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Harry
Harry

Reputation: 3822

What is a "translation unit" in C++?

I am reading at the time the "Effective C++" written by Scott Meyers and came across the term "translation unit".

Could somebody please give me an explanation of the following:

  1. What exactly is it?

  2. When should I consider using it while programming with C++?

  3. Is it C++ only, or can it be used with other programming languages?

I might already use it without knowing the term...

Upvotes: 343

Views: 119949

Answers (11)

Ana Betts
Ana Betts

Reputation: 74654

A translation unit is for all intents and purposes a file (.c/.cpp), after it's finished including all of the header files.

http://web.archive.org/web/20091213073754/http://msdn.microsoft.com/en-us/library/bxss3ska(VS.80).aspx

Upvotes: 95

Allan Stokes
Allan Stokes

Reputation: 535

In addition to the ODR, the translation unit is important in the definition of unnamed namespaces, which replaces one of the old uses of "static".

Upvotes: 7

anon
anon

Reputation:

A hard question to answer definitively. The C++ standard states:

The text of the program is kept in units called source files in this International Standard. A source file together with all the headers (17.4.1.2) and source files included (16.2) via the preprocessing directive #include, less any source lines skipped by any of the conditional inclusion (16.1) preprocessing directives, is called a translation unit. [Note: a C++ program need not all be translated at the same time. ]

So for most intents and purposes a translation unit is a single C++ source file and the header or other files it includes via the preprocessor #include mechanism.

Regarding your other questions:

  1. When should I consider using it when programming with C++

You can't not use it - translation units are the basis of a C++ program.

  1. If it is related only to C++, or it can be used with other programming languages

Other languages have similar concepts, but their semantics will be subtly different. Most other languages don't use a preprocessor, for example.

Upvotes: 37

JeffH
JeffH

Reputation: 10482

From here: (wayback machine link)

According to standard C++ (wayback machine link) : A translation unit is the basic unit of compilation in C++. It consists of the contents of a single source file, plus the contents of any header files directly or indirectly included by it, minus those lines that were ignored using conditional preprocessing statements.

A single translation unit can be compiled into an object file, library, or executable program.

The notion of a translation unit is most often mentioned in the contexts of the One Definition Rule, and templates.

Upvotes: 353

ganesh marmat
ganesh marmat

Reputation: 31

C and C++ programs consist of one or more source files, each of which contains some of the text of the program. A source file, together with its include files (files that are included using the #include preprocessor directive) but not including sections of code removed by conditional-compilation directives such as #if, is called a "translation unit."

Upvotes: 3

Yang
Yang

Reputation: 1

In my view, a "translation unit" is typically a single "post-preprocessing" source file. You can get more details on this MSDN page. http://msdn.microsoft.com/en-us/library/bxss3ska(v=vs.80).aspx

Upvotes: 0

rahul
rahul

Reputation: 91

According to MSDN: C and C++ programs consist of one or more source files, each of which contains some of the text of the program. A source file, together with its include files (files that are included using the #include preprocessor directive) but not including sections of code removed by conditional-compilation directives such as #if, is called a "translation unit."

Upvotes: 1

John Bode
John Bode

Reputation: 123468

As others have said, a translation unit is basically the contents of a source file after preprocessing. It's the topmost production in the language grammar; you would only need to worry about it if you were writing a C or C++ compiler.

Upvotes: 0

yesraaj
yesraaj

Reputation: 47900

Every cpp/c (implementation) file will be converted into a translation unit (ie.,object file (.obj)) headers in the cpp file will be replaced with the actual text from the header files.

Upvotes: 0

sigjuice
sigjuice

Reputation: 29759

A translation unit is code that is passed to the compiler proper. This typically means the output from running the preprocessor on the .c file.

Upvotes: 6

Ed Swangren
Ed Swangren

Reputation: 124642

The book makes it clear enough. When Meyers referes to a "translation Unit", he means a source code file.

Upvotes: 8

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