我的c++项目之路（三）

在之前两篇文章中我们初始化了一个cmake构建的项目并传入git库， 接下来我们改改代码并介绍前面用到的cmake

作为一个git项目， 得有README.md和.gitignore文件， README.md作为项目的介绍说明， 我们在案例中写入c++ demo， .gitignore文件的作用是告诉git有哪些文件可以忽略，例如我们忽略build目录文件和.vscode目录文件， 这样每次提交代码git会忽略这些目录

build/*
.vscode/*

接下来我们以一个开源项目的cmakelists.txt来说明一些常用的指令：

cmake_minimum_required(VERSION 3.1)

##
## PROJECT
## name and version
##
project(nlohmann_json VERSION 3.11.2 LANGUAGES CXX)

##
## MAIN_PROJECT CHECK
## determine if nlohmann_json is built as a subproject (using add_subdirectory) or if it is the main project
##
set(MAIN_PROJECT OFF)
if (CMAKE_CURRENT_SOURCE_DIR STREQUAL CMAKE_SOURCE_DIR)
    set(MAIN_PROJECT ON)
endif()

##
## INCLUDE
##
##
set(CMAKE_MODULE_PATH ${CMAKE_CURRENT_SOURCE_DIR}/cmake ${CMAKE_MODULE_PATH})
include(ExternalProject)

##
## OPTIONS
##

if (POLICY CMP0077)
    # Allow CMake 3.13+ to override options when using FetchContent / add_subdirectory.
    cmake_policy(SET CMP0077 NEW)
endif ()

# VERSION_GREATER_EQUAL is not available in CMake 3.1
if(${MAIN_PROJECT} AND (${CMAKE_VERSION} VERSION_EQUAL 3.13 OR ${CMAKE_VERSION} VERSION_GREATER 3.13))
    set(JSON_BuildTests_INIT ON)
else()
    set(JSON_BuildTests_INIT OFF)
endif()
option(JSON_BuildTests                     "Build the unit tests when BUILD_TESTING is enabled." ${JSON_BuildTests_INIT})
option(JSON_CI                             "Enable CI build targets." OFF)
option(JSON_Diagnostics                    "Use extended diagnostic messages." OFF)
option(JSON_GlobalUDLs                     "Place use-defined string literals in the global namespace." ON)
option(JSON_ImplicitConversions            "Enable implicit conversions." ON)
option(JSON_DisableEnumSerialization       "Disable default integer enum serialization." OFF)
option(JSON_LegacyDiscardedValueComparison "Enable legacy discarded value comparison." OFF)           
option(JSON_Install                        "Install CMake targets during install step." ${MAIN_PROJECT})
option(JSON_MultipleHeaders                "Use non-amalgamated version of the library." ON)
option(JSON_SystemInclude                  "Include as system headers (skip for clang-tidy)." OFF)

if (JSON_CI)
    include(ci)
endif ()

##
## CONFIGURATION
##
include(GNUInstallDirs)

set(NLOHMANN_JSON_TARGET_NAME               ${PROJECT_NAME})
set(NLOHMANN_JSON_CONFIG_INSTALL_DIR        "${CMAKE_INSTALL_DATADIR}/cmake/${PROJECT_NAME}" CACHE INTERNAL "")
set(NLOHMANN_JSON_INCLUDE_INSTALL_DIR       "${CMAKE_INSTALL_INCLUDEDIR}")
set(NLOHMANN_JSON_TARGETS_EXPORT_NAME       "${PROJECT_NAME}Targets")
set(NLOHMANN_JSON_CMAKE_CONFIG_TEMPLATE     "cmake/config.cmake.in")
set(NLOHMANN_JSON_CMAKE_CONFIG_DIR          "${CMAKE_CURRENT_BINARY_DIR}")
set(NLOHMANN_JSON_CMAKE_VERSION_CONFIG_FILE "${NLOHMANN_JSON_CMAKE_CONFIG_DIR}/${PROJECT_NAME}ConfigVersion.cmake")
set(NLOHMANN_JSON_CMAKE_PROJECT_CONFIG_FILE "${NLOHMANN_JSON_CMAKE_CONFIG_DIR}/${PROJECT_NAME}Config.cmake")
set(NLOHMANN_JSON_CMAKE_PROJECT_TARGETS_FILE "${NLOHMANN_JSON_CMAKE_CONFIG_DIR}/${PROJECT_NAME}Targets.cmake")
set(NLOHMANN_JSON_PKGCONFIG_INSTALL_DIR     "${CMAKE_INSTALL_DATADIR}/pkgconfig")

if (JSON_MultipleHeaders)
    set(NLOHMANN_JSON_INCLUDE_BUILD_DIR "${PROJECT_SOURCE_DIR}/include/")
    message(STATUS "Using the multi-header code from ${NLOHMANN_JSON_INCLUDE_BUILD_DIR}")
else()
    set(NLOHMANN_JSON_INCLUDE_BUILD_DIR "${PROJECT_SOURCE_DIR}/single_include/")
    message(STATUS "Using the single-header code from ${NLOHMANN_JSON_INCLUDE_BUILD_DIR}")
endif()

if (NOT JSON_ImplicitConversions)
    message(STATUS "Implicit conversions are disabled")
endif()

if (JSON_DisableEnumSerialization)
    message(STATUS "Enum integer serialization is disabled")
endif()

if (JSON_LegacyDiscardedValueComparison)
    message(STATUS "Legacy discarded value comparison enabled")
endif()

if (JSON_Diagnostics)
    message(STATUS "Diagnostics enabled")
endif()

if (JSON_SystemInclude)
    set(NLOHMANN_JSON_SYSTEM_INCLUDE "SYSTEM")
endif()

