制作DLL

// calc.h
#ifdef EXPORT_DLL
#define CALC_API __declspec(dllexport)
#else
#define CALC_API __declspec(dllimport)
#endif

extern "C"
{
    CALC_API int 加(int a, int b);
    CALC_API int 减(int a, int b);
    CALC_API int 乘(int a, int b);
    CALC_API double 除(double a, double b);
}

// calc.cpp
#define EXPORT_DLL
#include "calc.h"

extern "C"
{
    CALC_API int 加(int a, int b)
    {
        return a + b;
    }

    CALC_API int 减(int a, int b)
    {
        return a - b;
    }

    CALC_API int 乘(int a, int b)
    {
        return a * b;
    }

    CALC_API double 除(double a, double b)
    {
        return a / b;
    }
}

// CMakeLists.txt
cmake_minimum_required(VERSION 3.10)

project(calc)

set(CMAKE_CXX_STANDARD 11)

if(MSVC)
    add_compile_options("/source-charset:utf-8")
endif()

include_directories(./include)

if(MSVC)
    add_compile_options("/source-charset:utf-8")
endif()

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

add_library(${PROJECT_NAME} SHARED ${SOURCES})

使用DLL

// useCalc.cpp
#include <iostream>
#include "calc.h"
using namespace std;

int main()
{
    cout << 加(3, 4) << endl;
}

// CMakeLists.txt
cmake_minimum_required(VERSION 3.10)

project(useCalc)

set(CMAKE_CXX_STANDARD 11)

if(MSVC)
    add_compile_options("/source-charset:utf-8")
endif()

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

include_directories(include)

add_executable(${PROJECT_NAME} ${SOURCES})

target_link_libraries(${PROJECT_NAME} calc)