Why Doesn't `std::vector::reference` Return a `bool&`?

Why vector::reference Doesn't Return Reference to Bool?

In C , the std::vector container is specialized for boolean values. This specialization introduces differences in behavior compared to vectors of other data types.

The Issue

In the provided example, a function callIfToggled is used to toggle a bool value and call a method on a struct. When attempting to pass a reference to an element of a std::vector to callIfToggled, a compilation error occurs. This is because the reference type returned by vector::reference is not the expected bool&.

The Reason

Within the bool vector specialization, vector::reference returns a reference to a char, not a bool. This is done to optimize performance by using a bitwise representation for Boolean values.

Solutions

• Use vector Instead: Replace the bool vector with a vector to work around the specialization issue.
• Use Boost Containers Library: The Boost Containers library provides an unspecialized version of vector that offers a true reference to bool values.
• Wrap Bool Values in a Structure: Create a wrapper struct around bool called MyBool. This allows passing a reference to the wrapper, which in turn provides indirect access to the underlying bool value.

Example (Vector of Char):

#include <vector>

struct A
{
    void foo() {}
};

template<typename T>
void callIfToggled(char v1, char &v2, T &t)
{
    if (v1 != v2)
    {
        v2 = v1;
        t.foo();
    }
}

int main()
{
    std::vector<char> v = { false, true, false };

    const char f = false;
    A a;

    callIfToggled(f, v[0], a);
    callIfToggled(f, v[1], a);
    callIfToggled(f, v[2], a);
}

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