apollo_best_coding_practice.md

Apollo Best Coding Practice

1. Always build, test, and lint all.

   ./apollo.sh check

2. Always write unit tests and put them along with the source files.

   foobar.h
   foobar.cc
   foobar_test.cc
   

3. A bazel target should contain at most one header and one source file.

   cc_library(
     name = "foobar",
     hdrs = ["foobar.h"],
     srcs = ["foobar.cc"],
     deps = [
       ...
     ],
   )
   
   cc_test(
     name = "foobar_test",
     srcs = ["foobar_test.cc"],
     deps = [
       ":foobar",
       ...
     ]
   )

   You can use scripts/buildifier.sh <path/to/BUILD> to fix BUILD file style issues.

4. In general, Apollo follows Google C++ coding style. You should run scripts/clang_format.sh <path/to/cpp/dirs/or/files> to fix C++ style issues.

5. Simple and unified function signature.

   // 1. For input objects, const reference guarantes that it is valid, while
   //    pointers might be NULL or wild. Don't give others the chance to break
   //    you.
   // 2. For input scalas, just pass by value, which gives better locality and
   //    thus performance.
   // 3. For output, it's callers' responsibility to make sure the pointers are
   //    valid. No need to do sanity check, and also no need to mark as
   //    "OutputType* const", because pointer redirection is never allowed.
   void FooBar(const InputObjectType& input1, const InputScalaType input2, ...,
               OutputType* output1, ...);
   
   // RVO machanism will help you avoid unnecessary object copy.
   // See https://en.wikipedia.org/wiki/Copy_elision#Return_value_optimization
   OutputType FooBar(const InputType& input);

6. Use const whenever possible.

   // Variables that don't change.
   const int current_size = vec.size();
   // Functions that have no side effect.
   const std::string& name() const;

7. Prefer C++ headers over C headers.

   We prefer using #include <ctime> over #include <time.h>, <cmath> over <math.h>, <cstdio> over <stdio.h>, <cstring> over <string.h>, etc.

8. Include necessary headers only. No more, no less.

   Please also pay attention to header orders. Again, you can use scripts/clang_format.sh to fix header order issues.

9. List only direct dependencies in deps section of a Bazel target.

   Generally, only targets to which the included headers belongs should be listed as a dependency.

   For example, suppose sandwich.h includes bread.h which in turn includes flour.h. Since sandwich.h doesn't include flour.h directly (who wants flour in their sandwich?), the BUILD file would look like this:

   cc_library(
    name = "sandwich",
    srcs = ["sandwich.cc"],
    hdrs = ["sandwich.h"],
    deps = [
        ":bread",
        # BAD practice to uncomment the line below
        # ":flour",
    ],
   )
   
    cc_library(
        name = "bread",
        srcs = ["bread.cc"],
        hdrs = ["bread.h"],
        deps = [":flour"],
    )
   
    cc_library(
        name = "flour",
        srcs = ["flour.cc"],
        hdrs = ["flour.h"],
    )

10. Conform to the DRY principle.

    Don't repeat yourself, in any way. Avoid duplicate classes, functions, const variables, or a simple piece of code. Some examples:

    • It's fine to refer a name with full path once, like apollo::common::util::Type, but better to make a short alias if you need to use twice or more: using apollo::common::util::Type;.
    • It's fine to access a sub-field of proto once in cascade style, like a_proto.field_1().field_2().field_3(), but better to save the reference of a common part first if you need to access it twice or more: const auto& field_2 = a_proto.field_1().field_2();.