Problems with BLE Mesh in a C++ project.

[AF1]

I am using NCS Connect SDK version 2.6.0

I am trying to create an array of models and found a problem with using Zephry macros in C++ projects. My main.cpp is as below

#include <zephyr/bluetooth/bluetooth.h>
#include <zephyr/bluetooth/mesh.h>
#include <zephyr/sys/printk.h>

int main(void)
{
        printk("Hello Worlds! %s\n", CONFIG_BOARD);
       
       static const struct bt_mesh_model models[] =
       {
            BT_MESH_MODEL_CFG_SRV,
       };
        
        return 0;
}

BT_MESH_MODEL_CFG_SRV is defined in cfg_srv.h as

#define BT_MESH_MODEL_CFG_SRV	\
		BT_MESH_MODEL_CNT_CB(BT_MESH_MODEL_ID_CFG_SRV,	\
			     bt_mesh_cfg_srv_op, NULL,	\
			     NULL, 1, 0, &bt_mesh_cfg_srv_cb)

BT_MESH_MODEL_CNT_CB is defined in access.h

/**
 *  @brief Composition data SIG model entry with callback functions
 *	   with specific number of keys & groups.
 *
 * This macro uses compound literal feature of C99 standard and thus is available only from C,
 * not C++.
 *
 *  @param _id        Model ID.
 *  @param _op        Array of model opcode handlers.
 *  @param _pub       Model publish parameters.
 *  @param _user_data User data for the model.
 *  @param _keys      Number of keys that can be bound to the model.
 *		      Shall not exceed @kconfig{CONFIG_BT_MESH_MODEL_KEY_COUNT}.
 *  @param _grps      Number of addresses that the model can be subscribed to.
 *		      Shall not exceed @kconfig{CONFIG_BT_MESH_MODEL_GROUP_COUNT}.
 *  @param _cb        Callback structure, or NULL to keep no callbacks.
 */
#define BT_MESH_MODEL_CNT_CB(_id, _op, _pub, _user_data, _keys, _grps, _cb)	\
{										\
	.id = (_id),								\
	.pub = _pub,								\
	.keys = (uint16_t []) BT_MESH_MODEL_KEYS_UNUSED(_keys),			\
	.keys_cnt = _keys,							\
	.groups = (uint16_t []) BT_MESH_MODEL_GROUPS_UNASSIGNED(_grps),		\
	.groups_cnt = _grps,							\
	BT_MESH_MODEL_UUIDS_UNASSIGNED()					\
	.op = _op,								\
	.cb = _cb,								\
	.user_data = _user_data,						\
}

Which is where the problem lies. The macro is defined for C language only and cannot be used with a C++ compiler as it uses compound literals.

Does anyone have any suggestion to resolve this impase

Appreciate any suggestions.

Thanks

[JONATHAN LL]

I have asked for this to be looked at infernally so that we can maybe try and find a more C++ friendly approached. 

Regards,
Jonathan

[AF1]

Any updates on my ticket??

[JONATHAN LL]

No update as of now, will ping internally to try and rush things along

[JONATHAN LL]

There has been a discussion about this before it seems, ref: Bluetooth: Mesh: Make elements as const by LingaoM · Pull Request #57215 · zephyrproject-rtos/zephyr · GitHub 

C++ syntax doesn't support compound literals. It is possible to use GNU extension to enable it for C++ but it has different semantic (https://gcc.gnu.org/onlinedocs/gcc/Compound-Literals.html).

As a consequence of mentioned above discussion the macros for C++ excluded : https://github.com/zephyrproject-rtos/zephyr/pull/58311

It is not possible to use them for C++ as is. So either customize them for C++ or find out another approach. 

What could be proposed is: 

We have already this issue as keys and groups are also initialized as compound literals. We propose surrounding this problematic defines with #ifndef __cplusplus. C++ users can have a C-file (compiled with a C-compiler) with these macros to define variables, and access them in C++ code using extern.

Regards,
Jonathan

[JONATHAN LL]

Adding some extra comment for clearity:

Yes it is only supported in C. The suggestion for anyone who wants to use .cpp is to create a .c file where they instantiate composition data and expose pointer to that composition data through a header file. Then use this pointer in main.cpp. Or, simpler, move the entire mesh initialization to a .c file and call some init function from main.cpp.