Can't convert to base class(?)

Question

I created the ServiceCommand class, from which I inherited UndoCommand and RedoCommand. Now I'm writing a try_create_service_command function that should return a pointer to the ServiceCommand. To do this, I created a variable std :: unique_ptr <ServiceCommand> cmd = nullptr in the function, and when parsing dto-objects, I do cmd = std ::make_unique<UndoCommand>(). try_create_service_command() implementation :

std::optional<std::unique_ptr<ServiceCommand>> CommandCreator::try_create_service_command() {
    if (dto_objects.empty()) {
        return {};
    }

    std::unique_ptr<ServiceCommand> cmd = nullptr;
    auto dto = dto_objects.front();
    auto name = dto.get_name();
    switch (name) {
        case Command::UNDO:
            cmd = std::make_unique<UndoCommand>();
            dto_objects.erase(dto_objects.begin());
            break;
        case Command::REDO:
            cmd = std::make_unique<RedoCommand>();
            dto_objects.erase(dto_objects.begin());
            break;
        default:
            return {};
    }
    return cmd;
}

At lines cmd = std::make_unique<UndoCommand>(); and cmd = std::make_unique<RedoCommand>(); the compiler says:

[ 23%] Building CXX object CMakeFiles/text_editor.dir/command_creator.cpp.o
[ 23%] Building CXX object CMakeFiles/text_editor.dir/undo.cpp.o
In file included from /Users/dilshod/CLionProjects/text_editor/redo.cpp:1:
In file included from /Users/dilshod/CLionProjects/text_editor/redo.h:3:
In file included from /Users/dilshod/CLionProjects/text_editor/service_command.h:3:
In file included from /Users/dilshod/CLionProjects/text_editor/editor.h:4:
/Users/dilshod/CLionProjects/text_editor/undo.h:4:34: error: expected class name
class UndoCommand final : public ServiceCommand{
                                 ^
/Users/dilshod/CLionProjects/text_editor/undo.h:6:18: error: unknown type name 'Editor'
    void execute(Editor& editor) const override;
                 ^
In file included from /Users/dilshod/CLionProjects/text_editor/undo.cpp:1:
In file included from /Users/dilshod/CLionProjects/text_editor/undo.h:2:
In file included from /Users/dilshod/CLionProjects/text_editor/service_command.h:3:
In file included from /Users/dilshod/CLionProjects/text_editor/editor.h:5:
/Users/dilshod/CLionProjects/text_editor/redo.h:6:34: error: expected class name
class RedoCommand final : public ServiceCommand{
                                 ^
/Users/dilshod/CLionProjects/text_editor/redo.h:8:18: error: unknown type name 'Editor'
    void execute(Editor& editor) const override;
                 ^
22 errors generated.
 errors generated.
make[3]: *** [CMakeFiles/text_editor.dir/undo.cpp.o] Error 1
make[3]: *** Waiting for unfinished jobs....
make[3]: *** [CMakeFiles/text_editor.dir/redo.cpp.o] Error 1
In file included from /Users/dilshod/CLionProjects/text_editor/command_creator.cpp:1:
In file included from /Users/dilshod/CLionProjects/text_editor/command_creator.h:12:
In file included from /Users/dilshod/CLionProjects/text_editor/service_command.h:3:
In file included from /Users/dilshod/CLionProjects/text_editor/editor.h:4:
/Users/dilshod/CLionProjects/text_editor/undo.h:4:34: error: expected class name
class UndoCommand final : public ServiceCommand{
                                 ^
/Users/dilshod/CLionProjects/text_editor/undo.h:6:18: error: unknown type name 'Editor'
    void execute(Editor& editor) const override;
                 ^
In file included from /Users/dilshod/CLionProjects/text_editor/command_creator.cpp:1:
In file included from /Users/dilshod/CLionProjects/text_editor/command_creator.h:12:
In file included from /Users/dilshod/CLionProjects/text_editor/service_command.h:3:
In file included from /Users/dilshod/CLionProjects/text_editor/editor.h:5:
/Users/dilshod/CLionProjects/text_editor/redo.h:6:34: error: expected class name
class RedoCommand final : public ServiceCommand{
                                 ^
/Users/dilshod/CLionProjects/text_editor/redo.h:8:18: error: unknown type name 'Editor'
    void execute(Editor& editor) const override;
                 ^
/Users/dilshod/CLionProjects/text_editor/command_creator.cpp:48:17: error: no viable overloaded '='
            cmd = std::make_unique<UndoCommand>();
            ~~~ ^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/Library/Developer/CommandLineTools/usr/bin/../include/c++/v1/memory:2426:28: note: candidate function (the implicit copy assignment operator) not viable: no known conversion from 'unique_ptr<UndoCommand, default_delete<UndoCommand>>' to 'const unique_ptr<ServiceCommand, default_delete<ServiceCommand>>' for 1st argument
class _LIBCPP_TEMPLATE_VIS unique_ptr {
                           ^
/Library/Developer/CommandLineTools/usr/bin/../include/c++/v1/memory:2541:15: note: candidate function not viable: no known conversion from 'unique_ptr<UndoCommand, default_delete<UndoCommand>>' to 'unique_ptr<ServiceCommand, default_delete<ServiceCommand>>' for 1st argument
  unique_ptr& operator=(unique_ptr&& __u) _NOEXCEPT {
              ^
/Library/Developer/CommandLineTools/usr/bin/../include/c++/v1/memory:2580:15: note: candidate function not viable: no known conversion from 'typename __unique_if<UndoCommand>::__unique_single' (aka 'unique_ptr<UndoCommand>') to 'std::nullptr_t' (aka 'nullptr_t') for 1st argument
  unique_ptr& operator=(nullptr_t) _NOEXCEPT {
              ^
/Library/Developer/CommandLineTools/usr/bin/../include/c++/v1/memory:2552:15: note: candidate template ignored: requirement 'is_convertible<UndoCommand *, ServiceCommand *>::value' was not satisfied [with _Up = UndoCommand, _Ep = std::__1::default_delete<UndoCommand>]
  unique_ptr& operator=(unique_ptr<_Up, _Ep>&& __u) _NOEXCEPT {
              ^
/Users/dilshod/CLionProjects/text_editor/command_creator.cpp:55:17: error: no viable overloaded '='
            cmd = std::make_unique<RedoCommand>();
            ~~~ ^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/Library/Developer/CommandLineTools/usr/bin/../include/c++/v1/memory:2426:28: note: candidate function (the implicit copy assignment operator) not viable: no known conversion from 'unique_ptr<RedoCommand, default_delete<RedoCommand>>' to 'const unique_ptr<ServiceCommand, default_delete<ServiceCommand>>' for 1st argument
class _LIBCPP_TEMPLATE_VIS unique_ptr {
                           ^
/Library/Developer/CommandLineTools/usr/bin/../include/c++/v1/memory:2541:15: note: candidate function not viable: no known conversion from 'unique_ptr<RedoCommand, default_delete<RedoCommand>>' to 'unique_ptr<ServiceCommand, default_delete<ServiceCommand>>' for 1st argument
  unique_ptr& operator=(unique_ptr&& __u) _NOEXCEPT {
              ^
/Library/Developer/CommandLineTools/usr/bin/../include/c++/v1/memory:2580:15: note: candidate function not viable: no known conversion from 'typename __unique_if<RedoCommand>::__unique_single' (aka 'unique_ptr<RedoCommand>') to 'std::nullptr_t' (aka 'nullptr_t') for 1st argument
  unique_ptr& operator=(nullptr_t) _NOEXCEPT {
              ^
/Library/Developer/CommandLineTools/usr/bin/../include/c++/v1/memory:2552:15: note: candidate template ignored: requirement 'is_convertible<RedoCommand *, ServiceCommand *>::value' was not satisfied [with _Up = RedoCommand, _Ep = std::__1::default_delete<RedoCommand>]
  unique_ptr& operator=(unique_ptr<_Up, _Ep>&& __u) _NOEXCEPT {
              ^
6 errors generated.
make[3]: *** [CMakeFiles/text_editor.dir/command_creator.cpp.o] Error 1
make[2]: *** [CMakeFiles/text_editor.dir/all] Error 2
make[1]: *** [CMakeFiles/text_editor.dir/rule] Error 2
make: *** [text_editor] Error 2

Classes:


#include "editor.h"

class Editor;

class ServiceCommand
{
public:
    ServiceCommand() = default;
    virtual ~ServiceCommand() = default;
    virtual void execute(Editor& editor) const = 0;
};

class RedoCommand final : public ServiceCommand{
public:
    void execute(Editor& editor) const override;
};

class UndoCommand final : public ServiceCommand{
public:
    void execute(Editor& editor) const override;
};

How to fix ?

If I remove the `dto` stuff and just keep the `cmd = ...`. it compiles fine for me (MSVC). — BoP, Dec 05 '21 at 13:02
Could you post __all__ compiler error and info messages verbatim into the question? Could you change `class` to `struct` and try again? I can't reproduce https://godbolt.org/z/rT3WTdqoY . || `std::optional — KamilCuk, Dec 05 '21 at 13:17
@DilshodNozimov - I removed the lines that don't compile without the whole class (those containing `dto` or `Command`) , and kept those containing `cmd`. And separately they compiled fine. — BoP, Dec 05 '21 at 15:45
Looks like `undo.h` doesn't `#include "service_command.h"` (or whatever the name is of the .h file that defines the `ServiceCommand` class) — Jeremy Friesner, Dec 06 '21 at 05:37
`error: unknown type name 'Editor'` Is your error. Because the class definition is not complete, you can't construct it, so you can't make_unique it. — KamilCuk, Dec 06 '21 at 08:01

Can't convert to base class(?)

0 Answers0