CMake基礎 第13節 構建子項目

介紹

此示例說明如何設置包含子項目的CMake項目。頂層CMakeLists.txt調用子目錄中的CMakeLists.txt以創建以下內容：

• sublibrary1 - 靜態庫
• sublibrary2 - 頭文件庫
• subbinary - 可執行文件

此示例中包含的文件包括：

$ tree
.
├── CMakeLists.txt
├── subbinary
│   ├── CMakeLists.txt
│   └── main.cpp
├── sublibrary1
│   ├── CMakeLists.txt
│   ├── include
│   │   └── sublib1
│   │       └── sublib1.h
│   └── src
│       └── sublib1.cpp
└── sublibrary2
    ├── CMakeLists.txt
    └── include
        └── sublib2
            └── sublib2.h

• [CMakeLists.txt] - 頂級CMakeLists.txt

  cmake_minimum_required (VERSION 3.5)
  
  project(subprojects)
  
  # Add sub directories
  add_subdirectory(sublibrary1)
  add_subdirectory(sublibrary2)
  add_subdirectory(subbinary)
  

• [subbinary/CMakeLists.txt] - 生成可執行文件

  project(subbinary)
  
  # Create the executable
  add_executable(${PROJECT_NAME} main.cpp)
  
  # Link the static library from subproject1 using it's alias sub::lib1
  # Link the header only library from subproject2 using it's alias sub::lib2
  # This will cause the include directories for that target to be added to this project
  target_link_libraries(${PROJECT_NAME}
      sub::lib1
      sub::lib2
  )
  

• [subbinary/main.cpp] - 可執行文件的源代碼

  #include "sublib1/sublib1.h"
  #include "sublib2/sublib2.h"
  
  int main(int argc, char *argv[])
  {
      sublib1 hi;
      hi.print();
  
      sublib2 howdy;
      howdy.print();
      
      return 0;
  }
  

• [sublibrary1/CMakeLists.txt] - 創建靜態庫

  # Set the project name
  project (sublibrary1)
  
  # Add a library with the above sources
  add_library(${PROJECT_NAME} src/sublib1.cpp)
  add_library(sub::lib1 ALIAS ${PROJECT_NAME})
  
  target_include_directories( ${PROJECT_NAME}
      PUBLIC ${PROJECT_SOURCE_DIR}/include
  )
  

• [sublibrary1/include/sublib1/sublib1.h]

  #ifndef __SUBLIB_1_H__
  #define __SUBLIB_1_H__
  
  class sublib1
  {
  public:
      void print();
  };
  
  #endif
  

• [sublibrary1/src/sublib1.cpp]

  #include <iostream>
  
  #include "sublib1/sublib1.h"
  
  void sublib1::print()
  {
      std::cout << "Hello sub-library 1!" << std::endl;
  }
  

• [sublibrary2/CMakeLists.txt] - 設置僅含頭文件的庫

  # Set the project name
  project (sublibrary2)
  
  add_library(${PROJECT_NAME} INTERFACE)
  add_library(sub::lib2 ALIAS ${PROJECT_NAME})
  
  target_include_directories(${PROJECT_NAME}
      INTERFACE
          ${PROJECT_SOURCE_DIR}/include
  )
  
  

• [sublibrary2/include/sublib2/sublib2.h]

  #ifndef __SUBLIB_2_H__
  #define __SUBLIB_2_H__
  
  #include <iostream>
  
  class sublib2
  {
  public:
      void print()
      {
          std::cout << "Hello header only sub-library 2!" << std::endl;
      }
  };
  
  #endif
  

Tip	在本例中，我將頭文件移動到每個項目include目錄下的一個子文件夾中，同時將目標include保留為根include文件夾。這是防止文件名沖突的好主意，因為你必須包含如下所示的文件：#include“subib1/subib1.h”。這也意味着，如果你為其他用戶安裝庫，默認安裝位置將是/usr/local/include/subib1/subib1.h。

概念

添加子目錄

CMakeLists.txt文件可以包含和調用含CMakeLists.txt的子目錄

add_subdirectory(sublibrary1)
add_subdirectory(sublibrary2)
add_subdirectory(subbinary)

引用子項目目錄

當使用project()命令創建項目時，CMake將自動創建許多變量，這些變量可用於引用有關項目的詳細信息。然后，其他子項目或主項目可以使用這些變量。例如，引用你可以使用的不同項目的源目錄。

    ${sublibrary1_SOURCE_DIR}
    ${sublibrary2_SOURCE_DIR}

CMake創建的變量包括：

Variable	Info
PROJECT_NAME	由當前project()設置的項目名稱
CMAKE_PROJECT_NAME	由project()命令設置的第一個項目的名稱，即頂級項目
PROJECT_SOURCE_DIR	當前項目的源目錄
PROJECT_BINARY_DIR	當前項目的生成目錄
name_SOURCE_DIR	名為“name”的項目的源目錄。在本例中，創建的源目錄將是sublibrary1_SOURCE_DIR、sublibrary2_SOURCE_DIR和subbinary_SOURCE_DIR
name_BINARY_DIR	名為“name”的項目的二進制目錄。在本例中，創建的二進制目錄為sublibrary1_BINARY_DIR 、sublibrary2_BINARY_DIR和subbinary_BINARY_DIR。

頭文件庫

如果你有一個被創建為只包含頭文件的庫，cmake支持接口目標，以允許在沒有任何構建輸出的情況下創建目標。有關更多詳細信息，請單擊此處。

add_library(${PROJECT_NAME} INTERFACE)

在創建目標時，你還可以使用INTERFACE作用域包含該目標的目錄。INTERFACE作用域用於制定目標要求，這些要求在鏈接此目標的任何庫中使用，但不用於目標本身的編譯。如下示例，鏈接至此目標的任何目標都將包含一個include目錄，但此目標本身並不進行編譯（即不產生任何實體內容）：

target_include_directories(${PROJECT_NAME}
    INTERFACE
        ${PROJECT_SOURCE_DIR}/include
)

從子項目中引用庫

如果某個子項目創建庫，則其他項目可以通過在target_link_library()命令中調用該項目的名稱來引用該庫。這意味着你不必引用新庫的完整路徑，它將作為依賴項被添加。

target_link_libraries(subbinary
    PUBLIC
        sublibrary1
)

或者，你可以創建一個別名目標，使你可以在只讀上下文中引用該目標。

要創建別名目標運行，請執行以下操作：

add_library(sublibrary2)
add_library(sub::lib2 ALIAS sublibrary2)

要引用別名，只需如下所示：

target_link_libraries(subbinary
    sub::lib2
)

包含來自子項目的頭文件目錄

當從子項目添加庫時，從cmake v3開始，不需要使用它們在二進制文件的include目錄中添加項目include目錄。

這由創建庫時target_include_directory()命令中的作用域控制。在本例中，因為子二進制可執行文件鏈接了subibrary1和subibrary2庫，所以它將自動包括${subibrary1_source_DIR}/include和${subibrary2_source_DIR}/include文件夾，因為它們是隨庫的PUBLIC和INTERFACE范圍導出的。

構建示例

$ 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/02-sub-projects/A-basic/build

$ make
Scanning dependencies of target sublibrary1
[ 50%] Building CXX object sublibrary1/CMakeFiles/sublibrary1.dir/src/sublib1.cpp.o
Linking CXX static library libsublibrary1.a
[ 50%] Built target sublibrary1
Scanning dependencies of target subbinary
[100%] Building CXX object subbinary/CMakeFiles/subbinary.dir/main.cpp.o
Linking CXX executable subbinary
[100%] Built target subbinary