CMake基础 第2节 分离编译

介绍

展示一个hello world示例，它使用不同的文件夹来存储源文件和头文件。

本教程中的文件包括：

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

• [CMakeLists.txt] - 包含你希望运行的 CMake 命令。

  # Set the minimum version of CMake that can be used
  # To find the cmake version run
  # $ cmake --version
  cmake_minimum_required(VERSION 3.5)
  
  # Set the project name
  project (hello_headers)
  
  # Create a sources variable with a link to all cpp files to compile
  set(SOURCES
      src/Hello.cpp
      src/main.cpp
  )
  
  # Add an executable with the above sources
  add_executable(hello_headers ${SOURCES})
  
  # Set the directories that should be included in the build command for this target
  # when running g++ these will be included as -I/directory/path/
  target_include_directories(hello_headers
      PRIVATE 
          ${PROJECT_SOURCE_DIR}/include
  )
  

• [include/Hello.h] - 要包含的标题文件。

  #ifndef __HELLO_H__
  #define __HELLO_H__
  
  class Hello
  {
  public:
      void print();
  };
  
  #endif
  

• [src/Hello.cpp] - 要编译的源文件。

  #include <iostream>
  
  #include "Hello.h"
  
  void Hello::print()
  {
      std::cout << "Hello Headers!" << std::endl;
  }
  

• [src/main.cpp] - 主源文件。

  #include "Hello.h"
  
  int main(int argc, char *argv[])
  {
      Hello hi;
      hi.print();
      return 0;
  }
  

概念

目录路径

CMake 语法指定了许多变量，这些变量可用于帮助在项目或源代码树中找到有用的目录。其中一些包括：

变量	信息
CMAKE_SOURCE_DIR	根源目录
CMAKE_CURRENT_SOURCE_DIR	如果使用子项目和目录，则为当前源目录。
PROJECT_SOURCE_DIR	当前 cmake 项目的源目录。
CMAKE_BINARY_DIR	根二进制文件生成目录。这是运行cmake命令的目录。
CMAKE_CURRENT_BINARY_DIR	你当前所处的生成目录。
PROJECT_BINARY_DIR	当前项目的生成目录。

源文件变量

创建包含源码文件的变量可使你更清楚地了解这些文件，并轻松将其添加到多个命令中，例如，add_executable()功能。

# 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	另一种替代方案是使用 GLOB 命令使用通配符模式匹配查找文件。

file(GLOB SOURCES "src/*.cpp")

Tip	对于现代的CMake，不建议对源代码使用变量。相反，通常直接在add_xxx函数中声明源代码。这对于GLOB命令尤其重要，如果你添加一个新的源代码文件，这些命令可能并不总是显示正确的结果。

头文件目录

当你有不同的头文件目录时，可以使用target_include_directories()函数让编译器知道它们编译此目标时，会将这些目录添加到带有-i标志的编译指令中，例如-i/directory/path

target_include_directories(target
    PRIVATE
        ${PROJECT_SOURCE_DIR}/include
)

PRIVATE标识符指定include的范围，这对库很重要，这些内容将在下一个示例中进行说明。有关该功能的更多详细信息，请访问这里

构建示例

标准输出

构建示例的标准输出内容如下所示：

$ 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!

详细输出

在前面的示例中，当运行make命令时，输出仅显示构建的状态。要查看用于调试目的的完整输出，可以在运行make时添加VERBOSE=1标志。

VERBOSE 输出如下所示。可以看到，include目录已经被添加到了编译命令中。

$ 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