How to include a c++ library in a c project using Cmake

Question

I'm a begginer on c, c++ and Cmake.

I've started from a project that allows me to send the data to Google Cloud IoT Core which is written in C: https://github.com/espressif/esp-google-iot/blob/master/examples/smart_outlet/README.md

As you can see I have a board with an ESP32 chip.

I bought an humidity sensor which I would like to use and the libraries are written in c++, so adding them to the C project has being very difficult. ( https://github.com/adafruit/DHT-sensor-library ) ( https://github.com/adafruit/Adafruit_Sensor )

I'm importing the libraries like this in my CMakeLists.txt:

cmake_minimum_required(VERSION 3.5)

include($ENV{IDF_PATH}/tools/cmake/project.cmake)

# This is some google iot librares that are being imported in the project
set (EXTRA_COMPONENT_DIRS "../..")


# Libraries in cpp that are not being compiled correctly
INCLUDE_DIRECTORIES(lib/Adafruit_Sensor)
LINK_DIRECTORIES(lib/Adafruit_Sensor)
INCLUDE_DIRECTORIES(lib/DHT-sensor-library)
LINK_DIRECTORIES(lib/DHT-sensor-library)

project(smart_outlet)

But I'm getting an error message when compiling the c++ libraries.

../lib/DHT-sensor-library/DHT_U.h:45:1: error: unknown type name 'class'

I think that the error is thrown because c++ classes are not supported on c.

This is the c++.h file that si throwing me the error:

#ifndef DHT_U_H
#define DHT_U_H

#include <Adafruit_Sensor.h>
#include <DHT.h>

#define DHT_SENSOR_VERSION 1 /**< Sensor Version */

/*!
 *  @brief  Class that stores state and functions for interacting with
 * DHT_Unified.
 */
class DHT_Unified {
public:
  DHT_Unified(uint8_t pin, uint8_t type, uint8_t count = 6,
              int32_t tempSensorId = -1, int32_t humiditySensorId = -1);
  void begin();

  /*!
   *  @brief  Class that stores state and functions about Temperature
   */
  class Temperature : public Adafruit_Sensor {
  public:
    Temperature(DHT_Unified *parent, int32_t id);
    bool getEvent(sensors_event_t *event);
    void getSensor(sensor_t *sensor);

  private:
    DHT_Unified *_parent;
    int32_t _id;
  };

  /*!
   *  @brief  Class that stores state and functions about Humidity
   */
  class Humidity : public Adafruit_Sensor {
  public:
    Humidity(DHT_Unified *parent, int32_t id);
    bool getEvent(sensors_event_t *event);
    void getSensor(sensor_t *sensor);

  private:
    DHT_Unified *_parent;
    int32_t _id;
  };

  /*!
   *  @brief  Returns temperature stored in _temp
   *  @return Temperature value
   */
  Temperature temperature() { return _temp; }

  /*!
   *  @brief  Returns humidity stored in _humidity
   *  @return Humidity value
   */
  Humidity humidity() { return _humidity; }

private:
  DHT _dht;
  uint8_t _type;
  Temperature _temp;
  Humidity _humidity;

  void setName(sensor_t *sensor);
  void setMinDelay(sensor_t *sensor);
};

#endif

As you can see in the github repos, the files are with.cpp and.h extensions.

The main project has a dependency of a google library which is written in embedded c and allows to communicate with Google Cloud, which hasn't been a problem so far: https://github.com/GoogleCloudPlatform/iot-device-sdk-embedded-c

Could anyone plis enlight me on how to do this?

Many thanks!

Answer 1

Yeah, my brain hurt too when trying to figure out how to link vs libraries via Cmake.

This is how I did it:

add_executable(exe_name main.cpp)

find_library(FLTK fltk /home/user/proj/fltk/fltk/lib)
find_library(FLTK_IMG fltk_images /home/user/proj/fltk/fltk/lib)

target_link_libraries(exe_name LINK_PUBLIC ${FLTK} ${FLTK_IMG} ${LINK_FLAGS})

So for you I'm thinking you need a find_library but you have to specify the library name:

find_library(AF_SENSOR Adafruit_Sensor lib)
find_library(DHT_SENSOR DHT_sensor-library lib)
target_link_libraries(exe_name LINK_PUBLIC ${AF_SENSOR} ${DHT_SENSOR})

I'm guessing that "Adafruit_Sensor" is the actual.lib file, etc. You may need the full path for the 3rd argument to find_library()...