Calling it from C
Firstly we need to declare to C our result returning C++ function:
// Declare our C++ function's returning result type. Only needs to be done once.
// This declares an `status_result<size_t, system_code>` which is an alias to
// `basic_result<size_t, status_code<erased<intptr_t>>>`.
//
// The first parameter is some unique identifier for this type which will be used
// whenever we reference this type in the future.
CXX_DECLARE_RESULT_SYSTEM(to_string_rettype, size_t)
// Tell C about our extern C++ function `to_string()`
extern CXX_RESULT_SYSTEM(to_string_rettype) _Z9to_stringPcmi(char *buffer, size_t bufferlen, int v);
Now let’s call the C++ function from C:
void print(int v)
{
char buffer[4];
CXX_RESULT_SYSTEM(to_string_rettype) res;
res = _Z9to_stringPcmi(buffer, sizeof(buffer), v);
if(CXX_RESULT_HAS_VALUE(res))
{
printf("to_string(%d) fills buffer with '%s' of %zu characters\n", v, buffer, res.value);
return;
}
// Is the error returned in the POSIX domain and thus an errno?
if(CXX_RESULT_ERROR_IS_ERRNO(res))
{
fprintf(stderr, "to_string(%d) failed with error code %d (%s)\n", v, (int) res.error.value, strerror((int) res.error.value));
return;
}
fprintf(stderr, "to_string(%d) failed with unknown error code %lld\n", v, (long long) res.error.value);
}
int main(void)
{
print(9);
print(99);
print(999);
print(9999);
return 0;
}
Running this C program yields:
to_string(9) fills buffer with '9' of 1 characters
to_string(99) fills buffer with '99' of 2 characters
to_string(999) fills buffer with '999' of 3 characters
to_string(9999) failed with error code 105 (No buffer space available)