C++ is a pretty interesting language because of different quirks that it has. One of such things is the fact that there is a difference between defined constructor, not defined constructor and deleted constructor, especially move constructor.<\/p>\n
Consider following code:<\/p>\n
\r\n#include \r\n\r\nstruct ExplicitMoveConstructor\r\n{\r\n ExplicitMoveConstructor()\r\n {\r\n std::cout << \"Default constructor\" << std::endl;\r\n }\r\n\r\n ExplicitMoveConstructor(ExplicitMoveConstructor const& src)\r\n {\r\n std::cout << \"Copy constructor\" << std::endl;\r\n }\r\n\r\n ExplicitMoveConstructor(ExplicitMoveConstructor && src)\r\n {\r\n std::cout << \"Move constructor\" << std::endl;\r\n }\r\n};\r\n\r\nstruct DeletedMoveConstructor\r\n{\r\n DeletedMoveConstructor()\r\n {\r\n std::cout << \"Default constructor\" << std::endl;\r\n }\r\n\r\n DeletedMoveConstructor(DeletedMoveConstructor const& src)\r\n {\r\n std::cout << \"Copy constructor\" << std::endl;\r\n }\r\n\r\n DeletedMoveConstructor(DeletedMoveConstructor && src) = delete;\r\n};\r\n\r\nstruct NotDefinedMoveConstructor\r\n{\r\n NotDefinedMoveConstructor()\r\n {\r\n std::cout << \"Default constructor\" << std::endl;\r\n }\r\n\r\n NotDefinedMoveConstructor(NotDefinedMoveConstructor const& src)\r\n {\r\n std::cout << \"Copy constructor\" << std::endl;\r\n }\r\n};\r\n\r\nauto returnExplicitMoveConstructorClass()\r\n{\r\n ExplicitMoveConstructor cm;\r\n return cm;\r\n}\r\n\r\nauto returnDeletedMoveConstructorClass()\r\n{\r\n DeletedMoveConstructor cm;\r\n return cm;\r\n}\r\n\r\nauto returnNotDefinedMoveConstructorClass()\r\n{\r\n NotDefinedMoveConstructor cm;\r\n return cm;\r\n}\r\n\r\nint main()\r\n{\r\n auto first = returnExplicitMoveConstructorClass();\r\n auto second = returnDeletedMoveConstructorClass();\r\n auto third = returnNotDefinedMoveConstructorClass();\r\n return 0;\r\n}\r\n<\/pre>\nThe presented code does not compile when trying to do so with following command:<\/p>\n
\r\nclang++ constructor.cpp --std=c++14\r\n<\/pre>\nThe error code is as follows:<\/p>\n
\r\nconstructor.cpp:70:7: error: call to deleted constructor of 'DeletedMoveConstructor'\r\n auto second = returnDeletedMoveConstructorClass();\r\n ^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~\r\nconstructor.cpp:33:2: note: 'DeletedMoveConstructor' has been explicitly marked deleted here\r\n DeletedMoveConstructor(DeletedMoveConstructor && src) = delete;\r\n ^\r\n1 error generated.\r\n<\/pre>\nThe error from the compiler says that the move constructor was called but it is marked as deleted. If compiled with g++ it's pretty much the same. <\/p>\n
Conclusions from this are two. The first one is that the deleted constructor is also a declared constructor so the compiler will use it as if it was defined by the user. The second one is that the value returned from function is returned by utilizing move semantic when move constructor is declared, even if it is deleted. <\/p>\n