= Hello Headers
:toc:
:toc-placement!:

toc::[]


# Introduction

Shows a hello world example which uses a different folder for source and include
files.

The files in this tutorial include:

```
B-hello-headers$ tree
.
├── CMakeLists.txt
├── include
│   └── Hello.h
└── src
    ├── Hello.cpp
    └── main.cpp
```

  * link:CMakeLists.txt[CMakeLists.txt] - Contains the CMake commands you wish to run.
  * link:include/Hello.h[include/Hello.h] - The header file to include.
  * link:src/Hello.cpp[src/Hello.cpp] - A source file to compile.
  * link:src/main.cpp[src/main.cpp] - The source file with main.


# Concepts

## Directory Paths

CMake syntax specifies a number of https://cmake.org/Wiki/CMake_Useful_Variables[variables]
which can be used to help find useful directories in your project or source tree.
Some of these include:

[cols=",",options="header",]
|=======================================================================
|Variable |Info
|CMAKE_SOURCE_DIR |The root source directory

|CMAKE_CURRENT_SOURCE_DIR |The current source directory if using
sub-projects and directories.

|PROJECT_SOURCE_DIR |The source directory of the current cmake project.

|CMAKE_BINARY_DIR |The root binary / build directory. This is the
directory where you ran the cmake command.

|CMAKE_CURRENT_BINARY_DIR |The build directory you are currently in.

|PROJECT_BINARY_DIR |The build directory for the current project.
|=======================================================================

## Source Files Variable

Creating a variable which includes the source files allows you to be
clearer about these files and easily add them to multiple commands, for example,
the +add_executable()+ function.

[source,cmake]
----
# Create a sources variable with a link to all cpp files to compile
set(SOURCES
    src/Hello.cpp
    src/main.cpp
)

add_executable(${PROJECT_NAME} ${SOURCES})
----

[NOTE]
====
An alternative to setting specific file names in the +SOURCES+ variable is
to use a GLOB command to find files using wildcard pattern matching.

[source,cmake]
----
file(GLOB SOURCES "src/*.cpp")
----
====


[TIP]
====
For modern CMake it is NOT recommended to use a variable for sources. Insead it is 
typical to directly declare the sources in the add_xxx function.

This is particularly important for glob commands which may not always show you the
correct results if you add a new source file.
====

## Including Directories

When you have different include folders, you can make your compiler aware of them using the
+target_include_directories()+ link:https://cmake.org/cmake/help/v3.0/command/target_include_directories.html[function]. When compiling this target this will add these directories to the compiler with the -I flag e.g. `-I/directory/path`

[source,cmake]
----
target_include_directories(target
    PRIVATE 
        ${PROJECT_SOURCE_DIR}/include
)
----

The +PRIVATE+ identifier specifies the scope of the include. This is important for libraries and is exlpained in the next example. More details on the function is available link:https://cmake.org/cmake/help/v3.0/command/target_include_directories.html[here]

# Building the Example

## Standard Output

The standard output from building this example is presented below.

[source,bash]
----
$ mkdir build

$ cd build

$ cmake ..
-- The C compiler identification is GNU 4.8.4
-- The CXX compiler identification is GNU 4.8.4
-- Check for working C compiler: /usr/bin/cc
-- Check for working C compiler: /usr/bin/cc -- works
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Check for working CXX compiler: /usr/bin/c++
-- Check for working CXX compiler: /usr/bin/c++ -- works
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Configuring done
-- Generating done
-- Build files have been written to: /home/matrim/workspace/cmake-examples/01-basic/hello_headers/build

$ make
Scanning dependencies of target hello_headers
[ 50%] Building CXX object CMakeFiles/hello_headers.dir/src/Hello.cpp.o
[100%] Building CXX object CMakeFiles/hello_headers.dir/src/main.cpp.o
Linking CXX executable hello_headers
[100%] Built target hello_headers

$ ./hello_headers
Hello Headers!
----


## Verbose Output

In the previous examples, when running the make command the output only
shows the status of the build. To see the full output for debugging
purposes you can add +VERBOSE=1+ flag when running make.

