# Weird C++ error cannot bind rvalue reference of type

**URL:** <https://forum.arduino.cc/t/weird-c-error-cannot-bind-rvalue-reference-of-type/1443954>\
**Category:** Programming\
**Created:** [May 14, 2026, 1:27am UTC](https://forum.arduino.cc/t/weird-c-error-cannot-bind-rvalue-reference-of-type/1443954 "2026-05-14T01:27:20Z")\
**Posts on this page:** 8\
**Page:** 1

<div class="post-metadata">

**Author:** ![westfw](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/westfw/32/213443_2.png) [@westfw](https://forum.arduino.cc/u/westfw)\
**Post date:** [May 14, 2026, 1:27am UTC](https://forum.arduino.cc/t/weird-c-error-cannot-bind-rvalue-reference-of-type/1443954/1 "2026-05-14T01:27:20Z")

</div>

Ok, you people who actally know C++:

I have this trivial program for the Adafruit Feather M4 CAN board (a SAME51):

```arduino
Oscctrl *osc = OSCCTRL;
void setup() {
  OSCCTRL_EVCTRL_Type evc = osc->EVCTRL;
}

void loop() {}

```

OSCCTRL is defined off in the CMSIS-Atmel .h files (inside "extern "C""):

```arduino
#define OSCCTRL ((Oscctrl *)0x40001000UL) /**< \brief (OSCCTRL) APB Base Address */

```

And also the `Oscctrl` type:

```arduino
typedef struct {
  __IO OSCCTRL_EVCTRL_Type EVCTRL; /**< \brief Offset: 0x00 (R/W 8) Event Control */
       RoReg8 Reserved1[0x3];
  __IO OSCCTRL_INTENCLR_Type INTENCLR; /**< \brief Offset: 0x04 (R/W 32) Interrupt Enable Clear */
  __IO OSCCTRL_INTENSET_Type INTENSET; /**< \brief Offset: 0x08 (R/W 32) Interrupt Enable Set */
  __IO OSCCTRL_INTFLAG_Type INTFLAG; /**< \brief Offset: 0x0C (R/W 32) Interrupt Flag Status and Clear */
  __I OSCCTRL_STATUS_Type STATUS; /**< \brief Offset: 0x10 (R/ 32) Status */
  __IO OSCCTRL_XOSCCTRL_Type XOSCCTRL[2]; /**< \brief Offset: 0x14 (R/W 32) External Multipurpose Crystal Oscillator Control */
  __IO OSCCTRL_DFLLCTRLA_Type DFLLCTRLA; /**< \brief Offset: 0x1C (R/W 8) DFLL48M Control A */
       RoReg8 Reserved2[0x3];
  __IO OSCCTRL_DFLLCTRLB_Type DFLLCTRLB; /**< \brief Offset: 0x20 (R/W 8) DFLL48M Control B */
       RoReg8 Reserved3[0x3];
  __IO OSCCTRL_DFLLVAL_Type DFLLVAL; /**< \brief Offset: 0x24 (R/W 32) DFLL48M Value */
  __IO OSCCTRL_DFLLMUL_Type DFLLMUL; /**< \brief Offset: 0x28 (R/W 32) DFLL48M Multiplier */
  __IO OSCCTRL_DFLLSYNC_Type DFLLSYNC; /**< \brief Offset: 0x2C (R/W 8) DFLL48M Synchronization */
       RoReg8 Reserved4[0x3];
       OscctrlDpll Dpll[2]; /**< \brief Offset: 0x30 OscctrlDpll groups [DPLLS_NUM] */
} Oscctrl;

