C++ is a statically typed, free-form, multi-paradigm, compiled, general-purpose programming language. It is regarded as an intermediate-level language, as it comprises a combination of both high-level and low-level language features. It was developed by Bjarne Stroustrup starting in 1979 at Bell...
, linkage describes how names can or can not refer to the same entity throughout the whole program or one single translation unit
Translation unit (programming)
In C programming language terminology, a translation unit is the ultimate input to a C compiler from which an object file gets generated.-Context:...
.
The static keyword is used in C to restrict the visibility of a function or variable to its translation unit. This is also valid in C++, although C++ deprecates this usage in favor of anonymous namespaces (which are not available in C). Also, C++ implicitly treats any const namespace-scope variable as having internal linkage unless it is explicitly declared extern, unlike C.
A name's linkage is related to, but distinct from, its scope
Scope (programming)
In computer programming, scope is an enclosing context where values and expressions are associated. Various programming languages have various types of scopes. The type of scope determines what kind of entities it can contain and how it affects them—or semantics...
. The scope of a name is the part of a translation unit where it is visible. For instance, a name with global scope (which is the same as file-scope in C and the same as the global namespace-scope in C++) is visible in any part of the file. Its scope will end at the end of the translation unit, whether or not that name has been given external or internal linkage.
If the name has external linkage, the entity that name denotes may be referred to from another translation unit using a distinct declaration for that same name, and from other scopes within the same translation unit using distinct declarations. Were the name given internal linkage, such a declaration would denote a distinct entity, although using the same name, but its entity could be referred to by distinct declarations within the same translation unit. A name that has no linkage at all cannot be referred to from declarations in different scopes, not even from within the same translation unit. Examples of such names are parameters of functions and local variables. The details differ between C (where only objects and functions - but not types have linkage) and C++ and between this simplified overview.
Linkage between languages must be done with some care, as different languages adorn
Name mangling
In compiler construction, name mangling is a technique used to solve various problems caused by the need to resolve unique names for programming entities in many modern programming languages....
their external symbols differently.
A common idiom uses extern "C" to link C++ and C code.
Linkage in C
Definition of 'linkage' quoted from ISO/IEC 9899:TC3 (C99 Standard). C uses the term "identifier" where this article uses name which is what C++ uses to formalize linkage:
An identifier declared in different scopes or in the same scope more than once can be made to refer to the same object or function by a process called linkage.
Function foo is declared in two files, with its function body defined in demo2.c. Via linkage, foo called in main inside demo1.c refers to foo in demo2.c. This is an example of external linkage for a function.
In computer software, an application binary interface describes the low-level interface between an application program and the operating system or another application.- Description :...
The C and C++ programming languages are closely related. C++ grew out of C, as it was designed to be source-and-link compatible with C. Due to this, C code is often developed with C++ IDEs, integrated with C++ code, and compiled in C++ compilers...
In compiler construction, name mangling is a technique used to solve various problems caused by the need to resolve unique names for programming entities in many modern programming languages....
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