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ODR

ODR的含义和implementation

  • ODR within a file
  • ODR within the whole program

compiler需要从多个层次来保证ODR：

1) ODR within translation unit，是由compiler来保证的

2) ODR within the whole target file，是由linker来保证的

在这种情况下，linker的作用对象是**symbol with external linkage**，对于**symbol with internal linkage**，由于它们是**translation local**的，linker是不会对它们进行检查的。

关于linkage，参见C-and-C++\From-source-code-to-exec\Link\Linkage。

ODR and multiple definition

ODR中的"O"的含义是one，而在C++中，有的时候允许出现multiple definition，显然这看似是矛盾的。上面结合implementation对此进行分析，显然:

在C++、C中，是允许同时存在多个translation中存在着**translation local**的definition的。

这个问题在learncpp 6.6 — Internal linkage # The one-definition rule and internal linkage 章节进行了总结：

The one-definition rule and internal linkage

In lesson 2.7 -- Forward declarations and definitions, we noted that the one-definition rule says that an object or function can’t have more than one definition, either within a file or a program.

However, it’s worth noting that internal objects (and functions) that are defined in different files are considered to be independent entities (even if their names and types are identical), so there is no violation of the one-definition rule. Each internal object only has one definition.

cppreference Definitions and ODR (One Definition Rule)#One Definition Rule

Only one definition of any variable, function, class type, enumeration type, concept (since C++20) or template is allowed in any one translation unit (some of these may have multiple declarations, but only one definition is allowed).

NOTE: 这就是前面描述的1) 单个translation unit内。

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.

NOTE: 这就是前面描述的整个target file内。

For an inline function or inline variable (since C++17), a definition is required in every translation unit where it is odr-used.

NOTE: 对于inline function、inline variable，在每个definition中都需要一个definition，C++语言规定这是没有违反ODR的；在这种情况下，compiler是有多种实现方式的:

1) 将definition设置为translation local/internal linkage

2) 将函数inline

One and only one definition of a class is required to appear in any translation unit where the class is used in a way that requires it to be complete.

There can be more than one definition in a program of each of the following: class type, enumeration type, inline function, inline variable (since C++17), templated entity (template or member of template, but not full template specialization), as long as all of the following is true:

NOTE: 需要注意的是，上面这段话所强调的是"in a program "

NOTE: 原文这里给出了一些列描述的规则

If all these requirements are satisfied, the program behaves as if there is only one definition in the entire program. Otherwise, the program is ill-formed, no diagnostic required.

NOTE: 这是C++语言的特殊规定，这种情况下实际上是有多个definition，但是C++语言规定将它们视为同一个definition。

wikipedia One Definition Rule

TODO

https://cpluspluspedia.com/en/tutorial/4907/one-definition-rule--odr-