```

and OSCCTRL\_EVCTRL\_Type:

```arduino
typedef union {
  struct {
    uint8_t CFDEO0:1; /*!< bit: 0 Clock 0 Failure Detector Event Output Enable */
    uint8_t CFDEO1:1; /*!< bit: 1 Clock 1 Failure Detector Event Output Enable */
    uint8_t :6; /*!< bit: 2.. 7 Reserved */
  } bit; /*!< Structure used for bit access */
  struct {
    uint8_t CFDEO:2; /*!< bit: 0.. 1 Clock x Failure Detector Event Output Enable */
    uint8_t :6; /*!< bit: 2.. 7 Reserved */
  } vec; /*!< Structure used for vec access */
  uint8_t reg; /*!< Type used for register access */
} OSCCTRL_EVCTRL_Type;

```

And I get:

```arduino
sketch_may13a:4:33: error: no matching function for call to 'OSCCTRL_EVCTRL_Type(volatile OSCCTRL_EVCTRL_Type&)'
    4 | OSCCTRL_EVCTRL_Type evc = osc->EVCTRL;
      | ^ ~~~~~
In file included from /Applications/Arduino-1.8.19.app/Contents/Java/portable/packages/adafruit/tools/CMSIS-Atmel/1.2.2/CMSIS/Device/ATMEL/same51/include/same51j19a.h:576,
                 from /Applications/Arduino-1.8.19.app/Contents/Java/portable/packages/adafruit/tools/CMSIS-Atmel/1.2.2/CMSIS/Device/ATMEL/same.h:60,
                 from /Applications/Arduino-1.8.19.app/Contents/Java/portable/packages/adafruit/tools/CMSIS-Atmel/1.2.2/CMSIS/Device/ATMEL/sam.h:561,
                 from /Applications/Arduino-1.8.19.app/Contents/Java/portable/packages/adafruit/hardware/samd/1.7.16/cores/arduino/Arduino.h:48,
                 from /tmp/Arduino1.8.19Build/sketch/sketch_may13a.ino.cpp:1:
/Applications/Arduino-1.8.19.app/Contents/Java/portable/packages/adafruit/tools/CMSIS-Atmel/1.2.2/CMSIS/Device/ATMEL/same51/include/component/oscctrl.h:55:3: note: candidate: 'constexpr OSCCTRL_EVCTRL_Type::OSCCTRL_EVCTRL_Type(const OSCCTRL_EVCTRL_Type&)' <near match>
   55 | } OSCCTRL_EVCTRL_Type;
      | ^ ~~~~~~~~~~~~~~~~~~
/Applications/Arduino-1.8.19.app/Contents/Java/portable/packages/adafruit/tools/CMSIS-Atmel/1.2.2/CMSIS/Device/ATMEL/same51/include/component/oscctrl.h:55:3: note: conversion of argument 1 would be ill-formed:
sketch_may13a:4:33: error: binding reference of type 'const OSCCTRL_EVCTRL_Type&' to 'volatile OSCCTRL_EVCTRL_Type' discards qualifiers
    4 | OSCCTRL_EVCTRL_Type evc = osc->EVCTRL;
      | ~~~~~^~~~~~
In file included from /Applications/Arduino-1.8.19.app/Contents/Java/portable/packages/adafruit/tools/CMSIS-Atmel/1.2.2/CMSIS/Device/ATMEL/same51/include/same51j19a.h:576,
                 from /Applications/Arduino-1.8.19.app/Contents/Java/portable/packages/adafruit/tools/CMSIS-Atmel/1.2.2/CMSIS/Device/ATMEL/same.h:60,
                 from /Applications/Arduino-1.8.19.app/Contents/Java/portable/packages/adafruit/tools/CMSIS-Atmel/1.2.2/CMSIS/Device/ATMEL/sam.h:561,
                 from /Applications/Arduino-1.8.19.app/Contents/Java/portable/packages/adafruit/hardware/samd/1.7.16/cores/arduino/Arduino.h:48,
                 from /tmp/Arduino1.8.19Build/sketch/sketch_may13a.ino.cpp:1:
/Applications/Arduino-1.8.19.app/Contents/Java/portable/packages/adafruit/tools/CMSIS-Atmel/1.2.2/CMSIS/Device/ATMEL/same51/include/component/oscctrl.h:55:3: note: candidate: 'constexpr OSCCTRL_EVCTRL_Type::OSCCTRL_EVCTRL_Type(OSCCTRL_EVCTRL_Type&&)' <near match>
   55 | } OSCCTRL_EVCTRL_Type;
      | ^ ~~~~~~~~~~~~~~~~~~
/Applications/Arduino-1.8.19.app/Contents/Java/portable/packages/adafruit/tools/CMSIS-Atmel/1.2.2/CMSIS/Device/ATMEL/same51/include/component/oscctrl.h:55:3: note: conversion of argument 1 would be ill-formed:
sketch_may13a:4:33: error: cannot bind rvalue reference of type 'OSCCTRL_EVCTRL_Type&&' to lvalue of type 'volatile OSCCTRL_EVCTRL_Type'
    4 | OSCCTRL_EVCTRL_Type evc = osc->EVCTRL;
      | ~~~~~^~~~~~
exit status 1
no matching function for call to 'OSCCTRL_EVCTRL_Type(volatile OSCCTRL_EVCTRL_Type&)'

