zoukankan      html  css  js  c++  java
  • Definitions

    Definitions and ODR

    Definitions are declarations that fully define the entity introduced by the declaration. Every declaration is a definition, except for the following:
    下面都是declaration而不是definition

    extern const int a; // declares, but doesn't define a
    extern const int b = 1; // defines b
    
    • A function declaration without a function body
    int f(int);			// declares, but doesn't define f
    
    • A parameter declaration in a function declaration that isn't a definition
    int f(int x); // declares, but doesn't define f and x
    int f(int x) { // defines f and x
         return x+a;
    }
    
    • Declaration of a static data member inside a class definition
    struct S {    // defines S
        int n;        // defines S::n
        static int i; // declares, but doesn't define S::i
    };
    int S::i; // defines S::i
    
    • Declaration of a class name (by forward declaration or by the use of the elaborated type specifier in another declaration)
    struct S; // declares, but doesn't define S
    class Y f(class T p); // declares, but doesn't define Y and T (and also f and p)
    
    • Declaration of a template parameter
    template<typename T>	// declares, but doesn's define T
    
    • An explicit specialization whose declaration is not a definition.
    template<> struct A<int>; // declares, but doesn't define A<int>
    
    • A typedef declaration
    typedef S S2; // declares, but doesn't define S2 (S may be incomplete)
    
    • An alias-declaration(since C++11)
    using S2 = S; // declares, but doesn't define S2 (S may be incomplete)
    
    • An opaque declaration of an enumeration(since C++11)
    enum Color : int; // declares, but doesn't define Color
    
    • A using-declaration
    using N::d;	// declares, but doesn't define d
    
    • A using-directive (does not define any entities)(since C++11)
    • A static_assert declaration (does not define any entities)
    • An attribute declaration (does not define any entities)
    • An explicit instantiation declaration (an "extern template")
      extern template f<int, char>; // declares, but doesn't define f<int, char>
    • An empty declaration (does not define any entities)

    Where necessary, the compiler may implicitly define the default constructor, copy constructor, move constructor, copy assignment operator, move assignment operator, and the destructor.

    One Definition Rule=ODR

    Only one definition of any variable, function, class type, enumeration type, or template is allowed in any one translation unit (some of these may have multiple declarations, but only one definition is allowed).

    One and only one definition of every non-inline function or variable that is odr-used (see below) is required to appear in the entire program (including any standard and user-defined libraries). The compiler is not required to diagnose this violation, but the behavior of the program that violates it is undefined.

    For an inline function, a definition is required in every translation unit where it is odr-used.

    In short, the ODR states that:

    1. In any translation unit, a template, type, function, or object can have no more than one definition. Some of these can have any number of declarations. A definition provides an instance.
    2. In the entire program, an object or non-inline function cannot have more than one definition; if an object or function is used, it must have exactly one definition. You can declare an object or function that is never used, in which case you don't have to provide a definition. In no event can there be more than one definition.
    3. Some things, like types, templates, and extern inline functions, can be defined in more than one translation unit. For a given entity, each definition must be the same. Non-extern objects and functions in different translation units are different entities, even if their names and types are the same.

    Some violations of the ODR must be diagnosed by the compiler. Other violations, particularly those that span translation units, are not required to be diagnosed.

  • 相关阅读:
    非常不错的漂浮广告代码(须调用外部JS文件)
    终于也进入了博客园
    ARM伪指令必读
    细说嵌入式Linux文件系统的制作方法
    使用OpenJTAG来检查硬件焊接问题
    应对艰难职场环境的五条策略
    成为高端人才必看的二十大箴言
    神奇的platform_get_resource函数
    女生奋斗励志篇?现代女孩都应该看看
    中国40位40岁以下的商界精英
  • 原文地址:https://www.cnblogs.com/Wojoin/p/5168066.html
Copyright © 2011-2022 走看看