initializer | Android App 初始化任务执行框架 | Job Scheduling library

overload resolution […] chooses a single best constructor […] to perform the initialization of an object of type X from the argument initializer list

so the fact that the initialization of B from {1} would be ill-formed is irrelevant.

There are several such places where implicit conversion sequences are more liberal than actual initialization, causing certain cases to be ambiguous even though some of the possibilities wouldn’t actually work. If the programmer was confused and thought one of those near misses was actually a better match, it’s a feature that the ambiguity is reported.

Yes, if you write a user defined copy constructor, then you must write an initialiser for every sub object - unless you wish to default initialise them, in which case you don't need any initialiser - or if you can use a default member initialiser.

the object contains some member variables which are pointers to its other member variables)

This is a design that should be avoided when possible. Not only does this force you to define custom copy and move assignment operators and constructors, but it is often unnecessarily inefficient.

But, in case that is necessary for some reason - or custom special member functions are needed for any other reason - you can achieve clean code by combining the normally copying parts into a separate dummy class. That way the the user defined constructor has only one sub object to initialise.

Like this:

This is also what the error message you posted, advises to do: Set the spring.mongodb.embedd ed.version property or define your own MongodConfig bean to use embedded MongoDB

The annotation @DataMongoTest is meta-annotated with @ImportAutoConfiguration and @AutoConfigureDataMongo, and is designed to trigger auto-configuration of MongoDB unless explicitly disabled as you do in the working configuration examples.

In your first configuration example, the annotation @EnableAutoConfiguration(exclude = EmbeddedMongoAutoConfiguration.class) does not override this effect of @DataMongoTest.

With Spring Boot 2.5.6, the auto-configured MongodConfig bean is most likely also part of the application context but not effectively used. But this depends on the rest of the code and in particular on the MongodbContainerInitializer.

The standard has this to say (emphasis mine):

Absent default member initializers, if any non-static data member of a union has a non-trivial default constructor, copy constructor, move constructor, copy assignment operator, move assignment operator, or destructor, the corresponding member function of the union must be user-provided or it will be implicitly deleted for the union.

But I think the wording is a bit wooly here. I think what they actually mean is that you must provide an initialiser for the member that has (in your example) a non-trivial constructor, like so: