GoogleTest - Google Testing and Mocking Framework (grpc protobuff依赖)
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177 lines
6.9 KiB
177 lines
6.9 KiB
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<b>P</b>ump is <b>U</b>seful for <b>M</b>eta <b>P</b>rogramming. |
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# The Problem # |
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Template and macro libraries often need to define many classes, |
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functions, or macros that vary only (or almost only) in the number of |
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arguments they take. It's a lot of repetitive, mechanical, and |
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error-prone work. |
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Variadic templates and variadic macros can alleviate the problem. |
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However, while both are being considered by the C++ committee, neither |
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is in the standard yet or widely supported by compilers. Thus they |
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are often not a good choice, especially when your code needs to be |
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portable. And their capabilities are still limited. |
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As a result, authors of such libraries often have to write scripts to |
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generate their implementation. However, our experience is that it's |
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tedious to write such scripts, which tend to reflect the structure of |
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the generated code poorly and are often hard to read and edit. For |
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example, a small change needed in the generated code may require some |
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non-intuitive, non-trivial changes in the script. This is especially |
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painful when experimenting with the code. |
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# Our Solution # |
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Pump (for Pump is Useful for Meta Programming, Pretty Useful for Meta |
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Programming, or Practical Utility for Meta Programming, whichever you |
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prefer) is a simple meta-programming tool for C++. The idea is that a |
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programmer writes a `foo.pump` file which contains C++ code plus meta |
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code that manipulates the C++ code. The meta code can handle |
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iterations over a range, nested iterations, local meta variable |
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definitions, simple arithmetic, and conditional expressions. You can |
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view it as a small Domain-Specific Language. The meta language is |
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designed to be non-intrusive (s.t. it won't confuse Emacs' C++ mode, |
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for example) and concise, making Pump code intuitive and easy to |
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maintain. |
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## Highlights ## |
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* The implementation is in a single Python script and thus ultra portable: no build or installation is needed and it works cross platforms. |
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* 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. |
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* The format is human-readable and more concise than XML. |
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* The format works relatively well with Emacs' C++ mode. |
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## Examples ## |
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The following Pump code (where meta keywords start with `$`, `[[` and `]]` are meta brackets, and `$$` starts a meta comment that ends with the line): |
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``` |
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$var n = 3 $$ Defines a meta variable n. |
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$range i 0..n $$ Declares the range of meta iterator i (inclusive). |
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$for i [[ |
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$$ Meta loop. |
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// Foo$i does blah for $i-ary predicates. |
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$range j 1..i |
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template <size_t N $for j [[, typename A$j]]> |
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class Foo$i { |
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$if i == 0 [[ |
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blah a; |
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]] $elif i <= 2 [[ |
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blah b; |
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]] $else [[ |
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blah c; |
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]] |
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}; |
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]] |
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``` |
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will be translated by the Pump compiler to: |
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``` |
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// Foo0 does blah for 0-ary predicates. |
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template <size_t N> |
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class Foo0 { |
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blah a; |
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}; |
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// Foo1 does blah for 1-ary predicates. |
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template <size_t N, typename A1> |
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class Foo1 { |
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blah b; |
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}; |
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// Foo2 does blah for 2-ary predicates. |
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template <size_t N, typename A1, typename A2> |
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class Foo2 { |
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blah b; |
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}; |
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// Foo3 does blah for 3-ary predicates. |
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template <size_t N, typename A1, typename A2, typename A3> |
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class Foo3 { |
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blah c; |
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}; |
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``` |
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In another example, |
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``` |
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$range i 1..n |
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Func($for i + [[a$i]]); |
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$$ The text between i and [[ is the separator between iterations. |
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``` |
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will generate one of the following lines (without the comments), depending on the value of `n`: |
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``` |
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Func(); // If n is 0. |
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Func(a1); // If n is 1. |
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Func(a1 + a2); // If n is 2. |
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Func(a1 + a2 + a3); // If n is 3. |
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// And so on... |
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``` |
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## Constructs ## |
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We support the following meta programming constructs: |
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| `$var id = exp` | Defines a named constant value. `$id` is valid util the end of the current meta lexical block. | |
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|:----------------|:-----------------------------------------------------------------------------------------------| |
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| `$range id exp..exp` | Sets the range of an iteration variable, which can be reused in multiple loops later. | |
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| `$for id sep [[ code ]]` | Iteration. The range of `id` must have been defined earlier. `$id` is valid in `code`. | |
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| `$($)` | Generates a single `$` character. | |
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| `$id` | Value of the named constant or iteration variable. | |
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| `$(exp)` | Value of the expression. | |
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| `$if exp [[ code ]] else_branch` | Conditional. | |
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| `[[ code ]]` | Meta lexical block. | |
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| `cpp_code` | Raw C++ code. | |
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| `$$ comment` | Meta comment. | |
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**Note:** To give the user some freedom in formatting the Pump source |
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code, Pump ignores a new-line character if it's right after `$for foo` |
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or next to `[[` or `]]`. Without this rule you'll often be forced to write |
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very long lines to get the desired output. Therefore sometimes you may |
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need to insert an extra new-line in such places for a new-line to show |
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up in your output. |
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## Grammar ## |
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``` |
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code ::= atomic_code* |
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atomic_code ::= $var id = exp |
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| $var id = [[ code ]] |
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| $range id exp..exp |
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| $for id sep [[ code ]] |
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| $($) |
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| $id |
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| $(exp) |
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| $if exp [[ code ]] else_branch |
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| [[ code ]] |
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| cpp_code |
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sep ::= cpp_code | empty_string |
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else_branch ::= $else [[ code ]] |
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| $elif exp [[ code ]] else_branch |
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| empty_string |
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exp ::= simple_expression_in_Python_syntax |
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``` |
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## Code ## |
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You can find the source code of Pump in [scripts/pump.py](../scripts/pump.py). It is still |
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very unpolished and lacks automated tests, although it has been |
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successfully used many times. If you find a chance to use it in your |
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project, please let us know what you think! We also welcome help on |
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improving Pump. |
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## Real Examples ## |
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You can find real-world applications of Pump in [Google Test](http://www.google.com/codesearch?q=file%3A\.pump%24+package%3Ahttp%3A%2F%2Fgoogletest\.googlecode\.com) and [Google Mock](http://www.google.com/codesearch?q=file%3A\.pump%24+package%3Ahttp%3A%2F%2Fgooglemock\.googlecode\.com). The source file `foo.h.pump` generates `foo.h`. |
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## Tips ## |
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* If a meta variable is followed by a letter or digit, you can separate them using `[[]]`, which inserts an empty string. For example `Foo$j[[]]Helper` generate `Foo1Helper` when `j` is 1. |
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* 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.
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