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CS253 Namespaces

Overview

• Namespaces contain symbols
  • Classes
  • Functions
  • Variables
  • Constants
  • Types
• The most popular namespace, std, pronounced “standard”, contains stuff defined by the C++ standard.
• Boost uses the boost namespace.
• You can create your own.

Namespace qualifiers

Namespace qualifiers look like class qualifiers:

#include <iostream>
int main() {
    std::cout << "alpha" << std::endl;
}

alpha

Use a :: prefix to access the global namespace:

int alpha = 42;
int main() {
    int beta = 17;
    std::cout << beta + ::alpha << '\n';
}

59

Note that beta is not in the global namespace.

General using

To embrace an entire namespace:

#include <iostream>
using namespace std;
int main() {
    cout << "gamma" << endl;
}

gamma

• Some people believe that using namespace std; is evil, and litter their code with std:: uses.
• These people are need professional help.

Specific using

To import just one symbol:

#include <iostream>
using std::cout;
int main() {
    cout << "delta" << endl;
}

c.cc:4: error: 'endl' was not declared in this scope

The global namespace

• The global namespace has a zero-length name, referred to by ::.
• It’s considered poor form to unnecessarily pollute it.
• However, certain things must go there, for instance, main.

C Compatibility

This is a valid C program. Compatibility demands that it be a valid C++ program, as well.

#include <stdio.h>
#include <math.h>

int main() {
    printf("The square root of ten is %f\n", sqrt(10.0));
}

The square root of ten is 3.162278

But, what about namespaces‽

C Compatibility

• <math.h> puts symbols such as sqrt into the global namespace, since that’s where C programs expect them.
• <cmath> puts the same symbols into the std namespace.

The file cmath looks something like this:

// This is cmath
namespace std {
    #include <math.h>
}

This avoids duplication of information.

Similar names

Don’t confuse these:

• <string.h> puts strlen, strcpy, … into the global namespace.
• <cstring> puts strlen, strcpy, … into the std namespace.
• <string> puts the C++ string class into the std namespace.

Classes are closed

Once you’ve defined a class, you can’t add to it—it’s closed. Consider:

class Foo {
  public:
    int a,b,c;
};

class Foo {
  public:
    int d;
};

int main() { }

c.cc:6: error: redefinition of 'class Foo'

Namespaces are open

On the other hand, you can add to a namespace—it’s open. Consider:

namespace Math {
    constexpr double PI = 3.14159;
}

namespace Math {
    constexpr double E = 2.71828;
}

int main() {
    cout << Math::PI * Math::E << '\n';
}

8.53972

std is closed

However, the C++ standard forbids you adding anything to the namespace std, so it’s effectively closed, anyway.

Generally, compilers won’t stop you, because they have to allow the implementation to put things into std, and it’s difficult to differentiate between you and the implementation, but it’s still against the rules, so don’t do it.

Nested namespaces

Namespaces can be nested.

namespace Pika {
    constexpr int public_value = 123;

    namespace Private {
        constexpr int none_of_your_business = 456;
    }
}

int main() {
    cout << Pika::public_value << '\n';
}

123

The anonymous namespace

If you don’t give your namespace a name, that’s an anonymous namespace. The compiler makes up a name and does an implicit using declaration:

    namespace {
        int foo;
    }

It’s pretty much the same as:

    namespace UNIQUE_NAME_42258107985915 {
        int foo;
    }
    using namespace UNIQUE_NAME_42258107985915;

It’s intended to replace top-level static declarations.

Namespace aliases

Long namespace names are good. For example, you wouldn’t want a namespace called HP—that’s a popular abbreviation:

• Hewlett-Packard
• Harry Potter
• Hit Points
• Hensel Phelps

So, the library implementors can define the namespace name to be Hewlett_Packard, and the user can create an easy-to-type alias:

    namespace HP = Hewlett_Packard;
    value = HP::handy_function();

Symbol Resolution & Namespace Ambiguity

What about conflicts? Consider two libraries:

Mathematics functions	Morality functions
cos(radians)	karma(action)
tan(radians)	fasting()
sin(radians)	sin(type)

Uh, oh! Each library has a sin() function!

Ambiguity

What will this do?

    using namespace Math;
    using namespace Morality;
    a = sin(b);

It will just plain fail. An unqualified sin is ambiguous, because there are two versions available. There is no priority order. Also, the global namespace has no priority advantage, either.

Solutions

You can explicitly disambiguate:

    using namespace Math;
    using namespace Morality;
    a = Morality::sin(b);

Or, you can only bring in the functions that you use:

    using Math::tan;
    using Morality::sin;
    a = sin(b);

Ambiguity example

#include <iostream>
using namespace std;
int dec = 10;
int main() {
    cout << dec << "°F\n";
    return 0;
}

c.cc:5: error: reference to 'dec' is ambiguous

Ambiguity example solutions

#include <iostream>
using namespace std;
int dec = 10;
int main() {
    cout << ::dec << "°F\n";
    return 0;
}

10°F

#include <iostream>
using std::cout;
int dec = 10;
int main() {
    cout << dec << "°F\n";
    return 0;
}

10°F

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