MPC Edit Menu compilation error when compiling for multi-hotends

[zeleps]

Description

Compiling the latest bugfix with MPC_EDIT_MENU, multiple hotends, produces the following errors:

Marlin\src\lcd\menu\menu_advanced.cpp:355:13: error: 'c' is not captured
Marlin\src\lcd\menu\menu_advanced.cpp:369:61: error: 'mpc_edit_hotend' is not captured

Lambdas need byref capturing to use variables c / mpc_edit_hotend, but capturing lambdas have a different signature and are not convertible to screenFunc_t type. Converted mpc_edit_hotend to a static function and created a static index to the currently edited hotend in order to fix the issue.

I'm not very experienced with C++ lambdas, if there's another way to fix this, I'd love to see it.

Related Items

PR #23984

[tombrazier]

This code is thinkyhead's improvement on what I wrote. I suspect mpc_edit_hotend should actually be a #define rather than a function. Menus are very clever but they still leave me a bit confused. What happens if you replace mpc_edit_hotend with

#define MPC_EDIT_HOTEND(N) \
  MPC_EDIT_DEFS(N); \
  START_MENU(); \
  BACK_ITEM(MSG_TEMPERATURE); \
  MPC_EDIT_ITEMS(N); \
  END_MENU();

and then use

#define MPC_ENTRY(N) SUBMENU_N(N, MSG_MPC_EDIT, []{ MPC_EDIT_HOTEND(MenuItemBase::itemIndex); });

[zeleps]

I'm getting this now:

Marlin\src\lcd\menu\menu_advanced.cpp:373:1: error: macro "SUBMENU_N_P" passed 4 arguments, but takes just 3

It seems there is an issue with the expansion of macros START_MENU() and END_MENU() inside the lambda body. Still, when I remove these, I'm getting local variable not captured errors.

[thinkyhead]

The only change needed is to use the same default capture for the outer lambda as the inner lambda.

[zeleps]

I tried that first, along with line 354, doesn't compile:

Marlin\src\lcd\menu\menu_item.h:275:40: error: cannot convert 'menu_advanced_temperature()::<lambda()>' to 'screenFunc_t' {aka 'void (*)()'}

[thinkyhead]

What happens if you replace mpc_edit_hotend with…

You end up with a distinct function for each hotend, instead of just one function that takes the hotend index.

[zeleps]

@thinkyhead, this still doesn't compile:

Marlin\src\lcd\menu\menu_advanced.cpp:355:54: error: 'c' is not captured
Marlin\src\lcd\menu\menu_item.h:275:40: error: cannot convert 'menu_advanced_temperature()::<lambda()>' to 'screenFunc_t' {aka 'void (*)()'}

[tombrazier]

Confirmed. I get the same compile error with MPC_EDIT_MENU and HAS_MULTI_HOTEND.

[thisiskeithb]

@zeleps: You'll need to submit a new PR since this one is already merged.

[thinkyhead]

Missing capture? Usually you can just change []{ ... } to [&]{ ... } and that solves it. But, that may not work for local autos.

- EDIT_ITEM_FAST_N(float43, N, MSG_MPC_AMBIENT_XFER_COEFF_FAN_E, &editable.decimal, 0, 1, []{ \
+ EDIT_ITEM_FAST_N(float43, N, MSG_MPC_AMBIENT_XFER_COEFF_FAN_E, &editable.decimal, 0, 1, [&]{ \

[zeleps]

Missing capture? Usually you can just change []{ ... } to [&]{ ... } and that solves it. But, that may not work for local autos.

- EDIT_ITEM_FAST_N(float43, N, MSG_MPC_AMBIENT_XFER_COEFF_FAN_E, &editable.decimal, 0, 1, []{ \
+ EDIT_ITEM_FAST_N(float43, N, MSG_MPC_AMBIENT_XFER_COEFF_FAN_E, &editable.decimal, 0, 1, [&]{ \

Of course I tried that (I mention it in the PR description), but it doesn't work:

Marlin\src\lcd\menu\menu_item.h:275:40: error: cannot convert 'menu_advanced_temperature()::<lambda()>' to 'screenFunc_t' {aka 'void (*)()'}

If I use [&] anywhere, the signature becomes incompatible with the ::action() method (explicitly casting it also fails). If I leave it as is, capture issues occur.

I did a lot of different tests before ending up with the static function as a last resort.

Configuration-wise, enable MPC edit menu and #define HAS_MULTI_HOTEND 1 at some point before the function (e.g. @256). This commit demonstrates the problem.