ceph/src/rapidjson/thirdparty/gtest/googletest/docs/PumpManual.md

   1
   2
   3 <b>P</b>ump is <b>U</b>seful for <b>M</b>eta <b>P</b>rogramming.
   4
   5 # The Problem #
   6
   7 Template and macro libraries often need to define many classes,
   8 functions, or macros that vary only (or almost only) in the number of
   9 arguments they take. It's a lot of repetitive, mechanical, and
  10 error-prone work.
  11
  12 Variadic templates and variadic macros can alleviate the problem.
  13 However, while both are being considered by the C++ committee, neither
  14 is in the standard yet or widely supported by compilers.  Thus they
  15 are often not a good choice, especially when your code needs to be
  16 portable. And their capabilities are still limited.
  17
  18 As a result, authors of such libraries often have to write scripts to
  19 generate their implementation. However, our experience is that it's
  20 tedious to write such scripts, which tend to reflect the structure of
  21 the generated code poorly and are often hard to read and edit. For
  22 example, a small change needed in the generated code may require some
  23 non-intuitive, non-trivial changes in the script. This is especially
  24 painful when experimenting with the code.
  25
  26 # Our Solution #
  27
  28 Pump (for Pump is Useful for Meta Programming, Pretty Useful for Meta
  29 Programming, or Practical Utility for Meta Programming, whichever you
  30 prefer) is a simple meta-programming tool for C++. The idea is that a
  31 programmer writes a `foo.pump` file which contains C++ code plus meta
  32 code that manipulates the C++ code. The meta code can handle
  33 iterations over a range, nested iterations, local meta variable
  34 definitions, simple arithmetic, and conditional expressions. You can
  35 view it as a small Domain-Specific Language. The meta language is
  36 designed to be non-intrusive (s.t. it won't confuse Emacs' C++ mode,
  37 for example) and concise, making Pump code intuitive and easy to
  38 maintain.
  39
  40 ## Highlights ##
  41
  42   * The implementation is in a single Python script and thus ultra portable: no build or installation is needed and it works cross platforms.
  43   * Pump tries to be smart with respect to [Google's style guide](http://code.google.com/p/google-styleguide/): it breaks long lines (easy to have when they are generated) at acceptable places to fit within 80 columns and indent the continuation lines correctly.
  44   * The format is human-readable and more concise than XML.
  45   * The format works relatively well with Emacs' C++ mode.
  46
  47 ## Examples ##
  48
  49 The following Pump code (where meta keywords start with `$`, `[[` and `]]` are meta brackets, and `$$` starts a meta comment that ends with the line):
  50
  51 ```
  52 $var n = 3     $$ Defines a meta variable n.
  53 $range i 0..n  $$ Declares the range of meta iterator i (inclusive).
  54 $for i [[
  55                $$ Meta loop.
  56 // Foo$i does blah for $i-ary predicates.
  57 $range j 1..i
  58 template <size_t N $for j [[, typename A$j]]>
  59 class Foo$i {
  60 $if i == 0 [[
  61   blah a;
  62 ]] $elif i <= 2 [[
  63   blah b;
  64 ]] $else [[
  65   blah c;
  66 ]]
  67 };
  68
  69 ]]
  70 ```
  71
  72 will be translated by the Pump compiler to:
  73
  74 ```
  75 // Foo0 does blah for 0-ary predicates.
  76 template <size_t N>
  77 class Foo0 {
  78   blah a;
  79 };
  80
  81 // Foo1 does blah for 1-ary predicates.
  82 template <size_t N, typename A1>
  83 class Foo1 {
  84   blah b;
  85 };
  86
  87 // Foo2 does blah for 2-ary predicates.
  88 template <size_t N, typename A1, typename A2>
  89 class Foo2 {
  90   blah b;
  91 };
  92
  93 // Foo3 does blah for 3-ary predicates.
