std::vector<bool> workaround in C++

Question

I've been trying to make an std::vector of boolean values and I got fed up with the specialization. To get around this, I made a wrapper class around bool and a testsuite to make sure that it compiles and works as expected.

It seems to do everything std::vector<bool> does, except, of course, flip(). It also satisfies the container rules so that you can take a reference or a pointer to an element.

On to the code:

vector_safe_bool.hpp:

#pragma once

// A wrapper class around bool that can be used in std::vector without breaking container rules

class vector_safe_bool {
    bool value;
public:
    vector_safe_bool() = default;
    vector_safe_bool(bool b) : value{b} {}

    bool *operator&() noexcept { return &value; }
    const bool *operator&() const noexcept { return &value; }

    operator const bool &() const noexcept { return value; }
    operator bool &() noexcept { return value; }
};

vsb_test.cpp:

#include "vector_safe_bool.hpp"

#include <iostream>
#include <vector>

#if USE_VECTOR_SAFE_BOOL
using which_bool = vector_safe_bool;
#define TEST_CONTAINER true
#else
using which_bool = bool;
#define TEST_CONTAINER false
#endif

int main()
{
// The commented lines work for neither bool nor vector_safe_bool
    const which_bool t1 = true;
//  bool *t1p = &t1;
    const bool *t1cp = &t1;
//  bool &t1r = t1;
    const bool &t1cr = t1;
//  bool &&t1rr = t1;

    which_bool t2 = true;
    bool *t2p = &t2;
    const bool *t2cp = &t2;
    bool &t2r = t2;
    const bool &t2cr = t2;
//  bool &&t2rr = t2;
    t2++;
    ++t2;

    std::vector<which_bool> bv(10, true);
#if TEST_CONTAINER
    for(auto &b : bv)
        ;
#endif
    for(const auto &b : bv)
        ;
    for(auto &&b : bv)
        ;
    const std::vector<which_bool> cbv(10, true);
#if TEST_CONTAINER
    for(auto &cb : cbv)
        ;
#endif
    for(const auto &cb : cbv)
        ;
    for(auto &&cb : cbv)
        ;
}

GNUmakefile:

CXX = g++
CXXFLAGS = -Wall -Wextra -Wno-unused-variable -Wno-deprecated -pedantic -std=c++11

all:

check test: vsb_test.cpp vector_safe_bool.hpp
    $(CXX) $(CXXFLAGS) -I. vsb_test.cpp -DUSE_VECTOR_SAFE_BOOL
    @rm -f a.out
    $(CXX) $(CXXFLAGS) -I. vsb_test.cpp
    @rm -f a.out

What I'm looking for:

• Can I make this more idiomatic? How?
• Can I make use of features from newer versions of C++?
• Did I forget any obscure corner-cases?
• Is there anything that I can make better in the GNUmakefile?
• Are there any other ways that I could improve this?

Answer 1

If you aren’t reinventing the wheel as a learning exercise, and want to code this productively, the simplest options are to use the Boost version of Boost::Container::Vector<bool>, which does not specialize to a bitfield, or something like std::vector<unsigned char> or perhaps std::vector<std::byte>, letting a type the same size as bool decay to bool. Other suggestions in the past have included using std::deque or specializing the allocator.

If you’re going to re-implement this vector_safe_bool type encapsulating bool, you probably want to define operator(bool) and vector_safe_bool& operator= (bool), which would let you assign bool values to and from the vector. You might also need to define bitwise operators for expressions such as v1[i] &= !v2[i] to work.

Additionally, there’s no point in passing or returning a type that fits into a register by const reference. It’s more efficient to pass by value. And you should allow this type to be constexpr, so you can use use it in the initializer-list of a constexpr std::vector<vector_safe_bool> in C++20.

Answer 2

Makefile review:

All makefiles should have .DELETE_ON_ERROR: to ensure erroneous output doesn't satisfy dependencies.

The targets all, check and test should be declared as dependencies of .PHONY.

It's unusual to have a Makefile for a header-only library, but I think we can still make use of default rules for the two tests:

clean:
    $(RM) vsb_test

check test:
    $(MAKE) -B CXXFLAGS+=-UUSE_VECTOR_SAFE_BOOL vsb_test clean
    $(MAKE) -B CXXFLAGS+=-DUSE_VECTOR_SAFE_BOOL vsb_test clean

Probably also a good idea to have the compiler auto-generate header dependencies:

CXXFLAGS += -MMD
-include vsb_test.d