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CS253 Namespace Pollution

Guiding principles

• A header file should only define the symbols that it’s supposed to define. No more.
• For example, if Zippy.h is documented to define the Zippy class, then that’s the only symbol that it should drop into the global namespace.
  • not another class ZippyHelper
  • not a bunch of constants
  • not some random function declarations
• If the class Zippy needs constants, or a helper class, or functions, then put them inside Zippy.

An example

Consider the files in ~cs253/pub/Example/Pollution:

• Graphics.h: A header file that defines a class Graphics
• main.cc: A main() that uses Graphics.

main.cc

#include "Graphics.h"
#include <string>

std::string fill = "gray";
std::string color = "red";

int main() {
    Graphics g(fill, color);
}

Fine—it defines a couple of strings, and initializes a Graphics object.

Graphics.h

#ifndef GRAPHICS_H_INCLUDED
#define GRAPHICS_H_INCLUDED

#include <string>
// 🦡 all over
using namespace std;

class Graphics {
  public:
    Graphics(const string &f, const string &p) : fill_color(f), pen(p) { }
  private:
    string fill_color, pen;
};

// Internal function to translate color name to int
int color(const string &name);

#endif /* GRAPHICS_H_INCLUDED */

It declares the class Graphics, and an auxillary function.

Try it out

% g++ -Wall ~cs253/Example/Pollution/main.cc .
/s/bach/a/class/cs253/Example/Pollution/main.cc:5:13: error: ‘std::__cxx11::string color’ redeclared as different kind of symbol
 std::string color = "red";
             ^~~~~
In file included from /s/bach/a/class/cs253/Example/Pollution/main.cc:1:
/s/bach/a/class/cs253/Example/Pollution/Graphics.h:16:5: note: previous declaration ‘int color(const string&)’
 int color(const string &name);
     ^~~~~
/s/bach/a/class/cs253/Example/Pollution/main.cc: In function ‘int main()’:
/s/bach/a/class/cs253/Example/Pollution/main.cc:8:16: error: reference to ‘fill’ is ambiguous
     Graphics g(fill, color);
                ^~~~
In file included from /usr/include/c++/8/bits/char_traits.h:39,
                 from /usr/include/c++/8/string:40,
                 from /s/bach/a/class/cs253/Example/Pollution/Graphics.h:4,
                 from /s/bach/a/class/cs253/Example/Pollution/main.cc:1:
/usr/include/c++/8/bits/stl_algobase.h:724:5: note: candidates are: ‘template<class _ForwardIterator, class _Tp> void std::fill(_ForwardIterator, _ForwardIterator, const _Tp&)’
     fill(_ForwardIterator __first, _ForwardIterator __last, const _Tp& __value)
     ^~~~
/s/bach/a/class/cs253/Example/Pollution/main.cc:4:13: note:                 ‘std::__cxx11::string fill’
 std::string fill = "gray";
             ^~~~

That didn’t go well

• The problems:
  • main.cc tried to define a variable named color, but Graphics.h already declared a function named color.
  • main.cc tried to use a variable named fill, but that failed, because there’s already a fill somewhere in the C++ standard library.
• The solutions:
  • Graphics.h has no business declaring the function color(), since its only job is to declare class Graphics.
    • color() should be a static method of class Graphics.
  • Graphics.h has no business doing using namespace std;, which introduces potential conflicts with every symbol in the standard library.
    • Instead, references to string in Graphics.h should be std::string.

Alternative to using namespace std;

Alas, not having using namespace std; in a header file makes it more verbose:

class Graphics {
  public:
    Graphics(const std::string &fill, const std::string &color);
};

so do this, instead:

class Graphics {
    using string = std::string;
  public:
    Graphics(const string &fill, const string &color);
};

The alias is inside the class, so it doesn’t pollute the global namespace. The alias is in the private section of the class, so it’s not visible to users.

Don’t misunderstand

• The command "don’t put using namespace std; in a header file” should not be misunderstood as to forbid it in other places.
• It’s fine in a *.cc file. Nobody else is going to #include your *.cc file, so do what you like there.
• A *.h file, on the other hand, is designed to be #included into other files. Therefore, *.h files have to be well-behaved.