引入在C++程式中我們經常見到如下，兩個問題：

1. 輸出當前程式的版本號

2. 通過cmake新增macro巨集定義 出入到原始檔，例如在編譯opencv/caffe的時候，我們通過cmake -DUSE_CUDA=ON/OFF來確定,是否使用cuda.

問題cmake中如何實現上述兩個功能的呢？

至於問題1：

我們可能想到直接在標頭檔案中定義，這樣也可以，但是如果直接在cmake中定義，然後傳遞給程式讀取生成更加靈活。

至於問題2：

可以在cmake中通過option函式以及add_definitions函式來實現。

cmake 傳遞版本號給原始檔

首先定義demo11Config.h.in標頭檔案接受CMakeLists.txt傳遞來的版本號引數，檔案內容如下：

// the configured options and settings for demo11
#define demo11_VERSION_MAJOR @[email protected]
#define demo11_VERSION_MINOR @[email protected]

demo11.cpp程式碼如下：

#include <stdio.h>
// demoConfig.h檔案是cmake生成的
#include "demoConfig.h"
 


int main(int argc, char* argv[]){

    printf("%s version: %d.%d \n", argv[0], demo11_VERSION_MAJOR, demo11_VERSION_MINOR);
}

注意的是demoConfig.h是生成的在專案結構中並不存在，專案目錄如下:

CMakeLists.txt如下：

cmake_minimum_required(VERSION 3.5)

project(demo11)

# The version number
set(demo11_VERSION_MAJOR 1)
set(demo11_VERSION_MINOR 0
 
)

# configure a header file to pass some of the CMake settings to
# source code(demo11Config.h.in), the file demoConfig.h does't exists 
configure_file(
        "${PROJECT_SOURCE_DIR}/demo11Config.h.in"
        "${PROJECT_BINARY_DIR}/demoConfig.h"
)

# add the binary tree to the search path for include files
# so that we will find demoConfig.h
include_directories("${PROJECT_BINARY_DIR}")

# add the exectuable
add_executable(demo11 demo11.cpp)

cmake 編譯如下：

root@xy:~/cmake_practice/cmake_build/build_demo11# cmake ../../cmake_tuorial/demo11/
-- The C compiler identification is GNU 5.4.0

可以看到生產了demoConfig.h：

drwxr-xr-x  3 root root  4096 Jun 29 08:42 ./
........
-rw-r--r--  1 root root   112 Jun 29 08:42 demoConfig.h
-rw-r--r--  1 root root  4865 Jun 29 08:42 Makefile

demoConfig.h內容如下：

// the configured options and settings for demo11
#define demo11_VERSION_MAJOR 1
#define demo11_VERSION_MINOR 0

make & run,輸出如下：

root@xy:~/cmake_practice/cmake_build/build_demo11# ./demo11
./demo11 version: 1.0

新增macro到src檔案中(add_definitions/option)

本例項在cmake中新增macro定義到原始檔中，從而選擇使用不同的lib 模組函式.

專案目錄如下：demo12.cpp根據cmake定義的macro USE_SPECIFIED決定使用模組SpecifiedFun(特定的函式)，還是模組CommonFun(一般的函式).

兩個模組都只有一個函式showSelectedFun，主要是輸出呼叫了哪個函式.
模組CommonFun,內容如下：

void showSelectedFun(){
    std::cout<<"Select to use common function"<<std::endl;
}

對應的cmake如下：

add_library(common_fun CommonFun.cpp)

模組SpecifiedFun,內容如下：

void showSelectedFun(){
    std::cout<<"Select to use specified function"<<std::endl;
}

對應的cmake如下：

add_library(specified_fun SpecifiedFun.cpp)

主CMakeLists.txt內容如下, 主要是通過option USE_SPECIFIED ON/OFF來決定使用新增macro add_definitions(-DUSE_SPECIFIED)：

cmake_minimum_required(VERSION 3.5)

# should we use specified function
# if not set, then will use common function
option(USE_SPECIFIED "select to use specified function" ON)

# select to use specified function, or common function
message("USE_SPECIFIED is ${USE_SPECIFIED}")

if(USE_SPECIFIED)
    add_definitions(-DUSE_SPECIFIED)
    include_directories("${PROJECT_SOURCE_DIR}/SpecifiedFun")
    add_subdirectory(SpecifiedFun)
    set(EXTRA_LIBS ${EXTRA_LIBS} specified_fun)
    message("select to use specified function, lib is: ${EXTRA_LIBS}")
else()
    include_directories("${PROJECT_SOURCE_DIR}/CommonFun")
    add_subdirectory(CommonFun)
    set(EXTRA_LIBS ${EXTRA_LIBS} common_fun)
    message("select to use common function, lib is: ${EXTRA_LIBS}")
endif(USE_SPECIFIED)

add_executable(demo12 demo12.cpp)
target_link_libraries(demo12 ${EXTRA_LIBS})

首先編譯不使用USE_SPECIFIED即 USE_SPECIFIED=OFF

root@xy:~/tmp/demo12/build# cmake -DUSE_SPECIFIED=OFF ..
..........
USE_SPECIFIED is OFF
select to use common function, lib is: common_fun
-- Configuring done
-- Generating done
-- Build files have been written to: /home/xy/tmp/demo12/build

編譯make:

root@xy:~/tmp/demo12/build# make 
Scanning dependencies of target common_fun
[ 25%] Building CXX object CommonFun/CMakeFiles/common_fun.dir/CommonFun.cpp.o
[ 50%] Linking CXX static library libcommon_fun.a
[ 50%] Built target common_fun
Scanning dependencies of target demo12
[ 75%] Building CXX object CMakeFiles/demo12.dir/demo12.cpp.o
[100%] Linking CXX executable demo12
[100%] Built target demo12

執行：

root@xy:~/tmp/demo12/build# ./demo12 
Select to use common function

使用macro USE_SPECIFIED直接使用cmake .. 即可
接下來的步驟一樣就不重複了