How to pass a pointer to vector of pointers? - c++

Question

This is a C++ version of a duplicate question. I know the pointers to vectors a bit much in the following, but I did this in order to duplicate a much larger project. The members of info_a are not properly printing from within print_b and I have been unable to properly pass the vector of pointers.
With what I have tried, the members of info_a are not properly printing from within print_b. The first element is fine, but the next two are not.
The structs and print_b are from a third-party api and I am trying to pass in what they are expecting.
Here's the working code ...
http://coliru.stacked-crooked.com/a/c3ad6af6da9409a5
Does anyone see where I am going wrong?
typedef struct {
uint16_t a1;
uint8_t a2;
uint8_t a3;
} info_a;
typedef struct {
uint16_t id;
unsigned int arr_sz;
info_a *arr;
} info_b;
void print_a(const info_a* a)
{
using namespace std;
cout <<
"a->a1 0x" << hex << setfill('0') << setw(4) << a->a1 << std::endl <<
"a->a2 0x" << hex << setfill('0') << setw(2) << a->a2 << std::endl <<
"a->a3 0x" << hex << setfill('0') << setw(2) << a->a3 << std::endl;
}
void print_b(const info_b* b)
{
using namespace std;
cout << "b->id 0x" << hex << setfill('0') << setw(4) << b->id << endl
<< "b->arr_sz " << hex << setfill('0') << setw(2) << b->arr_sz << endl;
for (unsigned int i = 0; i < b->arr_sz; ++i) {
const info_a *elem = &(b->arr[i]);
print_a(elem);
}
}
int main(int argc, char **argv)
{
std::shared_ptr<std::vector<std::shared_ptr<info_a>>> sp_info_a =
std::make_shared<std::vector<std::shared_ptr<info_a>>>();
std::shared_ptr<std::vector<std::shared_ptr<info_b>>> sp_info_b =
std::make_shared<std::vector<std::shared_ptr<info_b>>>();
int offset = 0;
for (uint16_t i = 1; i <= 1; ++i) {
for (uint16_t j = 1; j <= 3; ++j) {
std::shared_ptr<info_a> a_info =
std::make_shared<info_a>(info_a { j, 0x31, 0x32 } );
sp_info_a->push_back(a_info);
}
std::shared_ptr<info_b> b_info = std::make_shared<info_b>(
info_b {
static_cast<uint16_t>(i),
static_cast<unsigned int>(sp_info_a->size()),
(*sp_info_a)[offset].get()
});
sp_info_b->push_back(b_info);
print_b(b_info.get());
}
return 0;
}

Answer 1

It doesn't work because you interpret an array of std::shared_ptr<info_a> as an array of info_a. This will work:
int main(int argc, char **argv)
{
std::shared_ptr<std::vector<info_a>> sp_info_a =
std::make_shared<std::vector<info_a>>();
std::shared_ptr<std::vector<std::shared_ptr<info_b>>> sp_info_b =
std::make_shared<std::vector<std::shared_ptr<info_b>>>();
int offset = 0;
for (uint16_t i = 1; i <= 1; ++i) {
for (uint16_t j = 1; j <= 3; ++j) {
sp_info_a->push_back(info_a { j, 0x31, 0x32 });
}
std::shared_ptr<info_b> b_info = std::make_shared<info_b>(
info_b {
static_cast<uint16_t>(i),
static_cast<unsigned int>(sp_info_a->size()),
sp_info_a->data() + offset
});
sp_info_b->push_back(b_info);
print_b(b_info.get());
}
return 0;
}

Answer 2

I have been unable to properly pass the vector of pointers
This is the core of the problem. The C API you're calling does not expect a vector of pointers, it expects a vector of structs.
In particular, this bit ...
unsigned int arr_sz;
info_a *arr;
... expects to find arr_sz structs in memory, one after the other, starting from arr.
Your code allocates all structs separately, then stores the address of the first one in arr. The printing code then walks off the end of arr[0], expecting to find the next struct, but as you've allocated them separately, they're stored who knows where in memory.
In order to fix this, whatever you do, you must create the equivalent of a std::vector<info_a> (which you can then wrap an info_b around, as a sort of array view). This is what gives you the guarantee that the structs are actually adjacent in memory.