2.9 — Naming collisions and an introduction to namespaces

Let’s say you are driving to a friend’s house for the first time, and the address given to you is 245 Front Street in Mill City. Upon reaching Mill City, you pull up your map, only to discover that Mill City actually has two different Front Streets across town from each other! Which one would you go to? Unless there were some additional clue to help you decide (e.g. you remember his house is near the river) you’d have to call your friend and ask for more information. Because this would be confusing and inefficient (particularly for your mailman), in most countries, all street names and house addresses within a city are required to be unique.

Similarly, C++ requires that all identifiers be non-ambiguous. If two identical identifiers are introduced into the same program in a way that the compiler or linker can’t tell them apart, the compiler or linker will produce an error. This error is generally referred to as a naming collision (or naming conflict).

An example of a naming collision



When the compiler compiles this program, it will compile a.cpp and main.cpp independently, and each file will compile with no problems.

However, when the linker executes, it will link all the definitions in a.cpp and main.cpp together, and discover conflicting definitions for function myFcn. The linker will then abort with an error. Note that this error occurs even though myFcn is never called!

Most naming collisions occur in two cases:
1) Two (or more) definitions for a function (or global variable) are introduced into separate files that are compiled into the same program. This will result in a linker error, as shown above.
2) Two (or more) definitions for a function (or global variable) are introduced into the same file (often via an #include). This will result in a compiler error.

As programs get larger and use more identifiers, the odds of a naming collision being introduced increases significantly. The good news is that C++ provides plenty of mechanisms for avoiding naming collisions. Local scope, which keeps local variables defined inside functions from conflicting with each other, is one such mechanism. But local scope doesn’t work for functions names. So how do we keep function names from conflicting with each other?

What is a namespace?

A namespace is a region where names can be defined. Within a namespace, all names must be unique, otherwise a naming collision will result.

However, names defined in one namespace will not causing naming collisions with identical names defined in other namespaces. Thus namespaces can be used to partition identifiers in a large project to help ensure they don’t inadvertently collide with other identifiers. For example, if you put all your math functions in a namespace called Math, then your math functions won’t collide with identically named functions outside the Math namespace.

We’ll talk about how to create your own namespaces in a future lesson.

The global namespace

In C++, any name that is not defined inside a class, function, or namespace is considered to be part of an implicitly defined namespace called the global namespace (sometimes also called the global scope.

In the example at the top of the lesson, functions main() and both versions of myFcn() are defined inside the global namespace. The naming collision encountered in the example happens because both versions of myFcn() end up inside the global namespace, which violates the rule that all names in the namespace must be unique.

The std namespace

When C++ was originally designed, all of the identifiers in the C++ standard library (including std::cin and std::cout) were available to be used without the std:: prefix (they were part of the global namespace). However, this meant that any identifier in the standard library could potentially conflict with any name you picked for your own identifiers (also defined in the global namespace). Code that was working might suddenly have a naming conflict when you #included a new file from the standard library. Or worse, programs that would compile under one version of C++ might not compile under a future version of C++, as new identifiers introduced into the standard library could have a naming conflict with already written code. So C++ moved all of the functionality in the standard library into a namespace named “std” (short for standard).

It turns out that std::cout‘s name isn’t really std::cout. It’s actually just cout, and std is the name of the namespace that identifier cout is part of. Because cout is defined in the std namespace, the name cout won’t conflict with any objects or functions named cout that we create in the global namespace.

Back to our address analogy for a moment, having two Front Streets was only problematic because those streets existed within the same city. A Front Street in two different cities (e.g. Mill City and Jonesville) would be no issue, so long as you knew which city you were looking for Front street in. Namespaces function like the cities do in this analogy. Do note that in order to disambiguate which Front street you mean, you do need to include the city’s name (e.g. Mill City’s Front Street).

Similarly, when accessing an identifier that is defined in a namespace (e.g. std::cout) , you need to tell the compiler that we’re looking for an identifier defined inside the namespace (std).

Key insight

When you use an identifier that is defined inside a namespace (such as the std namespace), you have to tell the compiler that the identifier lives inside the namespace.

There are a few different ways to do this.

Explicit namespace qualifier std::

The most straightforward way to tell the compiler that we want to use cout from the std namespace is by explicitly using the std:: prefix. For example:

The :: symbol is an operator called the scope resolution operator. The identifier to the left of the :: symbol identifies the namespace that the name to the right of the :: symbol is contained within. If no identifier to the left of the :: symbol is provided, the global namespace is assumed.

So when we say std::cout, we’re saying “the cout that lives in namespace std“.

This is the safest way to use cout, because there’s no ambiguity about which cout we’re referencing (the one in the std namespace).

Best practice

Use explicit namespace prefixes to access identifiers defined in a namespace.

Using namespace std (and why to avoid it)

Another way to access identifiers inside a namespace is to use a using directive statement. Here’s our original “Hello world” program with a using directive:

A using directive tells the compiler to check a specified namespace when trying to resolve an identifier that has no namespace prefix. So in the above example, when the compiler goes to determine what identifier cout is, it will check both locally (where it is undefined) and in the std namespace (where it will match to std::cout).

Many texts, tutorials, and even some compilers recommend or use a using directive at the top of the program. However, used in this way, this is a bad practice, and highly discouraged.

Consider the following program:

The above program doesn’t compile, because the compiler now can’t tell whether we want the cout function that we defined, or the cout that is defined inside the std namespace.

When using a using directive in this manner, any identifier we define may conflict with any identically named identifier in the std namespace. Even worse, while an identifier name may not conflict today, it may conflict with new identifiers added to the std namespace in future language revisions. This was the whole point of moving all of the identifiers in the standard library into the std namespace in the first place!


Avoid using directives (such as using namespace std;) at the top of your program. They violate the reason why namespaces were added in the first place.

We’ll talk more about using statements (and how to use them responsibly) in lesson 7.8 -- Pointers.

2.10 -- Introduction to the preprocessor
2.8 -- Programs with multiple code files