Question

Is using bitset::operator[] equivalent to using bitset::test or is there some underlying optimization?

That is, are these two loops equivalent?

Using bitset::operator[]:

static const int UP = 0;
static const int DOWN = 1;

for(int i = 1; i < KEY_MAX; ++i) {
    if(_handler && (_prevKey[i] == UP && _curKey[i] == DOWN)) {
        _handler->EnqueueEvent(new KeyboardKeyDownEvent(i));
    }
    if(_handler && (_prevKey[i] == DOWN && _curKey[i] == DOWN)) {
        _handler->EnqueueEvent(new KeyboardKeyPressEvent(i));
    }
    if(_handler && (_prevKey[i] == DOWN && _curKey[i] == UP)) {
        _handler->EnqueueEvent(new KeyboardKeyUpEvent(i));
    }
}

Using bitset::test():

static const bool UP = false;
static const bool  DOWN = true;

for(int i = 1; i < KEY_MAX; ++i) {
    if(_handler && (_prevKey.test(i) == UP && _curKey.test(i) == DOWN)) {
        _handler->EnqueueEvent(new KeyboardKeyDownEvent(i));
    }
    if(_handler && (_prevKey.test(i) == DOWN && _curKey.test(i) == DOWN)) {
        _handler->EnqueueEvent(new KeyboardKeyPressEvent(i));
    }
    if(_handler && (_prevKey.test(i) == DOWN && _curKey.test(i) == UP)) {
        _handler->EnqueueEvent(new KeyboardKeyUpEvent(i));
    }
}

Answer 1

From the C++03 standard, §23.3.5.2/39-41:

bool test(size_t pos) const;

Requires: pos is valid
Throws: out_of_range if pos does not correspond to a valid bit position.
Returns: true if the bit at position pos in *this has the value one.

§23.3.5.2/46-48:

bool operator[](size_t pos) const;

Requires: pos is valid.
Throws: nothing.
Returns: test(pos).

§23.3.5.2/49-51:

bitset<N>::reference operator[](size_t pos);

Requires: pos is valid.
Throws: nothing.
Returns: An object of type bitset<N>::reference such that (*this)[pos] == this-
test(pos), and such that (*this)[pos] = val is equivalent to this->set(pos, val).

So when the object is const, they return the same value, excepting that when pos is invalid test throws out_of_range while operator[] throws nothing. When the object is not const, the operator returns a proxy object allowing one to mutate the object’s data.