```

Should be a union assigned from a volatile union, right? I don't mind discarding the "volatile" aspect (I was the current value), but what's the rest of the error mean, and how do I fix it? Why does it think the rvalue and lvalues are C++ "references" (and why don't they match?)

(The code is accepted in a .c file, BTW.)

---

<div class="post-metadata">

**Author:** ![van\_der\_decken](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/van_der_decken/32/1006976_2.png) [@van\_der\_decken](https://forum.arduino.cc/u/van_der_decken)\
**Post date:** [May 14, 2026, 2:00am UTC](https://forum.arduino.cc/t/weird-c-error-cannot-bind-rvalue-reference-of-type/1443954/2 "2026-05-14T02:00:31Z")

</div>

I think it's because the usual C++ compiler generated copy constructor doesn't include `volatile`, only `const`.

You could try making a reference to it rather than a copy.

```arduino
__IO OSCCTRL_EVCTRL_Type &evc = osc->EVCTRL;

```

Dollars to donuts `__IO` is `volatile`.

Edit: and here's the whys and wherefores:

> <https://stackoverflow.com/questions/17217300/why-am-i-not-provided-with-a-default-copy-constructor-from-a-volatile>

---

<div class="post-metadata">

**Author:** ![gfvalvo](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/gfvalvo/32/1013044_2.png) [@gfvalvo](https://forum.arduino.cc/u/gfvalvo)\
**Post date:** [May 14, 2026, 2:44am UTC](https://forum.arduino.cc/t/weird-c-error-cannot-bind-rvalue-reference-of-type/1443954/3 "2026-05-14T02:44:59Z")

</div>

> [@westfw](#):
>
> (The code is accepted in a .c file, BTW.)

Then I'd put it in a function in a .c file and call it from your C++ using "extern "C"" to tell your C++ code that the function has 'C' linkage.

---

<div class="post-metadata">

**Author:** ![van\_der\_decken](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/van_der_decken/32/1006976_2.png) [@van\_der\_decken](https://forum.arduino.cc/u/van_der_decken)\
**Post date:** [May 14, 2026, 2:45am UTC](https://forum.arduino.cc/t/weird-c-error-cannot-bind-rvalue-reference-of-type/1443954/4 "2026-05-14T02:45:28Z")

</div>

Try this:

```arduino
typedef union {
   unsigned char a;
   signed char b;
} Chars;

typedef struct {
   char c;
   volatile Chars d;
} Struct;

Struct s;
Struct *p = &s;

void setup() {
   Chars e = p->d;
}

void loop() {}

```

You'll see the same errors you're getting. Take the `volatile` off the `Chars` member and it'll sail through. You'll also get the same errors with `Chars e = s.d;`.

