CMake系列1 - 编译选项实现条件链接

本文中涉及到一个自定义库math_functions和一个主工程tutorial.cpp的组织。
如果定义了USE_MYMATH编译选项，就在编译时链接math_functions自定义库，如果没有定义这个选项，就不链接，使用系统给定的sqrt函数。
详细的文档如下，
Step 2: Adding a Library — CMake 3.23.0-rc1 Documentation
代码结构如下，

image.png

代码如下，
CMakeLists.txt

cmake_minimum_required(VERSION 3.10)

# 设置工程名称和版本
project(Tutorial VERSION 1.0)

# 指定C++标准
set(CMAKE_CXX_STANDARD 11)
set(CMAKE_CXX_STANDARD_REQUIRED true)

# 是否使用自定义的 math实现
option(USE_MYMATH "Use tutorial provided math implementation" ON)

# 配置生成header文件
configure_file(tutorial_config.h.in tutorial_config.h)

# 添加 math_functions 库
if(USE_MYMATH)
    add_subdirectory(math_functions)
    list(APPEND EXTRA_LIBS math_functions)
    list(APPEND EXTRA_INCLUDES "${PROJECT_SOURCE_DIR}/math_functions")
endif()

add_executable(Tutorial tutorial.cpp)

target_link_libraries(Tutorial PUBLIC ${EXTRA_LIBS})

target_include_directories(Tutorial PUBLIC "${PROJECT_BINARY_DIR}"
                            ${EXTRA_INCLUDES})

tutorial_config.h.in

// Tutorial的配置选项和设置
#define Tutorial_VERSION_MAJOR @Tutorial_VERSION_MAJOR@
#define Tutorial_VERSION_MINOR @Tutorial_VERSION_MINOR@
#cmakedefine USE_MYMATH

tutorial.cpp

#include "tutorial_config.h"

#ifdef USE_MYMATH
    #include "math_functions.h"
#endif

#include <cmath>
#include <iostream>
#include <string>


int main(int argc, char* argv[]) {
    if(argc < 2) {
        std::cout << argv[0] <<  " Version " << Tutorial_VERSION_MAJOR << "."
            << Tutorial_VERSION_MINOR << std::endl;
        std::cout << "Usage: " << argv[0] << " number" << std::endl;
        return EXIT_FAILURE;
    }

    double const input_val = std::stod(argv[1]);

#ifdef USE_MYMATH
    double const ouput_val = mysqrt(input_val);
#else
    double const ouput_val = sqrt(input_val);
#endif
    
    std::cout << "The sqrt root of " << input_val << " is: " << ouput_val << std::endl;
    return EXIT_SUCCESS;
}

math_functions/math_functions.h

#ifndef _FREDRIC_MATH_FUNCTIONS_H_
#define _FREDRIC_MATH_FUNCTIONS_H_

double mysqrt(double x);

#endif

math_functions/mysqrt.cpp

#include <iostream>

#include "math_functions.h"

// a hack square root calculation using simple operations
double mysqrt(double x) {
    if (x <= 0) {
        return 0;
    }

    double result = x;

    // do ten iterations
    for (int i = 0; i < 10; ++i) {
        if (result <= 0) {
            result = 0.1;
        }
        double delta = x - (result * result);
        result = result + 0.5 * delta / result;
        std::cout << "Computing sqrt of " << x << " to be " << result
                  << std::endl;
    }
    return result;
}

math_functions/CMakeLists.txt

add_library(math_functions mysqrt.cpp)

程序输出如下，

image.png