Added CMake configure-time download instructions to docs

Adds instructions for how to add gtest and gmock to
another CMake project directly. Downloading of the
googletest sources happens as configure time, allowing
it to be added to the main build directly via the
add_subdirectory() command. This ensures googletest is
built with the same compiler settings, etc. and will
typically result in a more robust and more convenient
build arrangement.

1 files changed, 105 insertions, 2 deletions

diff --git a/googletest/README.md b/googletest/README.md
index edd44080..0a3474c0 100644
--- a/googletest/README.md
+++ b/googletest/README.md
@@ -59,7 +59,13 @@ cross-platform.). If you don't have CMake installed already, you can
 download it for free from <http://www.cmake.org/>.
 
 CMake works by generating native makefiles or build projects that can
-be used in the compiler environment of your choice.  The typical
+be used in the compiler environment of your choice.  You can either
+build Google Test as a standalone project or it can be incorporated
+into an existing CMake build for another project.
+
+#### Standalone CMake Project ####
+
+When building Google Test as a standalone project, the typical
 workflow starts with:
 
     mkdir mybuild       # Create a directory to hold the build output.
@@ -80,13 +86,110 @@ using Visual Studio.
 
 On Mac OS X with Xcode installed, a `.xcodeproj` file will be generated.
 
+#### Incorporating Into An Existing CMake Project ####
+
+If you want to use gtest in a project which already uses CMake, then a
+more robust and flexible approach is to build gtest as part of that
+project directly. This is done by making the GoogleTest source code
+available to the main build and adding it using CMake's
+`add_subdirectory()` command. This has the significant advantage that
+the same compiler and linker settings are used between gtest and the
+rest of your project, so issues associated with using incompatible
+libraries (eg debug/release), etc. are avoided. This is particularly
+useful on Windows. Making GoogleTest's source code available to the
+main build can be done a few different ways:
+
+* Download the GoogleTest source code manually and place it at a
+  known location. This is the least flexible approach and can make
+  it more difficult to use with continuous integration systems, etc.
+* Embed the GoogleTest source code as a direct copy in the main
+  project's source tree. This is often the simplest approach, but is
+  also the hardest to keep up to date. Some organizations may not
+  permit this method.
+* Add GoogleTest as a git submodule or equivalent. This may not
+  always be possible or appropriate. Git submodules, for example,
+  have their own set of advantages and drawbacks.
+* Use CMake to download GoogleTest as part of the build's configure
+  step. This is just a little more complex, but doesn't have the
+  limitations of the other methods.
+
+The last of the above methods is implemented with a small piece
+of CMake code in a separate file (e.g. `CMakeLists.txt.in`) which
+is copied to the build area and then invoked as a sub-build
+_during the CMake stage_. That directory is then pulled into the
+main build with `add_subdirectory()`. For example:
+
+New file `CMakeLists.txt.in`:
+
+    cmake_minimum_required(VERSION 2.8.2)
+ 
+    project(googletest-download NONE)
+ 
+    include(ExternalProject)
+    ExternalProject_Add(googletest
+      GIT_REPOSITORY    https://github.com/google/googletest.git
+      GIT_TAG           master
+      SOURCE_DIR        "${CMAKE_BINARY_DIR}/googletest-src"
+      BINARY_DIR        "${CMAKE_BINARY_DIR}/googletest-build"
+      CONFIGURE_COMMAND ""
+      BUILD_COMMAND     ""
+      INSTALL_COMMAND   ""
+      TEST_COMMAND      ""
+    )
+    
+Existing build's `CMakeLists.txt`:
+
+    # Download and unpack googletest at configure time
+    configure_file(CMakeLists.txt.in googletest-download/CMakeLists.txt)
+    execute_process(COMMAND ${CMAKE_COMMAND} -G "${CMAKE_GENERATOR}" .
+      RESULT_VARIABLE result
+      WORKING_DIRECTORY ${CMAKE_BINARY_DIR}/googletest-download )
+    if(result)
+      message(FATAL_ERROR "CMake step for googletest failed: ${result}")
+    endif()
+    execute_process(COMMAND ${CMAKE_COMMAND} --build .
+      RESULT_VARIABLE result
+      WORKING_DIRECTORY ${CMAKE_BINARY_DIR}/googletest-download )
+    if(result)
+      message(FATAL_ERROR "Build step for googletest failed: ${result}")
+    endif()
+
+    # Prevent overriding the parent project's compiler/linker
+    # settings on Windows
+    set(gtest_force_shared_crt ON CACHE BOOL "" FORCE)
+    
+    # Add googletest directly to our build. This defines
+    # the gtest and gtest_main targets.
+    add_subdirectory(${CMAKE_BINARY_DIR}/googletest-src
+                     ${CMAKE_BINARY_DIR}/googletest-build)
+
+    # The gtest/gtest_main targets carry header search path
+    # dependencies automatically when using CMake 2.8.11 or
+    # later. Otherwise we have to add them here ourselves.
+    if (CMAKE_VERSION VERSION_LESS 2.8.11)
+      include_directories("${gtest_SOURCE_DIR}/include")
+    endif()
+
+    # Now simply link against gtest or gtest_main as needed. Eg
+    add_executable(example example.cpp)
+    target_link_libraries(example gtest_main)
+    add_test(NAME example_test COMMAND example)
+
+Note that this approach requires CMake 2.8.2 or later due to
+its use of the `ExternalProject_Add()` command. The above
+technique is discussed in more detail in 
+[this separate article](http://crascit.com/2015/07/25/cmake-gtest/)
+which also contains a link to a fully generalized implementation
+of the technique.
+
+
 ### Legacy Build Scripts ###
 
 Before settling on CMake, we have been providing hand-maintained build
 projects/scripts for Visual Studio, Xcode, and Autotools.  While we
 continue to provide them for convenience, they are not actively
 maintained any more.  We highly recommend that you follow the
-instructions in the previous two sections to integrate Google Test
+instructions in the above sections to integrate Google Test
 with your existing build system.
 
 If you still need to use the legacy build scripts, here's how: