struct with default members initializers cannot be inside class and used as default argument

Question

The following code does not compile:

#include <iostream>

struct toto {
  struct options final {
    bool option1 = false;
    bool option2 = false;
  };

  static void func(const options &opt = {}) {
    std::cout << "option1 = " << opt.option1 << std::endl;
    std::cout << "option2 = " << opt.option2 << std::endl;
  }
};

int main() {
  toto::func();
}

I get the following errors:

test-default-member-init.cc:12:42: error: default member initializer for 'option1' needed within definition of enclosing class 'toto' outside of member functions
  static void func(const options &opt = {}) {
                                         ^
test-default-member-init.cc:8:10: note: default member initializer declared here
    bool option1 = false;
         ^
test-default-member-init.cc:19:14: error: too few arguments to function call, single argument 'opt' was not specified
  toto::func();
  ~~~~~~~~~~ ^
test-default-member-init.cc:12:3: note: 'func' declared here
  static void func(const options &opt = {}) {
  ^
2 errors generated.

But if I move the options struct outside of my enclosing toto struct, it does compile:

#include <iostream>

struct options final {
  bool option1 = false;
  bool option2 = false;
};

struct toto {
  static void func(const options &opt = {}) {
    std::cout << "option1 = " << opt.option1 << std::endl;
    std::cout << "option2 = " << opt.option2 << std::endl;
  }
};

int main() {
  toto::func();
}

I'm using clang on OSX, and compile the code snippet this way:

$ clang --version
Apple clang version 12.0.0 (clang-1200.0.32.2)
Target: x86_64-apple-darwin19.6.0
Thread model: posix
InstalledDir: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin

$ c++ -std=c++17 -stdlib=libc++ -o test-default-member-init test-default-member-init.cc

Can someone explain me why the first code snipped it does not compile, and if there is any solution to get it working?

Answer 1

I had a similar issue, It only happens on XCode. The only workaround that I found is to have 2 functions, one with a parameter and another one without it. Call the second function/constructor from the first one with the default value.

Header

        struct CreateEntityParams
        {
            bool serializable = true;
        };
        explicit Entity(
            EntityId id,
            std::string name,
            Entity * parent = nullptr
        );
        explicit Entity(
            EntityId id,
            std::string name,
            CreateEntityParams const params
            Entity * parent = nullptr
        );

Cpp

    Entity::Entity(
        EntityId id,
        std::string name,
        Entity * parent
    )
        : Entity(id, name, parent, CreateEntityParams {})
    {}
    
    Entity::Entity(
        EntityId const id,
        std::string name,
        CreateEntityParams const & params,
        Entity * parent
    )
        : mId(id)
        , mName(std::move(name))
        , mParent(parent)
        , mSerializable(params.serializable)
    {}