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Legacy C++

Annex C compatibility

Invalid and Valid Code Between C++ 98/03 and C++ 11

• The banning of narrowing conversions during aggregate initialization in C++11

  int a[] = { 1.0 }; // invalid C++ 11
      

• >> on the right of template parameter is valid in C++11 but not in C++03
• std::swap are defined in <utility> in C++11 but <algorithm> in C++98.
• std::basic_ios has in C++11 explicit operator bool() const instead of a conversion that evaluates to bool.
• string literals do not convert to char* in C++11.

Different Behaviors of the Same Code Under C++ 98/03 and C++ 11

• Integer division always rounds to zero.
• a destructor is now by default noexcept(true) unless specified otherwise, but C++98 allows exceptions.
• Dependent function calls with internal linkage are considered in C++11

  static void f(int) { }
  void f(long) { }
  
  template<typename T>
  void g(T t) { f(t); }
  
  int main() { g(0); } // caslls f(int) in C++11 but f(long) in C++98
  
  // However, with instantiation, this still calls f(B)
  struct B { };
  struct A : B { };
  
  template<typename T>
  void g(T t) { f(t); }
  
  static void f(A) { }
  void f(B) { }
  
  int main() { A a; g(a); }
      

• std::vector<T> a(50) resolves to a separate constructor that construct the elements in place in C++11 while in C++03 a default value of the second parameter is copied to each slot.
  • in case where a copy constructor taking a default value is different from the default constructor, this behavior is a breaking change.

Later Changes Since C++11

• C++17 changed std::string::data to char* from const char* to allow easy in-place string data modification through a pointer.
• Unnamed classes with a typedef name for linkage purposes (i.e. C++ code) can contain only

C-compatible constructs since C++20. Note this does not forbid the use inside extern "C" blocks or structs that contains only data.

// forbidden since C++20
typedef struct {
   void member_function();
} m_struct;