Why are unnamed namespaces used and what are their benefits

Question

I just joined a new C   software project and I m trying to understand the design   The project makes frequent use of unnamed namespaces   For example  something like this may occur in a class definition file      newusertype cc namespace     const int SIZE OF ARRAY X    const int SIZE OF ARRAY Y    bool getState userType  otherUserType       newusertype  newusertype             What are the design considerations that might cause one to use an unnamed namespace   What are the advantages and disadvantages

User · Answer

Having something in an anonymous namespace means it's local to this translation unit (.cpp file and all its includes) this means that if another symbol with the same name is defined elsewhere there will not be a violation of the One Definition Rule (ODR).

This is the same as the C way of having a static global variable or static function but it can be used for class definitions as well (and should be used rather than static in C++).

All anonymous namespaces in the same file are treated as the same namespace and all anonymous namespaces in different files are distinct. An anonymous namespace is the equivalent of:

namespace __unique_compiler_generated_identifer0x42 {
    ...
}
using namespace __unique_compiler_generated_identifer0x42;

User · Answer

An anonymous namespace makes the enclosed variables  functions  classes  etc  available only inside that file   In your example it s a way to avoid global variables   There is no runtime or compile time performance difference   There isn t so much an advantage or disadvantage aside from  do I want this variable  function  class  etc  to be public or private

User · Answer

Having something in an anonymous namespace means it's local to this translation unit (.cpp file and all its includes) this means that if another symbol with the same name is defined elsewhere there will not be a violation of the One Definition Rule (ODR).

This is the same as the C way of having a static global variable or static function but it can be used for class definitions as well (and should be used rather than static in C++).

All anonymous namespaces in the same file are treated as the same namespace and all anonymous namespaces in different files are distinct. An anonymous namespace is the equivalent of:

namespace __unique_compiler_generated_identifer0x42 {
    ...
}
using namespace __unique_compiler_generated_identifer0x42;

User · Answer

Unnamed namespaces are a utility to make an identifier translation unit local  They behave as if you would choose a unique name per translation unit for a namespace   namespace unique      empty      using namespace unique  namespace unique      namespace body  stuff in here        The extra step using the empty body is important  so you can already refer within the namespace body to identifiers like   name that are defined in that namespace  since the using directive already took place    This means you can have free functions called  for example  help that can exist in multiple translation units  and they won t clash at link time  The effect is almost identical to using the static keyword used in C which you can put in in the declaration of identifiers  Unnamed namespaces are a superior alternative  being able to even make a type translation unit local    namespace   int a1    static int a2    Both a s are translation unit local and won t clash at link time  But the difference is that the a1 in the anonymous namespace gets a unique name   Read the excellent article at comeau-computing Why is an unnamed namespace used instead of static   Archive org mirror

User · Answer

In addition to the other answers to this question  using an anonymous namespace can also improve performance  As symbols within the namespace do not need any external linkage  the compiler is freer to perform aggressive optimization of the code within the namespace  For example  a function which is called multiple times once in a loop can be inlined without any impact on the code size   For example  on my system the following code takes around 70  of the run time if the anonymous namespace is used  x86-64 gcc-4 6 3 and -O2  note that the extra code in add val makes the compiler not want to include it twice     include  lt iostream gt   namespace     double a    void b double x          a -  x        void add val double x          a    x      if x  0 01  b 0       if x  0 02  b 0 6       if x  0 03  b -0 1       if x  0 04  b 0 4          int main       a   0    for int i 0  i lt 1000000000    i              add val i 1e-10           std  cout  lt  lt  a  lt  lt    n     return 0

User · Answer

An anonymous namespace makes the enclosed variables  functions  classes  etc  available only inside that file   In your example it s a way to avoid global variables   There is no runtime or compile time performance difference   There isn t so much an advantage or disadvantage aside from  do I want this variable  function  class  etc  to be public or private

User · Answer

An anonymous namespace makes the enclosed variables  functions  classes  etc  available only inside that file   In your example it s a way to avoid global variables   There is no runtime or compile time performance difference   There isn t so much an advantage or disadvantage aside from  do I want this variable  function  class  etc  to be public or private

User · Answer

Unnamed namespace limits access of class variable function and objects to the file in which it is defined  Unnamed namespace functionality is similar to static keyword in C C    static keyword limits access of global variable and function to the file in which they are defined  There is difference between unnamed namespace and static keyword because of which unnamed namespace has advantage over static  static keyword can be used with variable  function and objects but not with user defined class  For example   static int x      Correct    But   static class xyz    Body of class      Wrong static structure    Body of structure      Wrong   But same can be possible with unnamed namespace   For example    namespace              class xyz    Body of class               static structure    Body of structure          Correct

User · Answer

The example shows that the people in the project you joined don t understand anonymous namespaces     namespace       const int SIZE OF ARRAY X      const int SIZE OF ARRAY Y    These don t need to be in an anonymous namespace  since const object already have static linkage and therefore can t possibly conflict with identifiers of the same name in another translation unit       bool getState userType  otherUserType        And this is actually a pessimisation  getState   has external linkage  It is usually better to prefer static linkage  as that doesn t pollute the symbol table  It is better to write  static bool getState             here  I fell into the same trap  there s wording in the standard that suggest that file-statics are somehow deprecated in favour of anonymous namespaces   but working in a large C   project like KDE  you get lots of people that turn your head the right way around again