  94 template <size_t N, typename A1, typename A2, typename A3>
  95 class Foo3 {
  96   blah c;
  97 };
  98 ```
  99
 100 In another example,
 101
 102 ```
 103 $range i 1..n
 104 Func($for i + [[a$i]]);
 105 $$ The text between i and [[ is the separator between iterations.
 106 ```
 107
 108 will generate one of the following lines (without the comments), depending on the value of `n`:
 109
 110 ```
 111 Func();              // If n is 0.
 112 Func(a1);            // If n is 1.
 113 Func(a1 + a2);       // If n is 2.
 114 Func(a1 + a2 + a3);  // If n is 3.
 115 // And so on...
 116 ```
 117
 118 ## Constructs ##
 119
 120 We support the following meta programming constructs:
 121
 122 | `$var id = exp` | Defines a named constant value. `$id` is valid util the end of the current meta lexical block. |
 123 |:----------------|:-----------------------------------------------------------------------------------------------|
 124 | `$range id exp..exp` | Sets the range of an iteration variable, which can be reused in multiple loops later.          |
 125 | `$for id sep [[ code ]]` | Iteration. The range of `id` must have been defined earlier. `$id` is valid in `code`.         |
 126 | `$($)`          | Generates a single `$` character.                                                              |
 127 | `$id`           | Value of the named constant or iteration variable.                                             |
 128 | `$(exp)`        | Value of the expression.                                                                       |
 129 | `$if exp [[ code ]] else_branch` | Conditional.                                                                                   |
 130 | `[[ code ]]`    | Meta lexical block.                                                                            |
 131 | `cpp_code`      | Raw C++ code.                                                                                  |
 132 | `$$ comment`    | Meta comment.                                                                                  |
 133
 134 **Note:** To give the user some freedom in formatting the Pump source
 135 code, Pump ignores a new-line character if it's right after `$for foo`
 136 or next to `[[` or `]]`. Without this rule you'll often be forced to write
 137 very long lines to get the desired output. Therefore sometimes you may
 138 need to insert an extra new-line in such places for a new-line to show
 139 up in your output.
 140
 141 ## Grammar ##
 142
 143 ```
 144 code ::= atomic_code*
 145 atomic_code ::= $var id = exp
 146     | $var id = [[ code ]]
 147     | $range id exp..exp
 148     | $for id sep [[ code ]]
 149     | $($)
 150     | $id
 151     | $(exp)
 152     | $if exp [[ code ]] else_branch
 153     | [[ code ]]
 154     | cpp_code
 155 sep ::= cpp_code | empty_string
 156 else_branch ::= $else [[ code ]]
 157     | $elif exp [[ code ]] else_branch
 158     | empty_string
 159 exp ::= simple_expression_in_Python_syntax
 160 ```
 161
 162 ## Code ##
 163
 164 You can find the source code of Pump in [scripts/pump.py](../scripts/pump.py). It is still
 165 very unpolished and lacks automated tests, although it has been
 166 successfully used many times. If you find a chance to use it in your
 167 project, please let us know what you think!  We also welcome help on
 168 improving Pump.
 169
 170 ## Real Examples ##
 171
 172 You can find real-world applications of Pump in [Google Test](http://www.google.com/codesearch?q=file%3A\.pump%24+package%3Ahttp%3A%2F%2Fgoogletest\.googlecode\.com) and [Google Mock](http://www.google.com/codesearch?q=file%3A\.pump%24+package%3Ahttp%3A%2F%2Fgooglemock\.googlecode\.com).  The source file `foo.h.pump` generates `foo.h`.
 173
 174 ## Tips ##
 175
 176   * If a meta variable is followed by a letter or digit, you can separate them using `[[]]`, which inserts an empty string. For example `Foo$j[[]]Helper` generate `Foo1Helper` when `j` is 1.
 177   * To avoid extra-long Pump source lines, you can break a line anywhere you want by inserting `[[]]` followed by a new line. Since any new-line character next to `[[` or `]]` is ignored, the generated code won't contain this new line.