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6.2 — User-defined namespaces

In lesson 2.9 -- Naming collisions and an introduction to namespaces, we introduced the concept of naming collisions and namespaces. As a reminder, a naming collision occurs when two identical identifiers are introduced into the same scope, and the compiler can’t disambiguate which one to use. When this happens, compiler or linker will produce an error because they do not have enough information to resolve the ambiguity. As programs become larger, the number of identifiers increases linearly, which in turn causes the probability of a naming collision occurring to increase exponentially.

Let’s revisit an example of a naming collision, and then show how we can resolve it using namespaces. In the following example, foo.cpp and goo.cpp are the source files that contain functions that do different things but have the same name and parameters.

foo.cpp:

goo.cpp:

main.cpp:

If this project contains only foo.cpp or goo.cpp (but not both), it will compile and run without incident. However, by compiling both into the same program, we have now introduced two different functions with the same name and parameters into the same scope (the global scope), which causes a naming collision. As a result, the linker will issue an error:

goo.cpp:3: multiple definition of `doSomething(int, int)'; foo.cpp:3: first defined here

Note that this error happens at the point of redefinition, so it doesn’t matter whether function doSomething is ever called.

One way to resolve this would be to rename one of the functions, so the names no longer collide. But this would also require changing the names of all the function calls, which can be a pain, and is subject to error. A better way to avoid collisions is to put your functions into your own namespaces. For this reason the standard library was moved into the std namespace.

Defining your own namespaces

C++ allows us to define our own namespaces via the namespace keyword. Namespaces that you create for your own declarations are called user-defined namespaces. Namespaces provided by C++ (such as the global namespace) or by libraries (such as namespace std) are not considered user-defined namespaces.

Namespace identifiers are typically non-capitalized.

Here is an example of the files in the prior example rewritten using namespaces:

foo.cpp:

goo.cpp:

Now doSomething() inside of foo.cpp is inside the foo namespace, and the doSomething() inside of goo.cpp is inside the goo namespace. Let’s see what happens when we recompile our program.

main.cpp:

The answer is that we now get another error!

ConsoleApplication1.obj : error LNK2019: unresolved external symbol "int __cdecl doSomething(int,int)" (?doSomething@@YAHHH@Z) referenced in function _main

In this case, the compiler was satisfied (by our forward declaration), but the linker could not find a definition for doSomething in the global namespace. This is because both of our versions of doSomething are no longer in the global namespace!

There are two different ways to tell the compiler which version of doSomething() to use, via the scope resolution operator, or via using statements (which we’ll discuss in a later lesson in this chapter).

For the subsequent examples, we’ll collapse our examples down to a one-file solution for ease of reading.

Accessing a namespace with the scope resolution operator (::)

The best way to tell the compiler to look in a particular namespace for an identifier is to use the scope resolution operator (::). The scope resolution operator tells the compiler that the identifier specified by the right-hand operand should be looked for in the scope of the left-hand operand.

Here is an example of using the scope resolution operator to tell the compiler that we explicitly want to use the version of doSomething() that lives in the foo namespace:

This produces the expected result:

7

If we wanted to use the version of doSomething() that lives in goo instead:

This produces the result:

1

The scope resolution operator is great because it allows us to explicitly pick which namespace we want to look in, so there’s no potential ambiguity. We can even do the following:

This produces the result:

7
1

Scope resolution with no prefix

The scope resolution operator can also be used without any preceding namespace (eg. ::doSomething). In such a case, the identifier (e.g. doSomething) is looked for in the global namespace.

So why would you actually want to do this? In most cases, you won’t. But we’ll see examples where this becomes useful in future lessons.

Multiple namespace blocks allowed

It’s legal to declare namespace blocks in multiple locations (either across multiple files, or multiple places within the same file). All declarations within the namespace are considered part of the namespace.

circle.h:

growth.h:

main.cpp:

This works exactly as you would expect:

3.14
2.7

The standard library makes extensive use of this feature, as each standard library header file contains its declarations inside a namespace std block contained within that header file. Otherwise the entire standard library would have to be defined in a single header file!

Note that this capability also means you could add your own functionality to the std namespace. Doing so causes undefined behavior most of the time, because the std namespace has a special rule, prohibiting extension from user code.

Warning

Do not add custom functionality to the std namespace.

When you separate your code into multiple files, you’ll have to use a namespace in the header and source file.

add.h

add.cpp

main.cpp

If the namespace is omitted in the source file, the linker won’t find a definition of basicMath::add, because the source file only defines add (global namespace). If the namespace is omitted in the header file, “main.cpp” won’t be able to use basicMath::add, because it only sees a declaration for add (global namespace).

Nested namespaces

Namespaces can be nested inside other namespaces. For example:

Note that because namespace goo is inside of namespace foo, we access add as foo::goo::add.

