How to allow range-for loop on my class? [duplicate]

Question

I have a class like this:

class Foo {
private:
    int a,b,c,d;
    char bar;
    double m,n
public:
    //constructors here
};

I wanna allow range-for loop on my class, e.g.

Foo foo {/*...*/};
for(auto& f : foo) {
  //f will be a specific order such as c,b,d,(int)m,(int)bar,a,(int)n
}

How can I achieve this? I was looking at iterator but don't know what are the requirements for a range-for loop. (Please don't ask me to use array or STL type)

Answer 1

The loop is defined to be equivalent to:

for ( auto __begin = <begin-expr>,
           __end = <end-expr>;
      __begin != __end;
      ++__begin ) {
    auto& f = *__begin;
    // loop body
}

where <begin-expr> is foo.begin(), or begin(foo) if there isn't a suitable member function, and likewise for <end-expr>. (This is a simplification of the specification in C++11 6.5.4, for this particular case where the range is a lvalue of class type).

So you need to define an iterator type that supports pre-increment ++it, dereference *it and comparison i1 != i2; and either

• give foo public member functions begin() and end(); or
• define non-member functions begin(foo) and end(foo), in the same namespace as foo so that they can be found by argument-dependent lookup.

Answer 2

This seems fairly un-C++-like, and rather prone to breakage. What if the iteration order is changed (accidentally or not) during some update in the future? Clients relying on a specific order will break.

All that said if you wish to support this all you have to do is implement your own iterator and provide begin/end methods (or free functions with those names) to provide access. Then the iterator takes care of remembering which attribute it's currently looking at and provides it when dereferenced.