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The specialisation of std::vector<bool>, as specified in C++11 23.3.7/1, doesn't declare a data() member (e.g. mentioned here and here).
The question is: Why does a std::vector<bool> have no .data()? This is the very same question as why is a vector of bools not stored contiguously in memory. What are the benefits in not doing so?
Why can a pointer to an array of bools not be returned?
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vector data() function in C++ STL The std::vector::data() is an STL in C++ which returns a direct pointer to the memory array used internally by the vector to store its owned elements.
The vector<bool> class is a partial specialization of vector for elements of type bool . It has an allocator for the underlying type that's used by the specialization, which provides space optimization by storing one bool value per bit.
Vectors are the same as dynamic arrays with the ability to resize itself automatically when an element is inserted or deleted, with their storage being handled automatically by the container. Vector elements are placed in contiguous storage so that they can be accessed and traversed using iterators.
C++ is a statically-typed language. A vector will hold an object of a single type, and only a single type.
Why does a std::vector have no .data()?
Because std::vector<bool> stores multiple values in 1 byte.
Think about it like a compressed storage system, where every boolean value needs 1 bit. So, instead of having one element per memory block (one element per array cell), the memory layout may look like this:
Assuming that you want to index a block to get a value, how would you use operator []? It can't return bool& (since it will return one byte, which stores more than one bools), thus you couldn't assign a bool* to it. In other words bool *bool_ptr =&v[0]; is not valid code, and would result in a compilation error.
Moreover, a correct implementation might not have that specialization and don't do the memory optimization (compression). So data() would have to copy to the expected return type depending of implementation (or standard should force optimization instead of just allowing it).
Why can a pointer to an array of bools not be returned?
Because std::vector<bool> is not stored as an array of bools, thus no pointer can be returned in a straightforward way. It could do that by copying the data to an array and return that array, but it's a design choice not to do that (if they did, I would think that this works as the data() for all containers, which would be misleading).
What are the benefits in not doing so?
Memory optimization.
Usually 8 times less memory usage, since it stores multiple bits in a single byte. To be exact, CHAR_BIT times less.
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