How can I convert a C++ vector to C-style array?

Question

I have a class which stores an array and I need to write a method to return a pointer to that array so that other objects can access/modify it.

In an old version of my program, I did that by defining the array in C style. Ie, having a private element bool* list and then allocating memory in the constructor (and liberating that in the destructor). The method then was very simple:

bool* MyClass::getList() {
    return list;
}

Now, I have decided to rewrite the code and use std::vector<bool> instead of the classical array. The problem is that I have also modified the above method as:

bool* MyClass::getList() {
    return &(list[0]);
}

which seems to be the standard way of converting a C++ vector to C array. However, I cannot compile my code, I get the following error:

error: taking address of temporary [-fpermissive]
error: cannot convert ‘std::vector<bool>::reference* {aka std::_Bit_reference*}’ to ‘bool*’ in return

Could anyone help me out with this and tell me what shall I do?

(I have also read that another alternative is to use list.data() as the pointer but this would work only with the newest version of C++ compilers. I am not sure if this is a good idea or not).

Thanks,

Answer 1

Your course of thinking is right in general and taking a pointer of the first element would have been sufficient. However, std::vector<bool> is not really a vector of bool s as it is being implemented as a bit set. There is a general consensus among the C++ Standard Committee and the Library Working Group that vector<bool> should be deprecated and subsequently removed from the standard library, while the functionality will be reintroduced under a different name. But for now it is still there.

Getting back to your problem, you cannot really interpret a bitset as array of booleans. Therefore, there are two possible ways to go about this:

1. Replace std::vector<bool> with one of the alternatives that would give you the right behavior.
2. Convert std::vector<bool> to a C array manually when you need it. For example, you can use size() to pre-allocate memory, then iterate over a vector and fill our your array.

Hope it helps. Good Luck!

Answer 2

bool* MyClass::getList() {
    return &(list[0]);
}

This will work with any type, but std::vector<bool> has special implementation, where 8 elements are stored in 1 byte, so it doesn't seem possible to take raw pointer to it.

Answer 3

try string.copy():

bool array[100];
std::vector<bool>;
std::copy(vector.begin(), vector.end(), array);

Answer 4

std::vector<bool> uses an internal model where each bool is stored as a single bit. This means it's not a plain C array internally as you would like and you can't convert it to one effortlessly.

Either replace std::vector<bool> with std::deque<bool> and don't use the deque parts, this will give the expected behaviour.

The other solution is to use a std::vector<char> instead and treat the values as bools. This will give the expected behaviour as long as you cast the values properly when accessing it.