User · Answer

In addition to the other answers to this question  using an anonymous namespace can also improve performance  As symbols within the namespace do not need any external linkage  the compiler is freer to perform aggressive optimization of the code within the namespace  For example  a function which is called multiple times once in a loop can be inlined without any impact on the code size   For example  on my system the following code takes around 70  of the run time if the anonymous namespace is used  x86-64 gcc-4 6 3 and -O2  note that the extra code in add val makes the compiler not want to include it twice     include  lt iostream gt   namespace     double a    void b double x          a -  x        void add val double x          a    x      if x  0 01  b 0       if x  0 02  b 0 6       if x  0 03  b -0 1       if x  0 04  b 0 4          int main       a   0    for int i 0  i lt 1000000000    i              add val i 1e-10           std  cout  lt  lt  a  lt  lt    n     return 0

User · Answer

Unnamed namespaces are a utility to make an identifier translation unit local  They behave as if you would choose a unique name per translation unit for a namespace   namespace unique      empty      using namespace unique  namespace unique      namespace body  stuff in here        The extra step using the empty body is important  so you can already refer within the namespace body to identifiers like   name that are defined in that namespace  since the using directive already took place    This means you can have free functions called  for example  help that can exist in multiple translation units  and they won t clash at link time  The effect is almost identical to using the static keyword used in C which you can put in in the declaration of identifiers  Unnamed namespaces are a superior alternative  being able to even make a type translation unit local    namespace   int a1    static int a2    Both a s are translation unit local and won t clash at link time  But the difference is that the a1 in the anonymous namespace gets a unique name   Read the excellent article at comeau-computing Why is an unnamed namespace used instead of static   Archive org mirror

User · Answer

Unnamed namespaces are a utility to make an identifier translation unit local  They behave as if you would choose a unique name per translation unit for a namespace   namespace unique      empty      using namespace unique  namespace unique      namespace body  stuff in here        The extra step using the empty body is important  so you can already refer within the namespace body to identifiers like   name that are defined in that namespace  since the using directive already took place    This means you can have free functions called  for example  help that can exist in multiple translation units  and they won t clash at link time  The effect is almost identical to using the static keyword used in C which you can put in in the declaration of identifiers  Unnamed namespaces are a superior alternative  being able to even make a type translation unit local    namespace   int a1    static int a2    Both a s are translation unit local and won t clash at link time  But the difference is that the a1 in the anonymous namespace gets a unique name   Read the excellent article at comeau-computing Why is an unnamed namespace used instead of static   Archive org mirror

User · Answer

The example shows that the people in the project you joined don t understand anonymous namespaces     namespace       const int SIZE OF ARRAY X      const int SIZE OF ARRAY Y    These don t need to be in an anonymous namespace  since const object already have static linkage and therefore can t possibly conflict with identifiers of the same name in another translation unit       bool getState userType  otherUserType        And this is actually a pessimisation  getState   has external linkage  It is usually better to prefer static linkage  as that doesn t pollute the symbol table  It is better to write  static bool getState             here  I fell into the same trap  there s wording in the standard that suggest that file-statics are somehow deprecated in favour of anonymous namespaces   but working in a large C   project like KDE  you get lots of people that turn your head the right way around again

User · Answer

An anonymous namespace makes the enclosed variables  functions  classes  etc  available only inside that file   In your example it s a way to avoid global variables   There is no runtime or compile time performance difference   There isn t so much an advantage or disadvantage aside from  do I want this variable  function  class  etc  to be public or private

User · Answer

Having something in an anonymous namespace means it's local to this translation unit (.cpp file and all its includes) this means that if another symbol with the same name is defined elsewhere there will not be a violation of the One Definition Rule (ODR).

This is the same as the C way of having a static global variable or static function but it can be used for class definitions as well (and should be used rather than static in C++).

All anonymous namespaces in the same file are treated as the same namespace and all anonymous namespaces in different files are distinct. An anonymous namespace is the equivalent of:

namespace __unique_compiler_generated_identifer0x42 {
    ...
}
using namespace __unique_compiler_generated_identifer0x42;

User · Answer

Unnamed namespaces are a utility to make an identifier translation unit local  They behave as if you would choose a unique name per translation unit for a namespace   namespace unique      empty      using namespace unique  namespace unique      namespace body  stuff in here        The extra step using the empty body is important  so you can already refer within the namespace body to identifiers like   name that are defined in that namespace  since the using directive already took place    This means you can have free functions called  for example  help that can exist in multiple translation units  and they won t clash at link time  The effect is almost identical to using the static keyword used in C which you can put in in the declaration of identifiers  Unnamed namespaces are a superior alternative  being able to even make a type translation unit local    namespace   int a1    static int a2    Both a s are translation unit local and won t clash at link time  But the difference is that the a1 in the anonymous namespace gets a unique name   Read the excellent article at comeau-computing Why is an unnamed namespace used instead of static   Archive org mirror

User · Answer

Having something in an anonymous namespace means it's local to this translation unit (.cpp file and all its includes) this means that if another symbol with the same name is defined elsewhere there will not be a violation of the One Definition Rule (ODR).

This is the same as the C way of having a static global variable or static function but it can be used for class definitions as well (and should be used rather than static in C++).

All anonymous namespaces in the same file are treated as the same namespace and all anonymous namespaces in different files are distinct. An anonymous namespace is the equivalent of:

namespace __unique_compiler_generated_identifer0x42 {
    ...
}
using namespace __unique_compiler_generated_identifer0x42;

User · Answer

Unnamed namespace limits access of class variable function and objects to the file in which it is defined  Unnamed namespace functionality is similar to static keyword in C C    static keyword limits access of global variable and function to the file in which they are defined  There is difference between unnamed namespace and static keyword because of which unnamed namespace has advantage over static  static keyword can be used with variable  function and objects but not with user defined class  For example   static int x      Correct    But   static class xyz    Body of class      Wrong static structure    Body of structure      Wrong   But same can be possible with unnamed namespace   For example    namespace              class xyz    Body of class               static structure    Body of structure          Correct

[c++] Why are unnamed namespaces used and what are their benefits?

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