Why is there no multiple definition error when you define a class in a header file?

by

The one-definition rule (3.2, [basic.def.odr]) applies differently to classes and functions:

1 – No translation unit shall contain more than one definition of any variable, function, class type, enumeration type, or template.

[…]

4 – Every program shall contain exactly one definition of every non-inline function or variable that is odr-used in that program […]

So while (non-inline) functions may be defined at most once in the whole program (and exactly once if they are called or otherwise odr-used), classes may be defined as many times as you have translation units (source files), but no more than once per translation unit.

The reason for this is that since classes are types, their definitions are necessary to be able to share data between translation units. Originally, classes (structs in C) did not have any data requiring linker support; C++ introduces virtual member functions and virtual inheritance, which require linker support for the vtable, but this is usually worked around by attaching the vtable to (the definition of) a member function.

More Related Contents:

  1. how to write makefile when two header files include eachother in c++? [duplicate]
  2. What is the difference between #include and #include “filename”?
  3. When should I write the keyword ‘inline’ for a function/method?
  4. Splitting templated C++ classes into .hpp/.cpp files–is it possible?
  5. What is __gxx_personality_v0 for?
  6. Altering DLL search path for static linked DLL
  7. How to specify preference of library path?
  8. MinGW linker error: winsock
  9. What do C and Assembler actually compile to? [closed]
  10. Difference between and ?
  11. Undefined reference error for template method [duplicate]
  12. Is #pragma once part of the C++11 standard?
  13. scope of using declaration within a namespace
  14. Libraries in /usr/local/lib not found
  15. How to link using GCC without -l nor hardcoding path for a library that does not follow the libNAME.so naming convention?
  16. How does the linker handle identical template instantiations across translation units?
  17. Using pthread in c++
  18. How to solve ——-undefined reference to `__chkstk_ms’——-on mingw
  19. static variables in an inlined function
  20. Handling header files dependencies with cmake
  21. Can I link a plain file into my executable? [duplicate]
  22. Header file included only once in entire program?
  23. Can different GCC dialects be linked together?
  24. What am I allowed to do with a static, constexpr, in-class initialized data member?
  25. cmake – Global linker flag setting (for all targets in directory)
  26. Default member values best practice
  27. What does exactly the warning mean about hidden symbol being referenced by DSO?
  28. Using a static library in Qt Creator
  29. Is is a good practice to put the definition of C++ classes into the header file?
  30. Eclipse CDT: Unresolved inclusion of stl header

Leave a Comment