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#include <cstring>
#include <iostream>
#include <type_traits>
 
// TODO: static_assert to detect alignment errors.
 
template<typename T, typename U>
struct binary_concat_t {
    enum { total_size = sizeof(T) + sizeof(U) };
 
    binary_concat_t(const T & t, const U & u) : t(t), u(u) {}
 
    const char * data() const { return reinterpret_cast<const char*>(this); }
 
    T t;
    U u;
};
 
template<typename T, typename U>
binary_concat_t<T, U> binary_concat(const T & t, const U & u) {
    static_assert(std::is_pod<binary_concat_t<T, U>>::value, "Type must be POD!");
    return binary_concat_t<T, U>(t, u);
}
 
template<typename T, typename U, typename V>
binary_concat_t<binary_concat_t<T, U>, V> binary_concat(const T & t, const U & u, const V & v) {
    return binary_concat(binary_concat(t, u), v);
}
 
struct IPv4Header {
    char data[17];
};
 
struct TCPHeader {
    char data[20];
};
 
struct HTTPHeader {
    char data[23];
};
 
int main() {
    IPv4Header ipv4;
    TCPHeader tcp;
    HTTPHeader http;
    auto data = binary_concat(ipv4, tcp, http);
 
    std::cout << "sizeof(IPv4Header) = " << sizeof(IPv4Header) << std::endl;
    std::cout << "sizeof(TCPHeader) = " << sizeof(TCPHeader) << std::endl;
    std::cout << "sizeof(HTTPHeader) = " << sizeof(HTTPHeader) << std::endl;
    std::cout << "sizeof(data) = " << sizeof(data) << std::endl;
}

prog.cpp: In function 'binary_concat_t<T, U> binary_concat(const T&, const U&) [with T = IPv4Header, U = TCPHeader]':
prog.cpp:27:48:   instantiated from 'binary_concat_t<binary_concat_t<T, U>, V> binary_concat(const T&, const U&, const V&) [with T = IPv4Header, U = TCPHeader, V = HTTPHeader]'
prog.cpp:46:46:   instantiated from here
prog.cpp:21:5: error: static assertion failed: "Type must be POD!"
prog.cpp: In function 'binary_concat_t<T, U> binary_concat(const T&, const U&) [with T = binary_concat_t<IPv4Header, TCPHeader>, U = HTTPHeader]':
prog.cpp:27:48:   instantiated from 'binary_concat_t<binary_concat_t<T, U>, V> binary_concat(const T&, const U&, const V&) [with T = IPv4Header, U = TCPHeader, V = HTTPHeader]'
prog.cpp:46:46:   instantiated from here
prog.cpp:21:5: error: static assertion failed: "Type must be POD!"

Standard output is empty