bitwise manipulation via operator[] - c++11

Question

I'm writing a simple bitset wrapper to easily and efficiently set, clear and read bits from an 8-bit integer. I would do these three operations via operator[], but I'm stucked, and honestly I'm not sure it is possible without losing performances (really important for my purposes).
#include <stdint.h>
#include <iostream>
class BitMask {
private:
uint8_t mask;
public:
inline void set(int bit) { mask |= 1 << bit; } // deprecated
inline void clear(int bit) { mask &= ~(1 << bit); } // deprecated
inline int read(int bit) { return (mask >> bit) & 1; } // deprecated
bool operator[](int bit) const { return (mask >> bit) & 1; } // new way to read
??? // new way to write
friend std::ostream& operator<<(std::ostream& os, const BitMask& bitmask) {
for (int bit = 0; bit < 8; ++bit)
os << ((bitmask.mask >> bit) & 1 ? "1" : "0");
return os;
}
};
int main() {
BitMask bitmask1;
bitmask1.set(3);
bitmask1.clear(3);
bitmask1.read(3);
std::cout << bitmask1;
BitMask bitmask2;
bitmask2[3] = 1; // set
bitmask2[3] = 0; // clear
bitmask2[3]; // read
std::cout << bitmask2;
}
Any idea?

Answer 1

One way (the only way?) is to return a proxy object from your operator[] which will hold index for your bit, this way you will assign new value to your proxy object which will alter appropriate BitMask bit. For example see here: Vector, proxy class and dot operator in C++
As for a performance - it all depends how compiler will optimize your code, if your proxy class would have only inline methods then it should be fast.
Below is example how to fix your code:
#include <stdint.h>
#include <iostream>
class BitMask {
private:
uint8_t mask;
public:
inline void set(int bit) { mask |= 1 << bit; } // deprecated
inline void clear(int bit) { mask &= ~(1 << bit); } // deprecated
inline int read(int bit) const { return (mask >> bit) & 1; } // deprecated
struct proxy_bit
{
BitMask& bitmask;
int index;
proxy_bit(BitMask& p_bitmask, int p_index) : bitmask(p_bitmask), index(p_index) {}
proxy_bit& operator=(int rhs) {
if (rhs)
bitmask.set(index);
else
bitmask.clear(index);
return *this;
}
operator int() {
return bitmask.read(index);
}
};
proxy_bit operator[](int bit) { return proxy_bit(*this, bit); } // new way to read
int operator[](int bit) const { return read(bit); } // new way to read
friend std::ostream& operator<<(std::ostream& os, const BitMask& bitmask) {
for (int bit = 0; bit < 8; ++bit)
os << ((bitmask.mask >> bit) & 1 ? "1" : "0");
return os;
}
};
int main() {
BitMask bitmask1;
bitmask1.set(3);
bitmask1.clear(3);
bitmask1.read(3);
std::cout << bitmask1 << std::endl;
BitMask bitmask2;
bitmask2[3] = 1; // set
bitmask2[3] = 0; // clear
bitmask2[3]; // read
std::cout << bitmask2 << std::endl;
const BitMask bitmask3;
if (bitmask3[3]) {}
//bitmask3[3] = 1; // compile error - OK!
}