In C++17, nested namespaces can also be declared this way:

Namespace aliases

Because typing the fully qualified name of a variable or function inside a nested namespace can be painful, C++ allows you to create namespace aliases, which allow us to temporarily shorten a long sequence of namespaces into something shorter:

One nice advantage of namespace aliases: If you ever want to move the functionality within foo::goo to a different place, you can just update the boo alias to reflect the new destination, rather than having to find/replace every instance of foo::goo.

It’s worth noting that namespaces in C++ were not originally designed as a way to implement an information hierarchy -- they were designed primarily as a mechanism for preventing naming collisions. As evidence of this, note that the entirety of the standard library lives under the singular namespace std:: (with some nested namespaces used for newer library features). Some newer languages (such as C#) differ from C++ in this regard.

In general, you should avoid deeply nested namespaces.

When you should use namespaces

In applications, namespaces can be used to separate application-specific code from code that might be reusable later (e.g. math functions). For example, physical and math functions could go into one namespace (e.g. math::). Language and localization functions in another (e.g. lang::).

When you write a library or code that you want to distribute to others, always place your code inside a namespace. The code your library is used in may not follow best practices -- in such a case, if your library’s declarations aren’t in a namespace, there’s an elevated chance for naming conflicts to occur. As an additional advantage, placing library code inside a namespace also allows the user to see the contents of your library by using their editor’s auto-complete and suggestion feature.


6.3 -- Local variables
Index
6.1 -- Compound statements (blocks)

266 comments to 6.2 — User-defined namespaces

  • jalal

    Would you please give an example of or explain the following? I didn't catch it!

    'One nice advantage of namespace aliases: If you ever want to move the functionality within foo::goo to a different place, you can just update the boo alias to reflect the new destination, rather than having to find/replace every instance of foo::goo.'

  • jalal

    First I wanted to thank you for such an amazing tutorial!

    According to the following code,  if we want to define a variable belong to namespace 'foo' not 'goo' then how should they be defined?

    • nascardriver

      Either don't use the short syntax and next `goo` inside `foo`

      Or add another namespace black

      These are identical.

  • Zara

    Hey,

    What do you mean by "the source file", add.cpp or main.cpp?

    'If the namespace is omitted in the source file, the linker won’t find a definition of basicMath::add'

  • sami

    I didn't include "add.h" in the file add.cpp, but I didn't get any link error. Why?

    "If the namespace is omitted in the source file, the linker won’t find a definition of basicMath::add, because the source file only defines add (global namespace). If the namespace is omitted in the header file, “main.cpp” won’t be able to use basicMath::add, because it only sees a declaration for add (global namespace)."

    • nascardriver

      "add.cpp" doesn't need the forward declarations that are in the header, your code works fine without the include. The include is only in the .cpp file for reasons mentioned in the lesson about headers.
      The text your quoted is unrelated to this, you must be confusing something.

  • sami

    Hi,

    1) "Note that this capability also means you could add your own functionality to the std namespace.

    Do those sentences mean we could use a namespace named "std" for our own use, right?

    2) "...because the std namespace has a special rule, prohibiting extension from user code. "

    what does "prohibiting extension from user code" mean?
    I did that and I didn't get any warning or error!

    • nascardriver

      1) It would be the same namespace.
      2) Undefined behavior isn't required to cause any warnings or errors. It's undefined what happens. Anything can happen.

  • sami

    Hello,
    I have two question:

    1)In your very first example, if I wanted to still have two separate foo.cpp and goo.cpp source files, and make the compiler aware of those namespace inside main.cpp, what would we do?

    2)

    what does that sentence mean? Can we also have both definitions and declarations inside a namespace?

    • sami

      I think I should define the following in main.cpp:

      main.cpp:

      • nascardriver

        You should move line 1-4 into foo.h and line line 5-8 into goo.hpp. That way you can use the functions without having to write forward declarations everywhere.

  • Gregouze

    Hi ! Thanks for all those C++ tutorials, it really helps me ;)

    However, i noticed that there is no ambuigity for my really simple ( test ) program:

    I just don't really understand why i can avoid the std:: before calling the function sqrt() from cmath, could you explain me ? :)

    • nascardriver

      Some headers in C++ are backwards compatible with C (which didn't have namespaces), so you can use their contents without `std::`. Since we're not in C, we recommend using the `std::` prefix.

  • Abhishek

    I wanted to know if the error message : "goo.cpp:3: multiple definition of `doSomething(int, int)'; foo.cpp:3: first defined here",
    is compile error or linker error, if it is compile error, can you please correct the line "As a result, the linker will issue an error:" ?

    • nascardriver

      The compiler works on individual files (Including headers in that file), it doesn't know about anything in other files. The linker detects duplicate definitions across files.

