使用C++模板编程提取任意结构的字段类型

Question

If I define a struct with arbitrary data types like:

struct custom_type {
    int a;
    float b;
    char c; 
    float *d; // etc...
};

Is there a common pattern using template programming (C++) to extract the field types of this struct and map them to some type-specific code handler at compile time ?

Some context: I'm creating an api that allows for clients to define their own arbitrary custom types, and still allow those to integrate with the underlying system I have for managing and introspecting these types, doing automated memory management and other housekeeping.

The "wrapping" template or other mechanism will allow this integration without the underlying system knowing anything about the header file that defines the custom type. From the client's point of view, the code accesses the struct in the normal way, but the template allows generic handling of each field in the struct.

Thanks.

Answer 1

The behavior you describe is called reflection . The C++ language features which support this feature are very limited (yet). However, with some tricks it is possible to achieve some level of reflection.

One example is Boost.PFR (Precise and Flat Reflection) by Antony Polukhin. PFR is an official boost library since December 2020 (boost 1.75.0), but according to its documentation it is a

header only library that does not depend on Boost.

Here is one example from the README :

#include <iostream>
#include <string>

#include "boost/pfr.hpp"

struct some_person {
    std::string name;
    unsigned birth_year;
};

int main() {
    some_person val{"Edgar Allan Poe", 1809};

    std::cout << boost::pfr::get<0>(val)                // No macro!
        << " was born in " << boost::pfr::get<1>(val);  // Works with any aggregate initializables!

    std::cout << boost::pfr::io(val);                   // Outputs: {"Edgar Allan Poe", 1809}
}

In the chapter on Limitations and Configuration , the docs state that at least C++14 is required, but at least C++17 is recommended. One important requirement is that

T must be aggregate initializable without empty base classes

Some underlying techniques are explained in a recorded CppCon 2016 talk with the title "C++14 Reflections Without Macros, Markup nor External Tooling.." .

Answer 2

No, that's not possible using templates or any other features in the standard C++ language. Such a feature is known as introspection or reflection. If you search using those terms, you'll find some external tools that can provide such information at build time (eg clang); after generating it you can run custom tools performing code generation.