returning bool& from vector

Question

In the following code:

class SomeClass {
    vector<int> i;
    vector<bool> b;

public:
    int& geti() {return i[0];}
    bool& getb() {return b[0];}
};

If you comment out getb(), the code compiles fine. Apparently there's no problem returning a reference to an int that's stored in a vector, but you can't do it with a bool.

Why is this?

Taking into account existing replies, you can still "salvage" your code by using `std::vector::reference` as the return type for your function, instead of the explicit `bool&`. `std::vector::reference` would be that proxy class that acts as a generalized reference for packed `std::vector`. — AnT stands with Russia, Jul 02 '12 at 03:15

score 11 · Accepted Answer · answered Jul 02 '12 at 02:32

11

std::vector<bool> is "special." It stores its elements as a bit array, meaning that the elements are not individually addressable and you cannot obtain a reference to an element.

std::vector<bool> iterators, its operator[], and its other member functions return proxy objects that provide access to the elements without requiring actual bool objects to be stored.

If you need to be able to access individual elements, consider using a std::vector<char> or defining a bool-like enumeration backed by a char (or a signed char or unsigned char, if you care about signedness).

answered Jul 02 '12 at 02:32

James McNellis

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+1 Nice catch... That didn't even cross my mind. I spent a min. thinking about references to locals and stuff... – Mysticial Jul 02 '12 at 02:34
That's incredibly irritating. Thanks for the answer. – Michael Dorst Jul 02 '12 at 02:38
IIRC `std::vector` is deprecated and shouldn't be used. – Jesse Good Jul 02 '12 at 02:46
@JesseGood: `std::vector` is not deprecated. – James McNellis Jul 02 '12 at 02:47
2

Okay, after googling it, it seems it was proposed, but the C++ standard committee shot it down. – Jesse Good Jul 02 '12 at 02:49

konjac · Answer 2 · 2012-07-02T02:49:56.570

vector<bool> is a special class template specialization for the bool type.

This specialization is provided to optimize for space allocation: eight bool elements are combined to one byte and each bool element occupies only one bit.

Reference to one bit in certain one byte is not allowed.

So the function could not return the reference to the a bool type in vector<bool>.

Someone also thinks vector<bool> is not a container.

You can use deque<bool> instead.

returning bool& from vector

2 Answers2