Initializing an object as class member resulting in linking error?

Question

I'm a little dumbfounded as to why this isn't working. I keep getting the following linker error:

In file included from src\ac.cpp:1:0:
src/ac.h:9:21: error: expected identifier before numeric constant
 #define RECEIVE_PIN 0
                     ^
src/ac.h:14:26: note: in expansion of macro 'RECEIVE_PIN'
     static IRrecv irrecv(RECEIVE_PIN);
                          ^
src/ac.h:9:21: error: expected ',' or '...' before numeric constant
 #define RECEIVE_PIN 0
                     ^
src/ac.h:14:26: note: in expansion of macro 'RECEIVE_PIN'
     static IRrecv irrecv(RECEIVE_PIN);
                          ^
*** [.pio\build\esp32cam\src\ac.cpp.o] Error 1
*** [.pio\build\esp32cam\src\main.cpp.o] Error 1

I am simply trying to initialize the IR receiver with static IRrecv irrecv(RECEIVE_PIN);, but the compiler is refusing to link it together.

#ifndef _AC_H_
#define _AC_H_
#include <Arduino.h>
#include <IRremoteESP8266.h>
#include <IRrecv.h>
#include <IRutils.h>
#define RECEIVE_PIN 0
class IR
{
public:
    static IRrecv irrecv(RECEIVE_PIN);
    static decode_results results;
    void initRecv();
    bool waitForIR();
};
extern IR ir;
#endif

If I do static IRrecv irrecv; it successfully compiles, but I'm unsure if I can pass the pin variable after the fact.

score 2 · Accepted Answer · answered Jan 02 '22 at 18:04

Static data members have to be defined outside the class declaration. The initialization happens at the point where the members are defined:

file ac.h:

//...
class IR
{
public:
    // Static data members are declared only:
    static IRrecv irrecv;
    static decode_results results;
    void initRecv();
    bool waitForIR();
};

file ac.cpp:

#include "ac.h"
#define RECEIVE_PIN 0
// Definitions and initializations of static data members:
IRrecv IR::irrecv(RECEIVE_PIN);
decode_results IR::results;

Initializing an object as class member resulting in linking error?

1 Answers1