IBM XLC/C++ for AIX V11.1语言参考详尽指南

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本资源是IBM针对其C/C++编译器XLC/C++ for AIX V11.1的语言参考手册，提供了详尽的语言规范和指导。这份文档适用于版本11.1的Program 5724-X13，以及后续的所有补丁和修改，直到有新的版本更新说明。它强调了在使用前阅读" Notices"页面467中的注意事项的重要性，确保用户使用的是与产品等级相匹配的正确版本。 "About this information"部分会介绍文档的目的和范围，包括它旨在帮助什么样的读者，如开发人员、系统管理员或技术支持人员理解C/C++语言在AIX平台上的特性、用法和可能遇到的问题。这部分内容将涵盖语法、特性、库函数、错误处理等内容的基础知识。 "Who should read this information"部分明确了目标受众，包括那些正在使用或计划使用IBM XLC/C++的开发者，特别是那些需要了解高级编程技巧、性能优化或调试工具的专业人士。 "如何使用此信息"部分会指导读者如何有效地利用这本参考手册，包括如何查找特定主题、理解示例代码、查阅API文档以及处理遇到的技术问题。 "如何组织信息"会阐述文档结构，可能会包含章节目录、索引和主题分类，以便用户能够快速定位所需的信息。 "Conventions"章节则会介绍文档中的约定和术语，确保读者在阅读时能够理解和遵循标准格式。 "Related information"部分可能会链接到其他IBM相关的技术文档，比如编程指南、API文档或最佳实践，以便读者能够获得更全面的知识。 "IBM XLC/C++ information"详细涵盖了C/C++语言在IBM AIX环境下的特性和实现，可能涉及编译选项、内存管理、并行编程和面向对象编程等关键特性。 "Standards and specifications"部分列出了所依据的标准和规格，如ISO/IEC 9899:1999 (C99) 和 ISO/IEC 14882:2011 (C++11)，以确保兼容性和遵循行业最佳实践。 "Other IBM information"可能包括与AIX操作系统、工具链和其他IBM产品相关的链接，有助于用户在更大的技术生态系统中找到相关支持。这本IBM XLC/C++ for AIX V11.1语言参考手册是深入学习和掌握C/C++在IBM AIX平台上编程的宝贵资源，对于提升编程技能和解决实际问题具有重要作用。

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xiv XL C/C++: Language Reference

Chapter 1. Scope and linkage

Scope is the largest region of program text in which a name can potentially be used

without qualification to refer to an entity; that is, the largest region in which the

name is potentially valid. Broadly speaking, scope is the general context used to

differentiate the meanings of entity names. The rules for scope combined with

those for name resolution enable the compiler to determine whether a reference to

an identifier is legal at a given point in a file.

The scope of a declaration and the visibility of an identifier are related but distinct

concepts. Scope is the mechanism by which it is possible to limit the visibility of

declarations in a program. The visibility of an identifier is the region of program

text from which the object associated with the identifier can be legally accessed.

Scope can exceed visibility, but visibility cannot exceed scope. Scope exceeds

visibility when a duplicate identifier is used in an inner declarative region, thereby

hiding the object declared in the outer declarative region. The original identifier

cannot be used to access the first object until the scope of the duplicate identifier

(the lifetime of the second object) has ended.

Thus, the scope of an identifier is interrelated with the storage duration of the

identified object, which is the length of time that an object remains in an identified

region of storage. The lifetime of the object is influenced by its storage duration,

which in turn is affected by the scope of the object identifier.

Linkage refers to the use or availability of a name across multiple translation units

or within a single translation unit. The term translation unit refers to a source code

file plus all the header and other source files that are included after preprocessing

with the #include directive, minus any source lines skipped because of conditional

preprocessing directives. Linkage allows the correct association of each instance of

an identifier with one particular object or function.

Scope and linkage are distinguishable in that scope is for the benefit of the

compiler, whereas linkage is for the benefit of the linker. During the translation of

a source file to object code, the compiler keeps track of the identifiers that have

external linkage and eventually stores them in a table within the object file. The

linker is thereby able to determine which names have external linkage, but is

unaware of those with internal or no linkage.

The distinctions between the different types of scopes are discussed in “Scope” on

page 2. The different types of linkages are discussed in “Program linkage” on page

Parameter names for a function have the scope of the outermost block of that

function. Also, if the function is declared and not defined, these parameter names

have function prototype scope.

When one block is nested inside another, the variables from the outer block are

usually visible in the nested block. However, if the declaration of a variable in a

nested block has the same name as a variable that is declared in an enclosing

block, the declaration in the nested block hides the variable that was declared in

the enclosing block. The original declaration is restored when program control

returns to the outer block. This is called block visibility.

Name resolution in a local scope begins in the immediately enclosing scope in

which the name is used and continues outward with each enclosing scope. The

order in which scopes are searched during name resolution causes the

phenomenon of information hiding. A declaration in an enclosing scope is hidden

by a declaration of the same identifier in a nested scope.

Related reference

“Block statements” on page 190

Function scope

The only type of identifier with function scope is a label name. A label is implicitly

declared by its appearance in the program text and is visible throughout the

function that declares it.

A label can be used in a goto statement before the actual label is seen.

Related reference

“Labeled statements” on page 187

Function prototype scope

In a function declaration (also called a function prototype) or in any function

declarator—except the declarator of a function definition—parameter names have

function prototype scope. Function prototype scope terminates at the end of the

nearest enclosing function declarator.

Related reference

“Function declarations” on page 212

File/global scope

A name has file scope if the identifier's declaration appears outside of any

block. A name with file scope and internal linkage is visible from the point where

it is declared to the end of the translation unit.

Global scope or global namespace scope is the outermost namespace scope of

a program, in which objects, functions, types and templates can be defined. A

name has global namespace scope if the identifier's declaration appears outside of all

blocks, namespaces, and classes.

A name with global namespace scope and internal linkage is visible from the point

where it is declared to the end of the translation unit.

Chapter 1. Scope and linkage 3

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