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#include <cstdint>
#include <iostream>

// Only intended for storage, automatically promotes to 64-bit for evaluation
struct Int40
{
    Int40(int64_t x) { set(static_cast<uint64_t>(x)); } // implicit constructor
    operator int64_t() const { return get(); } // implicit conversion to 64-bit
private:
    void set(uint64_t x)
    {
        setb<0>(x); setb<1>(x); setb<2>(x); setb<3>(x); setb<4>(x);
    };
    int64_t get() const
    {
        return static_cast<int64_t>(getb<0>() | getb<1>() | getb<2>() | getb<3>() | getb<4>() | signx());
    };
    uint64_t signx() const
    {
        return (data[4] >> 7) * (uint64_t(((1 << 25) - 1)) << 39);
    };
    template <int idx> uint64_t getb() const
    {
        return static_cast<uint64_t>(data[idx]) << (8 * idx);
    }
    template <int idx> void setb(uint64_t x)
    {
        data[idx] = (x >> (8 * idx)) & 0xFF;
    }
  
    unsigned char data[5];
};

int main()
{
    Int40 a = -1;
    Int40 b = -2;
    Int40 c = 1 << 16;
    std::cout << "sizeof(Int40) = " << sizeof(Int40) << std::endl;
    std::cout << a << "+" << b << "=" << (a+b) << std::endl;
    std::cout << c << "*" << c << "=" << (c*c) << std::endl;
}

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Compilation time: 0.62 sec, absolute running time: 0.14 sec, cpu time: 0 sec, memory peak: 3 Mb, absolute service time: 0.77 sec