After reading the question, I know the differences between declaration and definition. So does it mean definition equals declaration plus initialization?
Declaration, generally, refers to the introduction of a new name in the program. For example, you can declare a new function by describing its "signature":
void xyz();
or declare an incomplete type:
class klass;
struct ztruct;
and last but not least, to declare an object:
int x;
It is described, in the C++ standard, at §3.1/1 as:
A declaration (Clause 7) may introduce one or more names into a translation unit or redeclare names introduced by previous declarations.
A definition is a definition of a previously declared name (or it can be both definition and declaration). For example:
int x;
void xyz() {...}
class klass {...};
struct ztruct {...};
enum { x, y, z };
Specifically the C++ standard defines it, at §3.1/1, as:
A declaration is a definition unless it declares a function without specifying the function’s body (8.4), it contains the extern specifier (7.1.1) or a linkage-specification25 (7.5) and neither an initializer nor a function- body, it declares a static data member in a class definition (9.2, 9.4), it is a class name declaration (9.1), it is an opaque-enum-declaration (7.2), it is a template-parameter (14.1), it is a parameter-declaration (8.3.5) in a function declarator that is not the declarator of a function-definition, or it is a typedef declaration (7.1.3), an alias-declaration (7.1.3), a using-declaration (7.3.3), a static_assert-declaration (Clause 7), an attribute- declaration (Clause 7), an empty-declaration (Clause 7), or a using-directive (7.3.4).
Initialization refers to the "assignment" of a value, at construction time. For a generic object of type T
, it's often in the form:
T x = i;
but in C++ it can be:
T x(i);
or even:
T x {i};
with C++11.
So does it mean definition equals declaration plus initialization?
It depends. On what you are talking about. If you are talking about an object, for example:
int x;
This is a definition without initialization. The following, instead, is a definition with initialization:
int x = 0;
In certain context, it doesn't make sense to talk about "initialization", "definition" and "declaration". If you are talking about a function, for example, initialization does not mean much.
So, the answer is no: definition does not automatically mean declaration plus initialization.