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+# Migrating from v2 to v3
+
+v3 is the next major version of Catch2 and brings three significant changes:
+ * Catch2 is now split into multiple headers
+ * Catch2 is now compiled as a static library
+ * C++14 is the minimum required C++ version
+
+There are many reasons why we decided to go from the old single-header
+distribution model to a more standard library distribution model. The
+big one is compile-time performance, but moving over to a split header
+distribution model also improves the future maintainability and
+extendability of the codebase. For example v3 adds a new kind of matchers
+without impacting the compilation times of users that do not use matchers
+in their tests. The new model is also more friendly towards package
+managers, such as vcpkg and Conan.
+
+The result of this move is a significant improvement in compilation
+times, e.g. the inclusion overhead of Catch2 in the common case has been
+reduced by roughly 80%. The improved ease of maintenance also led to
+various runtime performance improvements and the introduction of new features.
+For details, look at [the release notes of 3.0.1](release-notes.md#301).
+
+_Note that we still provide one header + one translation unit (TU)
+distribution but do not consider it the primarily supported option. You
+should also expect that the compilation times will be worse if you use
+this option._
+
+
+## How to migrate projects from v2 to v3
+
+To migrate to v3, there are two basic approaches to do so.
+
+1. Use `catch_amalgamated.hpp` and `catch_amalgamated.cpp`.
+2. Build Catch2 as a proper (static) library, and move to piecewise headers
+
+Doing 1 means downloading the [amalgamated header](/extras/catch_amalgamated.hpp)
+and the [amalgamated sources](/extras/catch_amalgamated.cpp) from `extras`,
+dropping them into your test project, and rewriting your includes from
+`<catch2/catch.hpp>` to `"catch_amalgamated.hpp"` (or something similar,
+based on how you set up your paths).
+
+The disadvantage of using this approach are increased compilation times,
+at least compared to the second approach, but it does let you avoid
+dealing with consuming libraries in your build system of choice.
+
+
+However, we recommend doing 2, and taking extra time to migrate to v3
+properly. This lets you reap the benefits of significantly improved
+compilation times in the v3 version. The basic steps to do so are:
+
+1. Change your CMakeLists.txt to link against `Catch2WithMain` target if
+you use Catch2's default main. (If you do not, keep linking against
+the `Catch2` target.). If you use pkg-config, change `pkg-config catch2` to
+`pkg-config catch2-with-main`.
+2. Delete TU with `CATCH_CONFIG_RUNNER` or `CATCH_CONFIG_MAIN` defined,
+as it is no longer needed.
+3. Change `#include <catch2/catch.hpp>` to `#include <catch2/catch_all.hpp>`
+4. Check that everything compiles. You might have to modify namespaces,
+or perform some other changes (see the
+[Things that can break during porting](#things-that-can-break-during-porting)
+section for the most common things).
+5. Start migrating your test TUs from including `<catch2/catch_all.hpp>`
+to piecemeal includes. You will likely want to start by including
+`<catch2/catch_test_macros.hpp>`, and then go from there. (see
+[other notes](#other-notes) for further ideas)
+
+## Other notes
+
+* The main test include is now `<catch2/catch_test_macros.hpp>`
+
+* Big "subparts" like Matchers, or Generators, have their own folder, and
+also their own "big header", so if you just want to include all matchers,
+you can include `<catch2/matchers/catch_matchers_all.hpp>`,
+or `<catch2/generators/catch_generators_all.hpp>`
+
+
+## Things that can break during porting
+
+* The namespaces of Matchers were flattened and cleaned up.
+
+Matchers are no longer declared deep within an internal namespace and
+then brought up into `Catch` namespace. All Matchers now live in the
+`Catch::Matchers` namespace.
+
+* The `Contains` string matcher was renamed to `ContainsSubstring`.
+
+* The reporter interfaces changed in a breaking manner.
+
+If you are using a custom reporter or listener, you will likely need to
+modify them to conform to the new interfaces. Unlike before in v2,
+the [interfaces](reporters.md#top) and the [events](reporter-events.md#top)
+are now documented.
+
+
+---
+
+[Home](Readme.md#top)