C++中declaration与define的区别

Many C++ books do not clearly state the difference between declaration and definition, or they only mention the need to support separate compilation, indicating that using the extern specifier is a declaration, while without it is a definition. In fact, I think the C++ standard describes the difference between declaration and definition more clearly.

In section 2.2.2 of “C++ Primer 5th,” it mentions the relationship between variable declarations and definitions:

To support separate compilation, the C++ language distinguishes between declarations and definitions. A declaration makes the name known to the program; a file must include the declaration of a name to use a name defined elsewhere. A definition is responsible for creating the entity associated with the name.
To declare a variable without defining it, add the extern keyword before the variable name and do not explicitly initialize the variable.

extern int i; // declares i rather than defines i
int j; // declares and defines j

Any declaration that includes an explicit initialization becomes a definition. We can give a variable marked with the extern keyword an initial value, but doing so negates the effect of extern. An extern statement containing an initializer is no longer a declaration but a definition.

extern double pi=3.14159; // definition

The above is the description of the relationship between variable declarations and definitions in C++ Primer. I think the description in the C++ standard is more comprehensive:

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-specification (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).

Let me briefly translate this.
A declaration is a definition unless:

• it declares a function but does not specify the function body.

int func(); // declaration

• it contains an extern specifier or a linkage-specification (e.g., extern “C”) and has neither an initializer nor a function body.

extern int x;
extern "c" void func();

• it declares a static data member within a class definition.

struct A{
	A(){};
	static int x; // declaration
	static const int y=11; // define
	// notice: non-const static data member must be initialized out of line
};
int A::x=12; // define

• it is a class name declaration.

struct A;

• it is an opaque-enum-declaration.

enum A:int;

• it is a template parameter.

template<typename T> T f(T); // T is a declaration

• it is a parameter declaration in a function declaration.

int func(int x);

• it is a typedef declaration.

typedef int INT;

• it is an alias declaration.

using INT=int;

• it is a using declaration.

namespace A{
	typedef int INT;
}

using A::INT;

• it is a static_assert declaration.

static_assert(foo());

• it is an attribute declaration.

[[noreturn]] void funv();

• it is an empty declaration.

// empty-declaration:
;

• it is a using directive.

using namespace std;

In the above cases, declarations are not definitions. In simple terms, a “declaration” provides name and parameter prototypes, while a “definition” provides storage allocation and the corresponding implementation.
Oh, I just realized that cppreference already has this: Define and ODR.