##
## TARGET
## create target and add include path
##
add_library(${NLOHMANN_JSON_TARGET_NAME} INTERFACE)
add_library(${PROJECT_NAME}::${NLOHMANN_JSON_TARGET_NAME} ALIAS ${NLOHMANN_JSON_TARGET_NAME})
if (${CMAKE_VERSION} VERSION_LESS "3.8.0")
    target_compile_features(${NLOHMANN_JSON_TARGET_NAME} INTERFACE cxx_range_for)
else()
    target_compile_features(${NLOHMANN_JSON_TARGET_NAME} INTERFACE cxx_std_11)
endif()

target_compile_definitions(
    ${NLOHMANN_JSON_TARGET_NAME}
    INTERFACE
    $<$>:JSON_USE_GLOBAL_UDLS=0>
    $<$>:JSON_USE_IMPLICIT_CONVERSIONS=0>
    $<$:JSON_DISABLE_ENUM_SERIALIZATION=1>
    $<$:JSON_DIAGNOSTICS=1>
    $<$:JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON=1>
)

target_include_directories(
    ${NLOHMANN_JSON_TARGET_NAME}
    ${NLOHMANN_JSON_SYSTEM_INCLUDE} INTERFACE
    $
    $
)

## add debug view definition file for msvc (natvis)
if (MSVC)
    set(NLOHMANN_ADD_NATVIS TRUE)
    set(NLOHMANN_NATVIS_FILE "nlohmann_json.natvis")
    target_sources(
        ${NLOHMANN_JSON_TARGET_NAME}
        INTERFACE
            $
            $
    )
endif()

# Install a pkg-config file, so other tools can find this.
CONFIGURE_FILE(
    "${CMAKE_CURRENT_SOURCE_DIR}/cmake/pkg-config.pc.in"
    "${CMAKE_CURRENT_BINARY_DIR}/${PROJECT_NAME}.pc"
)

##
## TESTS
## create and configure the unit test target
##
if (JSON_BuildTests)
    include(CTest)
    enable_testing()
    add_subdirectory(tests)
endif()

##
## INSTALL
## install header files, generate and install cmake config files for find_package()
##
include(CMakePackageConfigHelpers)
# use a custom package version config file instead of
# write_basic_package_version_file to ensure that it's architecture-independent
# https://github.com/nlohmann/json/issues/1697
configure_file(
    "cmake/nlohmann_jsonConfigVersion.cmake.in"
    ${NLOHMANN_JSON_CMAKE_VERSION_CONFIG_FILE}
    @ONLY
)
configure_file(
    ${NLOHMANN_JSON_CMAKE_CONFIG_TEMPLATE}
    ${NLOHMANN_JSON_CMAKE_PROJECT_CONFIG_FILE}
    @ONLY
)

if(JSON_Install)
    install(
        DIRECTORY ${NLOHMANN_JSON_INCLUDE_BUILD_DIR}
        DESTINATION ${NLOHMANN_JSON_INCLUDE_INSTALL_DIR}
    )
    install(
        FILES ${NLOHMANN_JSON_CMAKE_PROJECT_CONFIG_FILE} ${NLOHMANN_JSON_CMAKE_VERSION_CONFIG_FILE}
        DESTINATION ${NLOHMANN_JSON_CONFIG_INSTALL_DIR}
    )
    if (NLOHMANN_ADD_NATVIS)
        install(
            FILES ${NLOHMANN_NATVIS_FILE}
            DESTINATION .
    )
    endif()
    export(
        TARGETS ${NLOHMANN_JSON_TARGET_NAME}
        NAMESPACE ${PROJECT_NAME}::
        FILE ${NLOHMANN_JSON_CMAKE_PROJECT_TARGETS_FILE}
    )
    install(
        TARGETS ${NLOHMANN_JSON_TARGET_NAME}
        EXPORT ${NLOHMANN_JSON_TARGETS_EXPORT_NAME}
        INCLUDES DESTINATION ${NLOHMANN_JSON_INCLUDE_INSTALL_DIR}
    )
    install(
        EXPORT ${NLOHMANN_JSON_TARGETS_EXPORT_NAME}
        NAMESPACE ${PROJECT_NAME}::
        DESTINATION ${NLOHMANN_JSON_CONFIG_INSTALL_DIR}
    )
    install(
        FILES "${CMAKE_CURRENT_BINARY_DIR}/${PROJECT_NAME}.pc"
        DESTINATION ${NLOHMANN_JSON_PKGCONFIG_INSTALL_DIR}
    )
endif()

1. cmake_minimum_required(VERSION 3.1)   指定项目最低cmake版本为3.1

2. project(nlohmann_json VERSION 3.11.2 LANGUAGES CXX)  指定项目名称、版本号和编程语言

3. set(MAIN_PROJECT OFF)  设置变量 MAIN_PROJECT 为OFF

4. if (CMAKE_CURRENT_SOURCE_DIR STREQUAL CMAKE_SOURCE_DIR)  用于做一些选择性的事情

5. include(ExternalProject) 引入ExternalProject模块

6. option(JSON_CI   "Enable CI build targets." OFF)  设定选项开关是否打开或者关闭

7. message(STATUS "Using the multi-header code from ${NLOHMANN_JSON_INCLUDE_BUILD_DIR}")  打印message信息， 主要用于校验变量等操作

8. add_library(${NLOHMANN_JSON_TARGET_NAME} INTERFACE)  定义一个库并指定源文件，如果没有指定静态库或者动态库则默认静态库

9. target_include_directories 为目标添加头文件搜索路径