ceph/src/seastar/coding-style.md

   1 # Seastar Coding Style
   2
   3 ## Files
   4
   5 Header files have the `.hh` extension, source files use the `.cc` extension. All files must have a license and copyright blurb. Use `#pragma once` instead of an include guard.
   6
   7 Header files which contain a public part of the interface of Seastar go in the `include` directory. Internal header and source files which are private to the implementation go in the `src` directory.
   8
   9 ## Whitespace
  10
  11 Use spaces only; NEVER tabs. Rationale: tabs render differently on each system.
  12
  13 An _indent_ is four spaces. A double indent is eight spaces, a half-indent is two spaces.
  14
  15 ## Naming
  16
  17 We follow the C++ and Boost naming conventions: class names, variables, and functions are `words_separated_by_whitespace`.
  18
  19 Private data members are prefixed by an underscore:
  20
  21 ```c++
  22 class my_class {
  23     int _a_member;
  24 public:
  25     void foo() {
  26         _a_member = 3;
  27     }
  28 };
  29 ```
  30
  31 Think of the leading underscore as a shorthand for `this->`.
  32
  33 Template parameters and concepts use `CamelCase`
  34
  35 ## Including header files
  36
  37 In any file, to include a public header file (one in the `include` directory), use an absolute path with `<>` like this:
  38
  39 ```c++
  40 #include <seastar/core/future.hh>
  41 ```
  42
  43 In any private file, to include a private header file (one in the `src` directory), use an absolute path with `""` like this:
  44
  45 ```c++
  46 #include "core/future_impl.hh"
  47 ```
  48
  49 ## Braced blocks
  50
  51 All nested scopes are braced, even when the language allows omitting the braces (such as an if-statement), this makes patches simpler and is more consistent. The opening brace is merged with the line that opens the scope (class definition, function definition, if statement, etc.) and the body is indented.
  52
  53 ```c++
  54 void a_function() {
  55     if (some condition) {
  56         stmt;
  57     } else {
  58         stmt;
  59     }
  60 }
  61 ```
  62
  63 An exception is namespaces -- the body is _not_ indented, to prevent files that are almost 100% whitespace left margin.
  64
  65 When making a change, if you need to insert an indentation level, you can temporarily break the rules by insering a half-indent, so that the patch is easily reviewable:
  66
  67 ```c++
  68 void a_function() {
  69   while (something) {   // new line - half indent
  70     if (some condition) {
  71         stmt;
  72     } else {
  73         stmt;
  74     }
  75   }                      // new line
  76 }
  77 ```
  78
  79 A follow-up patch can restore the indents without any functional changes.
  80
  81 ## Function parameters
  82
  83 Avoid output parameters; use return values instead.  In/out parameters are tricky, but in some cases they are relatively standard, such as serialization/deserialization.
  84
  85 If a function accepts a lambda or an `std::function`, make it the last argument, so that it can be easily provided inline:
  86
  87 ```c++
  88 template <typename Func>
  89 void function_accepting_a_lambda(int a, int b, Func func);
  90
  91 int f() {
  92     return function_accepting_a_lambda(2, 3, [] (int x, int y) {
  93         return x + y;
  94     });
  95 }
  96 ```
  97
  98 ## Complex return types
  99
 100 If a function returns a complicated return type, put its return type on a separate line, otherwise it becomes hard to see where the return type ends and where the function name begins:
 101
 102 ```c++
 103 template <typename T1, T2>
 104 template <typename T3, T4>
 105 std::vector<typename a_struct<T1, T2>::some_nested_class<T3, T4>>  // I'm the return type
 106 a_struct<T1, T2>::a_function(T3 a, T4 b) {                         // And I'm the function name
 107     // ...
 108 }
 109 ```
 110
 111 ## Whitespace around operators
 112
 113 Whitespace around operators should match their precedence: high precedence = no spaces, low precedency = add spaces:
 114
 115 ```c++
 116      return *a + *b;  // good
 117      return * a+* b;  // bad
 118 ```
 119
 120 `if`, `while`, `return` (and `template`) are not function calls, so they get a space after the keyword.
 121
 122 ## Long lines
 123
 124 If a line becomes excessively long (>120 characters?), or is just complicated, break it into two or more lines.  The second (and succeeding lines) are _continuation lines_, and have a double indent:
 125
 126 ```c++
 127     if ((some_condition && some_other_condition)
 128             || (more complicated stuff here...)   // continuation line, double indent
 129             || (even more complicated stuff)) {   // another continuation line
 130         do_something();  // back to single indent
 131     }
 132 ```
 133
 134 Of course, long lines or complex conditions may indicate that refactoring is in order.
 135
 136 ## Generic lambdas and types
 137
 138 Generic lambdas (`[] (auto param)`) are discouraged where the type is known. Generic
 139 lambdas reduce the compiler's and other tools' ability to reason about the code.
 140 In case the actual type of `param` doesn't match the programmers expectations,
 141 the compiler will only detect an error in the lambda body, or perhaps
 142 even lower down the stack if more generic functions are called. In the case of an
 143 IDE, most of its functionality is disabled in a generic lambda, since it can't
 144 assume anything about that parameter.
 145
 146 Of course, when there is a need to support multiple types, genericity is the correct
 147 tool. Even then, type parameters should be constrained with concepts, in order to
 148 catch type mismatches early rather than deep in the instantiation chain.
 149
 150