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Asked: 2026-05-15T23:08:12+00:00

Consider the following class hierarchy: base class Object with a virtual method foo() an

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Consider the following class hierarchy:

  • base class Object with a virtual method foo()
  • an arbitrary hierarchy with multiple inheritance (virtual and non-virtual); each class is a subtype of Object; some of them override foo(), some don’t
  • a class X from this hierarchy, not overriding foo()

How to determine which method will be executed upon a call of foo() on an object of class X in C++?

(I’m looking for the algorithm, not any specific case.)

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1 Answer

  1. Editorial Team

    There is no MRO in C++ like Python. If a method is ambiguous, it is a compile-time error. Whether a method is virtual or not doesn’t affect it, but virtual inheritance will.

    The algorithm is described in the C++ standard §[class.member.lookup] (10.2). Basically it will find the closest unambiguous implementation in the superclass graph. The algorithm works like this:

    1. Suppose you want to look up a function f in class C.

    2. We define a look-up set S(f, C) being a pair of sets (Δ, Σ) representing all possibilities. (§10.2/3)

      • The set Δ is called the declaration set, which is basically all the possible f‘s.

      • The set Σ is called the subobject set, which contain the classes that these f‘s are found.

    3. Let S(f, C) include all f directly defined (or using-ed) in C, if any (§10.2/4):

      Δ = {f in C};
if (Δ != empty)
  Σ = {C};
else
  Σ = empty;
S(f, C) = (Δ, Σ);

    4. If S(f, C) is empty (§10.2/5),

      • Compute S(f, Bi) where Bi is a base class of C, for all i.

      • Merge each S(f, Bi) into S(f, C) one by one.

        if (S(f, C) == (empty, empty)) {
  B = base classes of C;
  for (Bi in B)
    S(f, C) = S(f, C) .Merge. S(f, Bi);
}

    5. Finally the declaration set is returned as the result of name resolution (§10.2/7).

      return S(f, C).Δ;

    6. The merge between two look-up sets (Δ1, Σ1) and (Δ2, Σ2) is defined as (§10.2/6):

      • If every class in Σ1 is a base class of at least one class in Σ2, return (Δ2, Σ2).
        (Similar for the reverse.)
      • Else if Δ1 ≠ Δ2, return (ambiguous, Σ1 ∪ Σ2).

      • Otherwise, return (Δ1, Σ1 ∪ Σ2)

        function Merge ( (Δ1, Σ1), (Δ2, Σ2) ) {

   function IsBaseOf(Σp, Σq) {
     for (B1 in Σp) {
       if (not any(B1 is base of C for (C in Σq)))
         return false;
     }
     return true;
   }

   if      (Σ1 .IsBaseOf. Σ2) return (Δ2, Σ2);
   else if (Σ2 .IsBaseOf. Σ1) return (Δ1, Σ1);
   else {
      Σ = Σ1 union Σ2;
      if (Δ1 != Δ2)
        Δ = ambiguous; 
      else
        Δ = Δ1;
      return (Δ, Σ);
   }
}

    For example (§10.2/10),

    struct V { int f(); };
struct W { int g(); };
struct B : W, virtual V { int f(); int g(); };
struct C : W, virtual V { };

struct D : B, C {
   void glorp () {
     f();
     g();
   }
};

    We compute that

    S(f, D) = S(f, B from D) .Merge. S(f, C from D)
        = ({B::f}, {B from D}) .Merge. S(f, W from C from D) .Merge. S(f, V)
        = ({B::f}, {B from D}) .Merge. empty .Merge. ({V::f}, {V})
        = ({B::f}, {B from D})   // fine, V is a base class of B.

    and

    S(g, D) = S(g, B from D) .Merge. S(g, C from D)
        = ({B::g}, {B from D}) .Merge. S(g, W from C from D) .Merge. S(g, V)
        = ({B::g}, {B from D}) .Merge. ({W::g}, {W from C from D}) .Merge. empty
        = (ambiguous, {B from D, W from C from D})  // the W from C is unrelated to B.
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