Weird memory issues when inheriting from small vector

[stertingen]

Consider the following snippet: (https://godbolt.org/z/ax7TqGWTz)

#include <boost/container/small_vector.hpp>
#include <cstdint>
#include <iostream>
#include <iomanip>

// inspired by can_frame
struct frame {
    std::uint32_t header;
    std::uint8_t len;
    std::uint8_t data[8];
};

// Contains data from multiple frames
struct S : boost::container::small_vector<std::uint8_t, 8> {
    S() : my_header(0) {}

    S(const frame& f)
        : boost::container::small_vector<std::uint8_t, 8>(f.data, f.data + f.len),
          my_header(f.header) {}

    std::uint32_t my_header;
};

int main() {
    frame f1;
    f1.header = 0xAAAAAAAA;
    f1.len = 8; // 8 bytes of data are filled
    f1.data[0] = 0x01;
    f1.data[1] = 0x23;
    f1.data[2] = 0x45;
    f1.data[3] = 0x67;
    f1.data[4] = 0x89;
    f1.data[5] = 0xAB;
    f1.data[6] = 0xCD;
    f1.data[7] = 0xEF;

    S s(f1); // convert frame into S

    std::cout << std::hex << "header: " << s.my_header << "\n";
    for (std::uint8_t c : s) {
        std::cout << unsigned(c) << " ";
    }
}

I do not expect payload data to be overwritten by the header.

When having small_vector as data member instead of base class, the issue does not occur.

Is it illegal to derive from small_vector?

[igaztanaga]

Hi,

The existing implementation was UB. Compilers can use the unused alignment bits in the base class to store new members. This was reported as a bug (https://github.com/boostorg/container/issues/218) and the implementation changed with the newly released Boost 1.80. Could you please try your code with Boost 1.80?

[stertingen]

Looks good: https://godbolt.org/z/Mx3cz1GnT (Showing Boost 1.79 vs. Boost 1.80)