Using C API under C++: error: taking address of rvalue

[GrauBlitz]

I am currently trying to use the C-API of flecs in an C++17 project to have full control over it (i dont want to use the C++ API).

I initiate my world with the following code:

ecs_world_t *world = ecs_init();
ecs_singleton_set(world, EcsUnit, {0});
ecs_singleton_set(world, EcsRest, {0});
ecs_singleton_set(world, EcsMonitor, {0});

But my compiler (GCC 12) gives multiple errors in this line, maybe the usage of {0} is only in C supported and not in C++ but i can't find any alternative...

project\src\prog\esc\esc.cpp: In function 'int esc::init()':
project\src\prog\esc\esc.cpp:10:39: error: taking address of rvalue [-fpermissive]
   10 |     ecs_singleton_set(world, EcsUnit, {0});
      |                                       ^~~
project/extern/flecs/include/flecs.h:8544:81: note: in definition of macro 'ecs_set'
 8544 |     ecs_set_id(world, entity, ecs_id(component), sizeof(component), &(component)__VA_ARGS__)
      |                                                                                 ^~~~~~~~~~~
project\src\prog\esc\esc.cpp:10:5: note: in expansion of macro 'ecs_singleton_set'
   10 |     ecs_singleton_set(world, EcsUnit, {0});
      |     ^~~~~~~~~~~~~~~~~
project\src\prog\esc\esc.cpp:11:39: error: taking address of rvalue [-fpermissive]
   11 |     ecs_singleton_set(world, EcsRest, {0});
      |                                       ^~~
project/extern/flecs/include/flecs.h:8544:81: note: in definition of macro 'ecs_set'
 8544 |     ecs_set_id(world, entity, ecs_id(component), sizeof(component), &(component)__VA_ARGS__)
      |                                                                                 ^~~~~~~~~~~
project\src\prog\esc\esc.cpp:11:5: note: in expansion of macro 'ecs_singleton_set'
   11 |     ecs_singleton_set(world, EcsRest, {0});
      |     ^~~~~~~~~~~~~~~~~
project/extern/flecs/include/flecs.h:626:19: error: 'FLECS__EEcsMonitor' was not declared in this scope; did you mean 'FLECS__EFlecsMonitor'?
  626 | #define ecs_id(T) FLECS__E##T
      |                   ^~~~~~~~
project/extern/flecs/include/flecs.h:8544:23: note: in definition of macro 'ecs_set'
 8544 |     ecs_set_id(world, entity, ecs_id(component), sizeof(component), &(component)__VA_ARGS__)
      |                       ^~~~~~
project/extern/flecs/include/flecs.h:8644:20: note: in expansion of macro 'ecs_id'
 8644 |     ecs_set(world, ecs_id(comp), comp, __VA_ARGS__)
      |                    ^~~~~~
project\src\prog\esc\esc.cpp:12:5: note: in expansion of macro 'ecs_singleton_set'
   12 |     ecs_singleton_set(world, EcsMonitor, {0});
      |     ^~~~~~~~~~~~~~~~~
project\src\prog\esc\esc.cpp:12:42: error: expected ')' before '{' token
   12 |     ecs_singleton_set(world, EcsMonitor, {0});
      |                                          ^
project/extern/flecs/include/flecs.h:8544:81: note: in definition of macro 'ecs_set'
 8544 |     ecs_set_id(world, entity, ecs_id(component), sizeof(component), &(component)__VA_ARGS__)
      |                                                                                 ^~~~~~~~~~~
project\src\prog\esc\esc.cpp:12:5: note: in expansion of macro 'ecs_singleton_set'
   12 |     ecs_singleton_set(world, EcsMonitor, {0});
      |     ^~~~~~~~~~~~~~~~~
project/extern/flecs/include/flecs.h:8544:15: note: to match this '('
 8544 |     ecs_set_id(world, entity, ecs_id(component), sizeof(component), &(component)__VA_ARGS__)
      |               ^
project/extern/flecs/include/flecs.h:8644:5: note: in expansion of macro 'ecs_set'
 8644 |     ecs_set(world, ecs_id(comp), comp, __VA_ARGS__)
      |     ^~~~~~~
project\src\prog\esc\esc.cpp:12:5: note: in expansion of macro 'ecs_singleton_set'
   12 |     ecs_singleton_set(world, EcsMonitor, {0});
      |     ^~~~~~~~~~~~~~~~~

[SanderMertens]

The reason this happens is because ecs_set is part of a set of macro helpers that provide a sort of type safety for the C API, but they rely on features that are not available in C++ (such as taking the address of a temporary).

If you'd really like to use the C API from C++, consider using the "naked" C functions in the include/flecs.h file, like ecs_set_id:
https://github.com/SanderMertens/flecs/blob/master/include/flecs.h#L2529

It's a bit more typing, but you'll be using plain C functions:

// Initialize component to 0
EcsRest r = {};

// ecs_id(EcsRest) returns the name of the global variable that holds the component id for EcsRest
// A singleton is a component that's assigned to itself, which is why entity and id are both ecs_id(EcsRest)
ecs_set_id(world, ecs_id(EcsRest), ecs_id(EcsRest), sizeof(EcsRest), &r);

[GrauBlitz]

That did solve it, i now decided to switch to the C++ API to remove some extern C markings and to better integrate it.

Thanks for the help!

[SanderMertens]

You're welcome!