Not able to instanciate class

Question

I am a beginner in C++. So please bear with me:

I try to write code for a temperature sensor which should post values to an API.

I am not able to instantiate my class ApiClient. I always get these errors:

• IDE: call of an object of a class type without appropriate operator() or conversion functions to pointer-to-function type
• IDE: no instance of overloaded function "ApiClient::ApiClient" matches the specified type
• COMPILER: no match for call to '(ApiClient) (String&)'

My code looks like this (slightly slimmed for easier reading):

main.cpp

#include <ApiClient.h>
ApiClient api;

void setup()
{
  Serial.begin(115200);
  String apiUrl = "https://myapi.com/api";
  api(apiUrl); // ERROR: raises call of an object of a class type without appropriate operator() or conversion functions to pointer-to-function type

}

void loop()
{

  String temp = "49";
  api.sendValue("temperature", temp);
}

ApiClient.h

#ifndef WebClient_h
#define WebClient_h

#include <Arduino.h>
#include <WiFi.h>
#include <esp_wps.h>
#include <HTTPClient.h>

class ApiClient
{
public:
  ApiClient(); // this is only added for testing
  ApiClient(String apiUrl); // it should always instanciate with url
  void sendValue(String key, String value);
  String _apiUrl;
private:
};

#endif

ApiClient.cpp

#include <ApiClient.h>

String _apiUrl;

HTTPClient http;

ApiClient::ApiClient(String apiUrl) // IDE says: "no instance of overloaded function "ApiClient::ApiClient" matches the specified type"
{
  this->_apiUrl = apiUrl;
  // some stuff to establish a connection
}

Answer 1

You are attempting to call a constructor on an already constructed object.

This line fails:

api(apiUrl);

The call you are attempting fails because the compiler is looking for one of these functions:

return_type APIClient::operator()(const String&);
return_type APIClient::operator()(String); 
return_type APIClient::operator()(String&);

Your statement should be a declaration if you want your constructor to be called.

ApiClient api(apiUrl);
// or better in c++11:
APIClient api{ apiUrl };

Alternatively, you could create an initializer in ApiClient, that takes a single String as a parameter, something like this:

class ApiClient
{
public:
    ApiClient() { ...}
    ApiClient(const String& s) { Initialize(s); }
    Initialize(const String& s) { do the work }

    ...
};

Be careful when providing both one-step and two steps initialization APIs in your classes. You'll need a proper reset function to avoid mix-ups when reinitializing with a new string.