  • Alek

    Hello,I got a question about nested namespaces .you generally introduced two ways to do so. I'm gonna use the first method in the example below:

    so if I utilize the second method(c++17 style) how must I define two different functions that each of them belongs to one of the namespaces(one to foo,and another to goo)?
    because there is no block {} separating goo from foo.
    thanks in advance!.

    • nascardriver

      The C++17 style doesn't replace the old, it's an addition. If you want the use the C++17 style in this example, you'd have 2 separate namespace blocks

  • Mike

    Hi nascardriver!

    Found two typos. In the first section, near the end, there's a sentence "But this would also requiring changing the names of all the function calls", should be "require" instead of "requiring".
    and the second one is in "defining your own namespaces", "Here is an example of the headers in the prior example rewritten using namespaces", they are source files, not headers.

    One other thing. Up until this lesson, whenever I created a new project, I'd disable my compiler extensions and set my language standard to the latest draft in project properties as suggested by you guys. But when I did this, I couldn't use conditional compilation directives "ifndef as header guard as suggested, giving me the error "C 1004: unexpected end-of-file found" . So I'd usually resort to #pragma. But this time around, I decided to not disable my language extensions and then recompiled the program (with language standard set to the latest). It worked just fine! Does this mean that #ifndef is not part of the C++ standard?

    The last thing I wanna mention is that when I wanted to do the first exercise, I put my goo and foo functions inside two namespaces named goo and foo respectively and then put them in different .cpp files. And to call foo from main.cpp, I created a namespace named foo, within which I put the function prototype for doSomething() and placed it inside main.cpp. Should I put the namespace with the prototype inside a header file or should I leave it just the way it is?
    This is what I did in main:

    namespace foo
    {
        int doSomething(int x, int y); // should I put this whole namespace inside a header?
    }

    int main()
    {
        //std::cout << doSomething(4, 3) << '\n'; this wouldn't compile, so I used the namespace above
        std::cout <<foo::doSomething(12, 11) << '\n';
    }

    By the way, I'm using Visual studio 2017.

    • nascardriver

      Typos fixed, thanks!

      `#pragma once` in non-standard. Your error message sounds like you forgot the `#endif` at the end of the file.

      The declarations should go in a header with the same name as the source file.

      • Mike

        You're welcome. I checked my code to see if I forgot #endif at the end.But I haven't.This is how I wrote my header:

        #ifndef GROWTH_H
        #define GROWTH_H

        namespace basicMath
        {
            constexpr double e{ 2.7 };
        }

        #endif

        Also, could you teach me how to put my code inside code tags? No matter what I do, it just doesn't work:(

        • nascardriver

          Does your code compile if you add an empty line after the `#endif`? The empty line is no longer required, but there might be a compiler option that makes your compiler complain about the missing line.

          [-code]
          mike
          [-/code]
          without the -

          If you edit your comment, code tags work after refreshing the page.

  • koe

    "In all of the examples above, we’ve shown functions defined in user-defined namespaces. But namespaces can also contain global variables. You can see examples of this in upcoming lesson 6.8 -- Global constants and inline variables."

    However, the sub-section "Multiple namespace blocks allowed" examples use inline constexpr variables.

    • nascardriver

      Thanks! That was my bad, I didn't read the end of the lesson when I added the example with variables. If you don't mind giving me a quick feedback, was the example with variables surprising or difficult to understand? If so, I'll change it to functions. For now, I've removed the sentence at the end of the lesson.

  • Jordan

    Thank you for these tutorials.. I've read a few C++ tutorials before but never go the hang of some of the concepts (like why bit manipulation even exists), but the writing and explanation style of these tutorials are excellent.  I noticed here you are using: #if !defined(ADD_H) in the header file - which I wasn't expecting as before you introduced only ifndef and you usually explain the options and advise which is best practice.  Just something I noticed & had to websearch. Thanks again..

  • Yolo

    Just wanted to point out a few things. In the example of "Scope Resolution with no prefix" , you add two colons before print(::print). Is there any reason of these colons?

    Lastly, in the last examples you didn't add arguments in the function call of add(). Just pointing out.

    • nascardriver

      You don't need `::print()` in this example, plain `print()` would do the same. You'd only need `::print()` if the code was inside of a scope where there's another entity called `print` (eg. inside the namespace `foo`).

      I added the missing arguments, thanks for pointing them out :)

  • Bobby

    Is the directive #include"add.h" in the file add.cpp required?
    (In "Multiple namespace blocks allowed)
    We dont need declaration for a definition file.or do we??

  • john

    "The standard library makes extensive use of this feature, as each standard library header file contains its declarations inside a namespace std block contained within that header file. Otherwise the entire standard library would have to be defined in a single header file!"
    I don't understand, why? What's the difference in using multiple header files for the library and using a single header file?

    • Alex

      With multiple files you only have to pull in what you need. If there were only a single header, you'd have to pull in (and compile) everything in the standard library whether you used it or not!

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