Why is copy constructor called instead of conversion constructor?

by 
B b2 = a;

This is known as Copy Initialization.

It does the following:

  1. Create an object of type B from a by using B (const A& a).
  2. Copy the created temporary object to b2 by using B (const B& b).
  3. Destroy the temporary object by using ~B().

The error you get is not at step 1 but rather at step 2.

Where is this in the standard?

C++03 8.5 Initializers
Para 14:

….
— If the destination type is a (possibly cv-qualified) class type:
…
…
— Otherwise (i.e., for the remaining copy-initialization cases), user-defined conversion sequences that can convert from the source type to the destination type or (when a conversion function is used) to a derived class thereof are enumerated as described in 13.3.1.4, and the best one is chosen through overload resolution (13.3). If the conversion cannot be done or is ambiguous, the initialization is ill-formed. The function selected is called with the initializer expression as its argument; if the function is a constructor, the call initializes a temporary of the destination type. The result of the call (which is the temporary for the constructor case) is then used to direct-initialize, according to the rules above, the object that is the destination of the copy-initialization. In certain cases, an implementation is permitted to eliminate the copying inherent in this direct-initialization by constructing the intermediate result directly into the object being initialized; see 12.2, 12.8.

More Related Contents:

  1. What is The Rule of Three?
  2. Conditions for automatic generation of default/copy/move ctor and copy/move assignment operator?
  3. Why should the copy constructor accept its parameter by reference in C++?
  4. What is a converting constructor in C++ ? What is it for?
  5. Why must the copy assignment operator return a reference/const reference?
  6. std::thread pass by reference calls copy constructor
  7. Copy constructor and = operator overload in C++: is a common function possible?
  8. When do we have to use copy constructors?
  9. Why is the copy constructor not called?
  10. C++ Compiler Error C2280 “attempting to reference a deleted function” in Visual Studio 2013 and 2015
  11. Copy constructor is not inherited
  12. Why is this code trying to call the copy constructor?
  13. The copy constructor and assignment operator
  14. How are C++ array members handled in copy control functions?
  15. C++ template copy constructor on template class
  16. How do I make this C++ object non-copyable?
  17. Can we return objects having a deleted/private copy/move constructor by value from a function?
  18. Disable copy constructor
  19. constructor invocation mechanism
  20. What’s the use of the private copy constructor in c++
  21. Why C++ copy constructor must use const object?
  22. Explicit copy constructor
  23. Calling assignment operator in copy constructor
  24. Why is the copy constructor called when we pass an object as an argument by value to a method?
  25. Why am I not provided with a default copy constructor from a volatile?
  26. Uniform initialization fails to copy when object has no data members
  27. Why aren’t copy constructors “chained” like default constructors and destructors?
  28. C++ Copy constructor gets called instead of initializer_list
  29. C++ copy-construct construct-and-assign question
  30. Why are iostreams not copyable?

Leave a Comment