecordT * members; // assume pointers are valid
   testRecord * memberFunctions;

// Need to be able to create a read function/template that would work. This would simplify the code generation a lot. I can generate either one below, as long as it is consistent in both cases.
   auto r = read(members->field1); // or read(members,field1)
   auto v = read(memberFunctions->field1); // or read(memberFunctions,field1)
}

The read functions or template functions should be consistent. Any pointers or thoughts would be helpful. I am using C++17 with gcc 7.3.1 .

Answer 1

You can use std::invoke for this. It can both call functions or access members.

auto r = std::invoke(&testRecordT::field1, members);
auto v = std::invoke(&testRecord::field1, memberFunctions);