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diff --git a/absl/FAQ.md b/absl/FAQ.md deleted file mode 100644 index af721307c2ec..000000000000 --- a/absl/FAQ.md +++ /dev/null @@ -1,144 +0,0 @@ -# Abseil FAQ - -## Is Abseil the right home for my utility library? - -Most often the answer to the question is "no." As both the [About -Abseil](https://abseil.io/about/) page and our [contributing -guidelines](https://github.com/abseil/abseil-cpp/blob/master/CONTRIBUTING.md#contribution-guidelines) -explain, Abseil contains a variety of core C++ library code that is widely used -at [Google](https://www.google.com/). As such, Abseil's primary purpose is to be -used as a dependency by Google's open source C++ projects. While we do hope that -Abseil is also useful to the C++ community at large, this added constraint also -means that we are unlikely to accept a contribution of utility code that isn't -already widely used by Google. - -## How to I set the C++ dialect used to build Abseil? - -The short answer is that whatever mechanism you choose, you need to make sure -that you set this option consistently at the global level for your entire -project. If, for example, you want to set the C++ dialect to C++17, with -[Bazel](https://bazel/build/) as the build system and `gcc` or `clang` as the -compiler, there several ways to do this: -* Pass `--cxxopt=-std=c++17` on the command line (for example, `bazel build - --cxxopt=-std=c++17 ...`) -* Set the environment variable `BAZEL_CXXOPTS` (for example, - `BAZEL_CXXOPTS=-std=c++17`) -* Add `build --cxxopt=-std=c++17` to your [`.bazelrc` - file](https://docs.bazel.build/versions/master/guide.html#bazelrc) - -If you are using CMake as the build system, you'll need to add a line like -`set(CMAKE_CXX_STANDARD 17)` to your top level `CMakeLists.txt` file. See the -[CMake build -instructions](https://github.com/abseil/abseil-cpp/blob/master/CMake/README.md) -for more information. - -For a longer answer to this question and to understand why some other approaches -don't work, see the answer to "What is ABI and why don't you recommend using a -pre-compiled version of Abseil?" - -## What is ABI and why don't you recommend using a pre-compiled version of Abseil? - -For the purposes of this discussion, you can think of -[ABI](https://en.wikipedia.org/wiki/Application_binary_interface) as the -compiled representation of the interfaces in code. This is in contrast to -[API](https://en.wikipedia.org/wiki/Application_programming_interface), which -you can think of as the interfaces as defined by the code itself. [Abseil has a -strong promise of API compatibility, but does not make any promise of ABI -compatibility](https://abseil.io/about/compatibility). Let's take a look at what -this means in practice. - -You might be tempted to do something like this in a -[Bazel](https://bazel.build/) `BUILD` file: - -``` -# DON'T DO THIS!!! -cc_library( - name = "my_library", - srcs = ["my_library.cc"], - copts = ["-std=c++17"], # May create a mixed-mode compile! - deps = ["@com_google_absl//absl/strings"], -) -``` - -Applying `-std=c++17` to an individual target in your `BUILD` file is going to -compile that specific target in C++17 mode, but it isn't going to ensure the -Abseil library is built in C++17 mode, since the Abseil library itself is a -different build target. If your code includes an Abseil header, then your -program may contain conflicting definitions of the same -class/function/variable/enum, etc. As a rule, all compile options that affect -the ABI of a program need to be applied to the entire build on a global basis. - -C++ has something called the [One Definition -Rule](https://en.wikipedia.org/wiki/One_Definition_Rule) (ODR). C++ doesn't -allow multiple definitions of the same class/function/variable/enum, etc. ODR -violations sometimes result in linker errors, but linkers do not always catch -violations. Uncaught ODR violations can result in strange runtime behaviors or -crashes that can be hard to debug. - -If you build the Abseil library and your code using different compile options -that affect ABI, there is a good chance you will run afoul of the One Definition -Rule. Examples of GCC compile options that affect ABI include (but aren't -limited to) language dialect (e.g. `-std=`), optimization level (e.g. `-O2`), -code generation flags (e.g. `-fexceptions`), and preprocessor defines -(e.g. `-DNDEBUG`). - -If you use a pre-compiled version of Abseil, (for example, from your Linux -distribution package manager or from something like -[vcpkg](https://github.com/microsoft/vcpkg)) you have to be very careful to -ensure ABI compatibility across the components of your program. The only way you -can be sure your program is going to be correct regarding ABI is to ensure -you've used the exact same compile options as were used to build the -pre-compiled library. This does not mean that Abseil cannot work as part of a -Linux distribution since a knowledgeable binary packager will have ensured that -all packages have been built with consistent compile options. This is one of the -reasons we warn against - though do not outright reject - using Abseil as a -pre-compiled library. - -Another possible way that you might afoul of ABI issues is if you accidentally -include two versions of Abseil in your program. Multiple versions of Abseil can -end up within the same binary if your program uses the Abseil library and -another library also transitively depends on Abseil (resulting in what is -sometimes called the diamond dependency problem). In cases such as this you must -structure your build so that all libraries use the same version of Abseil. -[Abseil's strong promise of API compatibility between -releases](https://abseil.io/about/compatibility) means the latest "HEAD" release -of Abseil is almost certainly the right choice if you are doing as we recommend -and building all of your code from source. - -For these reasons we recommend you avoid pre-compiled code and build the Abseil -library yourself in a consistent manner with the rest of your code. - -## What is "live at head" and how do I do it? - -From Abseil's point-of-view, "live at head" means that every Abseil source -release (which happens on an almost daily basis) is either API compatible with -the previous release, or comes with an automated tool that you can run over code -to make it compatible. In practice, the need to use an automated tool is -extremely rare. This means that upgrading from one source release to another -should be a routine practice that can and should be performed often. - -We recommend you update to the latest release of Abseil as often as -possible. Not only will you pick up bug fixes more quickly, but if you have good -automated testing, you will catch and be able to fix any [Hyrum's -Law](https://www.hyrumslaw.com/) dependency problems on an incremental basis -instead of being overwhelmed by them and having difficulty isolating them if you -wait longer between updates. - -If you are using the [Bazel](https://bazel.build/) build system and its -[external dependencies](https://docs.bazel.build/versions/master/external.html) -feature, updating the -[`http_archive`](https://docs.bazel.build/versions/master/repo/http.html#http_archive) -rule in your -[`WORKSPACE`](https://docs.bazel.build/versions/master/be/workspace.html) for -`com_google_abseil` to point to the latest release is all you need to do. You -can commit the updated `WORKSPACE` file to your source control every time you -update, and if you have good automated testing, you might even consider -automating this. - -One thing we don't recommend is using GitHub's `master.zip` files (for example -[https://github.com/abseil/abseil-cpp/archive/master.zip](https://github.com/abseil/abseil-cpp/archive/master.zip)), -which are always the latest commit in the `master` branch, to implement live at -head. Since these `master.zip` URLs are not versioned, you will lose build -reproducibility. In addition, some build systems, including Bazel, will simply -cache this file, which means you won't actually be updating to the latest -release until your cache is cleared or invalidated. |