问题
I had always thought that creating a new object would always call the default constructor on an object, and whether the constructor was explicit or automatically generated by the compiler made no difference. According to this highly regarded answer to a different question, this changed in a subtle way between C++98 and C++03 and now works like so:
struct B { ~B(); int m; }; // non-POD, compiler generated default ctor
new B; // default-initializes (leaves B::m uninitialized)
new B(); // value-initializes B which zero-initializes all fields since its default ctor is compiler generated as opposed to user-defined.
Can anyone tell me:
- Why was the standard changed, i.e. what advantage does this give or what is now possible that wasn't before;
- What exacly do the terms "default-initialize" and "value-initialize" represent?
- What's the relevant part of the standard?
回答1:
I do not know what the rationales around the change (or how the standard was before), but on how it is, basically default-initialization is either calling a user defined constructor or doing nothing (lots of hand-waving here: this is recursively applied to each subobject, which means that the subobjects with a default constructor will be initialized, the subobjects with no user defined constructors will be left uninitialized).
This falls within the only pay for what you want philosophy of the language and is compatible with C in all the types that are C compatible. On the other hand, you can request value-initialization, and that is the equivalent to calling the default constructor for objects that have it or initializing to 0 converted to the appropriate type for the rest of the subobjects.
This is described in §8.5 Initializers, and it is not trivial to navigate through. The definitions for zero-initialize, default-initialize and value-initialize are the 5th paragraph:
To zero-initialize an object of type T means:
— if T is a scalar type (3.9), the object is set to the value of 0 (zero) converted to T;
— if T is a non-union class type, each nonstatic data member and each base-class subobject is zeroinitialized;
— if T is a union type, the object’s first named data member89) is zero-initialized;
— if T is an array type, each element is zero-initialized;
— if T is a reference type, no initialization is performed.
To default-initialize an object of type T means:
— if T is a non-POD class type (clause 9), the default constructor for T is called (and the initialization is ill-formed if T has no accessible default constructor);
— if T is an array type, each element is default-initialized;
— otherwise, the object is zero-initialized.
To value-initialize an object of type T means:
— if T is a class type (clause 9) with a user-declared constructor (12.1), then the default constructor for T is called (and the initialization is ill-formed if T has no accessible default constructor);
— if T is a non-union class type without a user-declared constructor, then every non-static data member and base-class component of T is value-initialized;
— if T is an array type, then each element is value-initialized;
— otherwise, the object is zero-initialized
A program that calls for default-initialization or value-initialization of an entity of reference type is illformed. If T is a cv-qualified type, the cv-unqualified version of T is used for these definitions of zeroinitialization, default-initialization, and value-initialization.
来源：https://stackoverflow.com/questions/7084831/difference-between-default-initialize-and-value-initialize-in-c03