The VERBOSE output is show below, and a examination of the output shows
the include directories being added to the c++ compiler command.

[source,bash]
----
$ make clean

$ make VERBOSE=1
/usr/bin/cmake -H/home/matrim/workspace/cmake-examples/01-basic/hello_headers -B/home/matrim/workspace/cmake-examples/01-basic/hello_headers/build --check-build-system CMakeFiles/Makefile.cmake 0
/usr/bin/cmake -E cmake_progress_start /home/matrim/workspace/cmake-examples/01-basic/hello_headers/build/CMakeFiles /home/matrim/workspace/cmake-examples/01-basic/hello_headers/build/CMakeFiles/progress.marks
make -f CMakeFiles/Makefile2 all
make[1]: Entering directory `/home/matrim/workspace/cmake-examples/01-basic/hello_headers/build'
make -f CMakeFiles/hello_headers.dir/build.make CMakeFiles/hello_headers.dir/depend
make[2]: Entering directory `/home/matrim/workspace/cmake-examples/01-basic/hello_headers/build'
cd /home/matrim/workspace/cmake-examples/01-basic/hello_headers/build && /usr/bin/cmake -E cmake_depends "Unix Makefiles" /home/matrim/workspace/cmake-examples/01-basic/hello_headers /home/matrim/workspace/cmake-examples/01-basic/hello_headers /home/matrim/workspace/cmake-examples/01-basic/hello_headers/build /home/matrim/workspace/cmake-examples/01-basic/hello_headers/build /home/matrim/workspace/cmake-examples/01-basic/hello_headers/build/CMakeFiles/hello_headers.dir/DependInfo.cmake --color=
make[2]: Leaving directory `/home/matrim/workspace/cmake-examples/01-basic/hello_headers/build'
make -f CMakeFiles/hello_headers.dir/build.make CMakeFiles/hello_headers.dir/build
make[2]: Entering directory `/home/matrim/workspace/cmake-examples/01-basic/hello_headers/build'
/usr/bin/cmake -E cmake_progress_report /home/matrim/workspace/cmake-examples/01-basic/hello_headers/build/CMakeFiles 1
[ 50%] Building CXX object CMakeFiles/hello_headers.dir/src/Hello.cpp.o
/usr/bin/c++    -I/home/matrim/workspace/cmake-examples/01-basic/hello_headers/include    -o CMakeFiles/hello_headers.dir/src/Hello.cpp.o -c /home/matrim/workspace/cmake-examples/01-basic/hello_headers/src/Hello.cpp
/usr/bin/cmake -E cmake_progress_report /home/matrim/workspace/cmake-examples/01-basic/hello_headers/build/CMakeFiles 2
[100%] Building CXX object CMakeFiles/hello_headers.dir/src/main.cpp.o
/usr/bin/c++    -I/home/matrim/workspace/cmake-examples/01-basic/hello_headers/include    -o CMakeFiles/hello_headers.dir/src/main.cpp.o -c /home/matrim/workspace/cmake-examples/01-basic/hello_headers/src/main.cpp
Linking CXX executable hello_headers
/usr/bin/cmake -E cmake_link_script CMakeFiles/hello_headers.dir/link.txt --verbose=1
/usr/bin/c++       CMakeFiles/hello_headers.dir/src/Hello.cpp.o CMakeFiles/hello_headers.dir/src/main.cpp.o  -o hello_headers -rdynamic
make[2]: Leaving directory `/home/matrim/workspace/cmake-examples/01-basic/hello_headers/build'
/usr/bin/cmake -E cmake_progress_report /home/matrim/workspace/cmake-examples/01-basic/hello_headers/build/CMakeFiles  1 2
[100%] Built target hello_headers
make[1]: Leaving directory `/home/matrim/workspace/cmake-examples/01-basic/hello_headers/build'
/usr/bin/cmake -E cmake_progress_start /home/matrim/workspace/cmake-examples/01-basic/hello_headers/build/CMakeFiles 0
----
