Merge some cosmetic patches.

[ginac.git] / doc / CodingStyle

GiNaC Coding Style
==================

This document attempts to describe the preferred coding style for GiNaC.


0. Preface
----------

Different people have different ideas of how source code should be formatted
to be most beautiful and/or useful to work with. GiNaC itself was developed
by a group of people whose ideas on these matters differed in some details.
It also evolved over the course of the years, and received contributions
from outside. As a result, the GiNaC source is not in all places 100%
consistent with the rules laid out in this document. Old code will
eventually be converted, however. New code should always be in this style.

Sometimes it's also not possible (or desirable) to give precise rules for
every single occasion. Should you write "2*x+y", "2*x + y" or "2 * x + y"?
We don't know. We also don't care (much). Use your own discretion.

This document does not intend to codify the "perfect" programming style, but
please try to follow these rules. It will make our (and your) lives easier. :-)


1. General Conventions
----------------------

Any code in GiNaC should comply to the C++ standard, as defined by ISO/IEC
14882:2011(E). Don't use compiler-specific language extensions unless they
are surrounded by appropriate "#ifdef"s, and you also provide a compliant
version of the code in question for other compilers.

Source code formatting assumes a fixed-width font. You do not need to restrict
yourself to 80 columns, except in comment blocks, but try to avoid overlong
lines.


2. Indentation and Alignment
----------------------------

Short version: Indentation and alignment should be tab-size independent. Use
one Tab for each indentation level. Use spaces for aligning portions of code.

Explanation: "Indentation" and "alignment" are two related, but different
things. To indent means to set a section of code in from the left margin,
to facilitate distinguishing "contained" sections (such as a sequence of lines
inside an "if" statement) from their "containers" (in this case, the "if"
statement itself). For nested constructs, indentation is carried out in
several levels:

No indentation
Begin 1
        One level of indentation
        Begin 2
                Two levels of indentation
                etc.
        End 2
End 1

To align, on the other hand, means to make parts of the text start on the
same column on the screen, to make it look more tidy and clear.

Here is an example that features both indentation and alignment:

class person {
        string name;  // person's full name
        int age;      // age in years
};

The inner part of the class definition (the part between the braces) is
_indented_ by one level. The comments at the end of these two lines are
_aligned_ to put them directly below each other on the screen.

Now why are we making such a fuss about the difference between these two?
This is where the "tab-size independent" part comes in.

Both indentation and alignment are often done with Tab characters ('\t' or
ASCII 0x09). And in theory, that would be the best and logical choice.
Unfortunately, there is no general agreement about the placement of the
tabulator stops in effect.

Traditionally, tab stops are every 8 characters. Many programmers indent with
Tabs because it's only one keypress, but they feel that a tab-size of 8
pushes the code too far to the right, so they change it to 4 characters (or
some other value) in their editors. When alignment is also performed with
tabs this results in misaligned code unless the tab-size is set to the
exact same value the author of the code used.

Take the "person" class definition from above as an example (here and in
the following, ':' represents a Tab, while '.' represents a Space). Assume
that we had done both indentation and alignment with Tabs, with a tab-size
of 8:

|-------|-------|-------|-------|-------|-------|------- <- tab stops
class person {
::::::::string name;::::// person's full name
::::::::int age;::::::::// age in years
};

Now somebody who prefers a tab-size of 4 looks at the code:

|---|---|---|---|---|---|---|---|---|---|---|---|---|--- <- tab stops
class person {
::::string name;::::// person's full name
::::int age;::::// age in years
};

The indentation is still correct, but the two comments are now misaligned.

The default indentation mode of the Emacs editor is even worse: it mixes
Tabs (which it assumes to be of size 4) and spaces for both indentation and
alignment, with an effective amount of 2 character widths per indentation
level. The resulting code usually looks like a complete mess for any tab-size
setting other than 4.

So, how do you make it tab-size independent? One solution would be to not
use any Tab characters at all. This, however, would hard-code the amount
of space used for indentation, something which so many people disagree
about.

Instead, we adopted a different approach in GiNaC: Tabs are used exclusively
for indentation (one Tab per level); spaces are used for alignment. This
gets you the best of both worlds: It allows every programmer to change the
tab-size (and thus, the visual amount of indentation) to his/her own desire,
but the code still looks OK at any setting.

This is how our class definition should be entered using this scheme
(remember, ':' are Tabs, '.' are Spaces):

|-------|-------|-------|-------|-------|-------|------- <- tab stops
class person {
::::::::string name;..// person's full name
::::::::int age;......// age in years
};

8 characters indentation are too much for you? No problem. Just change the
tab-size, and it still looks good:

|---|---|---|---|---|---|---|---|---|---|---|---|---|--- <- tab stops
class person {
::::string name;..// person's full name
::::int age;......// age in years
};

Some more examples (shown with a tab-size of 4):

// here, we have aligned the parameter declarations
int foo(int i1, int i2, int i3,
........string s1, string s2,
........vector<int> &result)
{
::::// inside the function, one level of indentation
::::if (i1 == i2) {
::::::::// inside the "if", two levels of indentation
::::::::return 0;
::::}
::::// outside the "if", one level again

::::// indentation is also used here:
::::static int fibonacci[] = {
::::::::1, 2, 3, 5, 8, 13,
::::::::21, 34, 55, 89, 144
::::};

::::// and here:
::::int x = bar(
::::::::i1 - i2,
::::::::i2 - i3,
::::::::i3 - i1
::::);

::::// continuation lines, however, are aligned, not indented:
::::cout << "i1 = " << i1 << ", i2 = " << i2 << ", i3 = " << i3
::::.....<< ", string1 = " << s1
::::.....<< ", string2 = " << s2 << endl;

::::if (s1 == s2)
::::::::return i1;.......// these two comments
::::else
::::::::return i2 + i3;..// are also aligned
}


3. Whitespace
-------------

A ',' is always followed by one space:

        int a[] = {1, 2, 3};

There is no space between a function name and the following opening
parenthesis. There are no spaces after the opening and before the closing
parentheses, either:

        x = foo(i1, i2, i3);

There is, however, one space after "if", "for", "while", "switch", and
"catch" (these are not functions, after all):

        if (i1 == i2)

You should place one space before and behind any binary operator (except
'::', '[]', '.', '.*', '->' and '->*'; for ',' see above). There is no space
after (or before, in the case of postfix '++' and '--') unary operators:

        a = b[i] + *p++;
        x = -(y + z) / 2;

There are no spaces around the '<' and '>' used to designate template
parameters:

        vector<int> vi;
        vector<list<int>> vli;

'*' and '&' in the declaration of pointer-to and reference-to variables
have a space before, but not after them:

        int *p;
        int &r;

There is still an ongoing debate amongst GiNaC developers whether reference
parameters should be written as "foo(string &s)" or "foo(string & s)". :-)

The following section has additional examples for the proper use of
whitespace.


4. Braces
---------

One word: K&R, also known as "One True Brace Style", suitably extended for
C++. The opening brace goes at the end of the line, except for function
bodies. Really short functions can be written in one single line.

        if (a == b) {
                // do something
        } else if (a > b) {
                // do something else
        } else {
                // must be a < b
        }

        for (int i = 0; i < 5; ++i) {
                // "++i" is preferred over "i++" because, in the case of
                // overloaded operators, the prefix "++" is the simpler one
                // (the postfix "++" usually has to use a temporary variable
                // to save the previous state of the object for returning
                // it to the caller)
        }

        while (a < b) {
                // loop body
        }

        do {
                // loop body
        } while (a < b);

        switch (x) {
        case 0:
                // first case
                break;
        case 1:
                // second case
                break;
        default:
                // default case
                break;
        }

        try {
                // do something dangerous
        } catch (std::exception &e) {
                // we're caught
        } catch (...) {
                // catchall
        }

        class foo {
        public:
                foo(int i) : x(i)
                {
                        // under construction
                }

                int get_x() const { return x; }

        protected:
                void schwupp(char c);

        private:
                int x;
        };

        namespace bar {
                // a foo by any other name...
        }

        void foo::schwupp(char c)
        {
                // diwupp
        }

Also take note of the use of whitespace in the above examples.


5. Naming
---------

C++ identifiers (names of classes, types, functions, variables, etc.) should
not contain any uppercase characters. Preprocessor macros (anything that is
"#define"d), on the other hand, should not contain any lowercase characters.
There are some exceptions however, like "Li()" and "is_ex_the_function()"
(a macro). Not to mention the "GiNaC" namespace...

Names that consist of multiple words should use underscores to separate the
words (for example, "construct_from_int").

Don't use naming conventions such as "Hungarian notation" where the type,
scope, or context of an identifier is encoded in its name as a prefix,
like "T" or "C" for classes ("TList"), "f" or "m" for member variables, etc.
Try to follow the existing naming conventions in GiNaC.

Names of C++ source files end in ".cpp" (not ".C", ".cc", or ".cxx").
Names of header files end in ".h" (not ".hpp", ".H", ".hh", or ".hxx").


6. Namespaces
-------------

Don't place "using namespace std;", "using std::vector;" or anything like this
into public library header files. Doing so would force the import of all or
parts of the "std" namespace upon all users of the library, even if they don't
want it. Always fully qualify identifiers in the headers.

Definitions that are only used internally within the library but have to be
placed in a public header file for one reason or another should be put into
a namespace called "internal" inside the "GiNaC" namespace.


7. Miscellaneous Conventions
----------------------------

Don't put the expression after a "return" statement in parentheses. It's
"return x;", not "return (x);" or "return(x);".

Don't put an empty "return;" statement at the end of a function that doesn't
return a value.

Try to put declarations of local variables close to the point where they are
used for the first time. C++ is not like C, where all declarations have to
be at the beginning of a block.

It's "const string &s", not "string const &s".

"goto" labels (if you have to use them) always start at column 1.

Don't deliberately modify code to dodge compiler warnings, unless it
clarifies the code in question to a human reader. This includes the use
of "UNUSED()" macros and similar contraptions.

Don't use more than two consecutive empty lines. Use single empty lines to
separate logical blocks of code (preferably, each of these blocks also has an
explanatory comment in front). Use two lines when you feel that one line is
not enough (to separate two functions, for example). But not more.


8. Documentation
----------------

Every class, class member, and function definition should have a comment
in doxygen format (Javadoc style) in front of it, explaining the object's
purpose.

Comments inside functions should give the reader a general idea of the
algorithms used in the function, and describe the pragmatics behind the
code. To quote Linus Torvalds: "You want your comments to tell WHAT your
code does, not HOW.".

If your algorithms are covered in detail in some paper or thesis, it's a
good idea to put in a short bibliographical note.