Insert vector data as bytea in postgres using libpqxx

Question

I have a vector which needs to be saved as bytea type in Postgres.

I have seen solutions using binarystring for doing that. But binary string is now deprecated in the latest versions (I am using 7.7.5).

std::vector<int> params_;
params_.push_back(156);
params_.push_back(256*256);

// Creating basic_string for saving data to DB 
std::basic_string<std::byte>a {
            reinterpret_cast<std::byte const *>(std::data(md.params_)), 
2*sizeof(int)};

// Converting back to vector<int>, during reads 
for (i = 0; i < std::size(a); i += 1) {
            int const  z{int(a.at(i))};
            std::cout << "int at: " << i << " " << z << '\n';
            std::cout << "bits at: " << i << " " << a.at(i) << '\n';
        }

Response for the code is:

int at: 0 156
bits at: 0 10011100
int at: 1 0
bits at: 1 00000000
int at: 2 0
bits at: 2 00000000
int at: 3 0
bits at: 3 00000000
int at: 4 0
bits at: 4 00000000
int at: 5 0
bits at: 5 00000000
int at: 6 1
bits at: 6 00000001
int at: 7 0
bits at: 7 00000000

I tried using basic_string (mentioned in docs), But the byte information shown in the above response is bit odd.

For the 156 value in the vector, the byte information at index 0 should be at index 3 and the same goes for another element (some endianess issue).

Is there a sample code for storing bytea data using basic_string or any other way?

Is there anything mistake in the above code?

Answer 1

You should transform the vector before taking its data. Since you are talking about the byte order, the chosen type int is not suitable. It's also unclear, why std::basic_string<std::byte> instead of std::vector<std::byte> is chosen.

std::vector<unsigned int> params_;
params_.push_back(156);
params_.push_back(256*256);

std::transform(params_.begin(), params_.end(), params_.begin(), [](unsigned int v) { return htonl(v); });

// Creating basic_string for saving data to DB 
std::basic_string<std::byte>a {
            reinterpret_cast<std::byte const *>(std::data(md.params_)), 
2*sizeof(int)};

// Converting back to vector<int>, during reads 
for (i = 0; i < std::size(a); i += 1) {
    int const  z{int(a.at(i))};
    std::cout << "int at: " << i << " " << z << '\n';
    std::cout << "bits at: " << i << " " << a.at(i) << '\n';
}