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

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

Consider the following example:

#include <vector>

struct A
{
    void foo() {}
};

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

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

    const bool f = false;
    A a;

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

This code fails to compile with the following error:

dk2.cpp:29:28: error: no matching function for call to 'callIfToggled(const bool&, std::vector<bool>::reference, A&amp;)'

The issue arises because 'std::vector' is a specialized template for the 'bool' type.

Vector Specialization for 'bool'

In the case of 'std::vector', 'std::vector' is specialized for the 'bool' type. This means that the underlying data storage and access mechanisms specifically handle the binary nature of 'bool' values. As a result, 'std::vector::reference' does not return a reference to a 'bool' object but rather a reference to an internal representation optimized for 'bool' values.

'fixed_vector' and 'Boost Containers'

To mitigate this issue, you can use the following strategy:

1. 'fixed_vector' Template Metaprogramming: Define a custom template metaprogram called 'fixed_vector' that specializes 'std::vector' for 'bool' and uses 'char' as the underlying storage type. This allows you to retrieve references to 'char' values, which can be interpreted as 'bool' values.
2. 'Boost Containers' Library: Use the 'Boost Containers' library, which provides an unspecialized version of 'std::vector' and supports references to 'bool' values.

Example Using 'fixed_vector':

template<typename t, typename... p>
using fixed_vector = std::vector<typename foo<t>::type, p...>;

int main()
{
    fixed_vector<bool> v = {false, true, false};

    const bool f = false;
    A a;

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

Example Using 'Boost Containers':

#include <boost/container/vector.hpp>

int main()
{
    boost::container::vector<bool> v = {false, true, false};

    const bool f = false;
    A a;

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

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