---

<div class="post-metadata">

**Author:** ![westfw](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/westfw/32/213443_2.png) [@westfw](https://forum.arduino.cc/u/westfw)\
**Post date:** [May 14, 2026, 6:55am UTC](https://forum.arduino.cc/t/weird-c-error-cannot-bind-rvalue-reference-of-type/1443954/5 "2026-05-14T06:55:56Z")

</div>

> [@van\_der\_decken](#):
>
> I think it's because the usual C++ compiler generated copy constructor doesn't include `volatile`, only `const`.

Yes \_\_IO is `volatile`  
So why does it work OK for simple types, like `uint8_t reg = (osc->EVCTRL.reg);`

Does this mean that there's no way to have a local variable of a volatile bitfield struct in C++? (maybe that's why CMSIS doesn't like such unions?)

I want to do something like:

```arduino
void print_evctrl(OSCCTRL_EVCTRL_Type evctrl) {
  printf("EVCTRL (Event Control) @ 0x00\n");
  printf(" ├─ CFDEO0 (Clock 0 Failure Detector Event Output): %s\n",
         evctrl.bit.CFDEO0 ? "ENABLED" : "DISABLED");
  printf(" └─ CFDEO1 (Clock 1 Failure Detector Event Output): %s\n",
         evctrl.bit.CFDEO1 ? "ENABLED" : "DISABLED");
  printf(" Raw: 0x%02X\n\n", evctrl.reg);
}

```

---

<div class="post-metadata">

**Author:** ![gfvalvo](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/gfvalvo/32/1013044_2.png) [@gfvalvo](https://forum.arduino.cc/u/gfvalvo)\
**Post date:** [May 14, 2026, 2:04pm UTC](https://forum.arduino.cc/t/weird-c-error-cannot-bind-rvalue-reference-of-type/1443954/6 "2026-05-14T14:04:46Z")

</div>

> [@westfw](#):
>
> Does this mean that there's no way to have a local variable of a volatile bitfield struct in C++? (maybe that's why CMSIS doesn't like such unions?)

Modifying @van_der_decken's example code, the below compiles without error. But remember type punning via a Union is not allowed in C++. That's probably why this SAMD51 core stuff (and the SAMD21 core stuff before it) is written in C, not C++.

```arduino
typedef union {
  unsigned char a;
  signed char b;
} Chars;

typedef struct {
  char c;
  volatile Chars d;
} Struct;

Struct s;
Struct *p = &s;

void setup() {
  Chars e = * (const_cast<Chars *> (&(p->d)));
}

void loop() {}

```

---

<div class="post-metadata">

**Author:** ![gfvalvo](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/gfvalvo/32/1013044_2.png) [@gfvalvo](https://forum.arduino.cc/u/gfvalvo)\
**Post date:** [May 14, 2026, 2:13pm UTC](https://forum.arduino.cc/t/weird-c-error-cannot-bind-rvalue-reference-of-type/1443954/7 "2026-05-14T14:13:43Z")

</div>

> [@gfvalvo](#):
>
> Modifying @van_der_decken's example code, the below compiles without error.

And here's the modified @westfw code that compiles (but is untested):

```arduino
Oscctrl *osc = OSCCTRL;
void setup() {
  OSCCTRL_EVCTRL_Type evc = *(const_cast<OSCCTRL_EVCTRL_Type *>(&(osc->EVCTRL)));
}

void loop() {}

```

---

<div class="post-metadata">

**Author:** ![van\_der\_decken](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/van_der_decken/32/1006976_2.png) [@van\_der\_decken](https://forum.arduino.cc/u/van_der_decken)\
**Post date:** [May 14, 2026, 2:26pm UTC](https://forum.arduino.cc/t/weird-c-error-cannot-bind-rvalue-reference-of-type/1443954/8 "2026-05-14T14:26:59Z")

</div>

> [@westfw](#):
>
> So why does it work OK for simple types

I imagine because the compiler doesn't have to generate a copy constructor for simple types.