basic_def_odr.md

• There can be more than one definition of a class type (Clause 12), enumeration type (10.2), inline function with external linkage (10.1.6), inline variable with external linkage (10.1.6), class template (Clause 17), non-static function template (17.6.6), concept (17.6.8), static data member of a class template (17.6.1.3), member function of a class template (17.6.1.1), or template specialization for which some template parameters are not specified (17.8, 17.6.5) in a program provided that each definition appears in a different translation unit, and provided the definitions satisfy the following requirements. Given such an entity named D defined in more than one translation unit, then ... If the definitions of D do not satisfy these requirements, then the behavior is undefined. [basic.def.odr]/p12

  • Examples from: DCL60-CPP. Obey the one-definition rule

  // a.cpp
  struct S {
    int a;
  };
  
  // b.cpp
  // S in b.cpp does not consist of the same sequence of token as S in a.cpp
  class S {
  public:
    int a;
  };

  const int n = 42;
  
  int g(const int &lhs, const int &rhs);
  
  inline int f(int k) {
    return g(k, n);   // f() has external linkage 
           // n has internal linkage but is our-used by g() 
                              // n will not be identical in all transition units
  }

  • Rationale:
    • Devirtualization in C++, part 7 (Enforcing One Definition Rule)