To be short, I want to achieve something like “templatize” the operation of “getting an attribute of a object” as a generalized template function, like the function foo below.

template <
    typename T, 
    // something like
    "attribute" obj // or T::obj
>
"sth refer to type of obj" foo(T t)
{
    return t.obj; 
}

What I hope to achieve is, with any simple struct/class like

struct A
{
    int b; 
    type_of_c c; 
    ...
} a; // for any class/struct A with (non-function) attribute b in any type

foo<A, A::b>(a) may return a.b, and foo<A, A::c>(a) may return a.c.

So my question is, how can I express T::obj to set the information of “specifying attribute obj of class T” as the parameter of template?

Some limitations in my programming:

• Methods that unfriendly to compile and IDE auto-hints are not encouraged. e.g. sending attribute name in form of string instead of attribute itself, or involving macro.
• Add type of T::obj to the template is acceptable, but not encouraged.
• Tricks in any version of c++ is acceptable. Using newly published versions like C++20 is definitely ok.
• The solution is expected to work also for class template, if possible.