small cmake introduction

Cmake Intro/.gitignore

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+/build

Cmake Intro/CMakeLists.txt

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+# set a minimum CMake version required
+cmake_minimum_required(VERSION 2.4)
+
+# start a new CMake project
+project(HelloWorld)
+
+# create build target app
+add_executable(app hello.cpp)

Cmake Intro/hello.cpp

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+#include <iostream>
+
+int main()
+{
+    std::cout << "Hello World!\n";
+    return 0;
+}

Cmake Intro/readme.md

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+# C++ with CMAKE
+The most basic project is an executable built from source code files. 
+For simple projects, a three line `CMakeLists.txt` file is all that is required. Inside this file are all commands for CMAKE. 
+
+You can build and run our project now. First, run the cmake executable (or cmake-gui) to configure the project and then build it with your chosen build tool.
+
+Move to the CMake source code tree and create a build directory:
+```bash
+mkdir build
+```
+Next, navigate to the build directory and run CMake to configure the project and generate a native build system:
+```bash
+cd build
+cmake ..
+```
+Then call that build system to actually compile/link the project:
+```bash
+cmake --build .
+```
+
+## Version Numbering and Header Files
+Provide the executable and project with a version number, using CMakeLists.txt provides more flexibility. This can be done by using the `VERSION` append to the project name.
+```cmake
+cmake_minimum_required(VERSION 3.10)
+
+# set the project name and version
+project(HelloWorld VERSION 1.0)
+```
+
+You can also configure header files to pass version numbers to the source code. Here a header file `Hello.h.in` will be read and the version numbers passed to get `Hello.h`
+```bash
+configure_file(Hello.h.in Hello.h)
+```
+As the configured file will be written into the binary tree, add that directory to the list of paths to search for include files.
+```bash
+target_include_directories(HelloWorld PUBLIC
+                           "${PROJECT_BINARY_DIR}"
+                           )
+```
+When CMake configures header files the values inside `@...@` will be replaced.
+
+## Specify the C++ Standard
+Sometimes you need to explicitly state in the CMake code that it should use the correct flags. The easiest way to enable support for a specific C++ standard in CMake is by using the `CMAKE_CXX_STANDARD` variable. Make sure to add the `CMAKE_CXX_STANDARD` declarations above the call to `add_executable`.
+```cmake
+cmake_minimum_required(VERSION 3.10)
+
+# set the project name and version
+project(HelloWorld VERSION 1.0)
+
+# specify the C++ standard
+set(CMAKE_CXX_STANDARD 11)
+set(CMAKE_CXX_STANDARD_REQUIRED True)
+```
+
+## Rebuild
+Build our project again. As you already created the bild directory skip this step and proceed:
+```bash
+cd build
+cmake --build .
+```