std::vector 的 SINGLE BIT 上的按位运算<bool> . 为什么会出错？

Question

I have come up with a set of rules to transform one set of bits into another.

For these to work, I basically need to xor a certain number of bits in the source set of bits (say, result[i]=source[foo(i)]^source[bar(i)] , where foo and bar are bounds-checked). Since I want to be able to change the size of the set, I decided to go with a std::vector<bool> .

Thus, I end up with:

int foo(int i);
int bar(int i);
void baz(std::vector<bool> in, std::vector<bool>& out){
   out.clear();
   for(int i=0;i<in.size();i++){
    if(foo(i)>0 && foo(i)<in.size())
      out[i]^=in[foo(i)]
    if(bar(i)>0 && bar(i)<in.size())
      out[i]^=in[bar(i)]
   }
}

However, this gives me an error:

No viable overloaded ^=

What can I do in order to be able to do this stuff?

Answer 1

std::vector<bool> is messy in the best case and an abomination in the worst. Its operator[] does not return a bool& but rather a proxy object ( see here ). This proxy does not have overloads vor all possible bitwise operations, hence you have to use ifs, its operator=(bool) or its flip() .

EDIT: I just read your code and not your description what you want to do. Setting the xor of two bits is no problem at all:

out[i] = in[j] ^ in[k]; // Whatever the right indices are.

I think what you actually want to do is along the lines of:

bool tmp = false;
if(foo(i)>0 && foo(i)<in.size())
 tmp^=in[foo(i)];
if(bar(i)>0 && bar(i)<in.size())
 out[i]=tmp^in[bar(i)];

That is, if you want to return in[bar(i)]^in[foo(i)] if both pass the bounds check, return in[j] if only one (here denoted by j ) passes the bounds check and false if none of them pass it.

Answer 2

std::vector<bool> is a special container. It's like a std::vector<int> , but the standard allows for std::vector<bool> to pack its elements into single bits which means it returns a proxy object when you use operator[] . That object doesn't have an overload for operator ^= so you can't use it. What you can do is write out the long form of ^= like

out[i] = out[i] ^ in[foo(i)]

Answer 3

std::vector<bool>::operator[] returns a proxy class. You can see the documentation on cppreference .

You can use static_cast<bool> so get an actual bool value and use that in your XOR.

out[i] = static_cast<bool>(out[i]) ^ in[foo(i)];

Edit: Or as pointed out by Max, simply rewriting it will do an implicit conversion so the cast is redundant.

out[i] = out[i] ^ in[foo(i)];