How is compilation unit defined in c++?

Question

Possible Duplicate:
What is a “translation unit” in C++

It is often said that the static variables declared in C/C++ are not visible across compilation units ? Does this mean that each .c or .cpp file is a separate compilation unit ? What about a .h file and the static variables declared in the .h file ? Is .h file also considered as a separate compilation unit ?

Possible duplicate: http://stackoverflow.com/questions/1106149/what-is-a-translation-unit-in-c — Flexo, Feb 14 '11 at 12:44
Technically a duplicate, but that presumes that you know that a "compilation unit" is the same as a "translation unit". — MSalters, Feb 14 '11 at 12:55

score 42 · Accepted Answer · edited May 15 '23 at 05:01

42

Header files have no separate life, only their content is #included into .c or .cpp files. But since #include is handled by the preprocessor, the compiler has no knowledge about distinct header files; it only sees the resulting code listing as input. This is what is called a compilation unit: a source file with all its #include directives replaced by the content of the relevant header files and all other preprocessor statements processed (like #define, #ifdef etc.).

edited May 15 '23 at 05:01

Thomas Weller

55,411
20
125
222

answered Feb 14 '11 at 12:44

Péter Török

114,404
31
268
329

1

This question could be interpreted as duplicated, but the answer is much more instructive than the answer of the original question – user5193682 Jun 21 '16 at 04:45
2

... minus any lines excluded by conditional preprocessor directives. – Eric Towers Feb 08 '18 at 19:56

score 12 · Answer 2 · edited Sep 27 '14 at 13:48

C and C++ compilation is (usually) divided in three independent steps:

Preprocessing, involving macro and #include expansions.
Compiling, converting source code to binary code and generating intermediante object files.
Linking, joining the object files in a single ELF or EXE file.

Wherever there is an #include or a macro, the preprocessor expands that expression with the actual value. In the case of an #include that entire line is replaced with the .h file contents.

The actual compiler is (usually) not aware of any header file, it sees a compilation unit as a big .c or .cpp file.

The "usually" part comes from the fact that some compilers optimizes header inclusion by storing a precompiled header in some sort of cache, but the effect is the same.

score 5 · Answer 3 · answered Feb 14 '11 at 12:45

The compiler only processes source files, usually with the extension .c or .cpp. The compiler doesn't really care about the files that are included: as far as the compiler is usually implemented, each .c/.cpp file is processed anew, whatever .h files are read (courtesy of the preprocessor).

This is why we talk about 'compilation units': something that is compiled in one go, the results of which may subsequently be linked together into executables.

How is compilation unit defined in c++?

3 Answers3

Linked