boolean_cast

• Download demo project - 18.9 Kb

Motivation

We can't ignore compiler warnings, but they are sometimes annoying:

void motivation(HWND hWnd)
{
  bool b = ::IsWindow(hWnd); // warning C4800
}

The above code produces the warning:

warning C4800: 'BOOL' : forcing value to bool 'true' 
                       or 'false' (performance warning)

static_cast makes the same warning as the one given above. Then you write:

void wrong(HWND hWnd)
{
  bool b = (::IsWindow(hWnd) == TRUE);
}

The behavior is undefined, because the Win32 document says:

If the window handle identifies an existing window, 
                             the return value is nonzero.
If the window handle does not identify an existing window, 
                             the return value is zero.

You may think that BOOL is defined as TRUE or FALSE, but it seems wrong. Then you write:

void right(HWND hWnd)
{
  bool b = (::IsWindow(hWnd) != 0);
}

Your job is completed (you can write FALSE as 0). Anyway, you must be thinking it is still annoying. So that is where boolean_cast comes in.

Requirements

• Microsoft Visual C++ .NET version 7.1
• The newest version of BooleanCast

boolean_cast

boolean_cast that is defined in the namespace poost safely converts values between bool, BOOL and VARIANT_BOOL types:

void good(HWND hWnd)
{
  bool b = boolean_cast<bool>(::IsWindow(hWnd));
}

VARIANT_TRUE is defined as ((short)-1), in fact, the implicit cast is dangerous:

void good_save()
{
  VARIANT_BOOL vb = TRUE;
  ATLASSERT( vb != VARIANT_TRUE ); // wow!

  vb = boolean_cast<VARIANT_BOOL>(TRUE);
  ATLASSERT( vb == VARIANT_TRUE );
}

boolean_cast increases the safety and readability. But well, why should you write target types? The compilers must know the types.

auto_boolean

auto_boolean uses the type of the assigned object as the target type:

void best(HWND hWnd)
{
  bool b = auto_boolean(::IsWindow(hWnd));
}

That's what you want. No warning, no annoying. Note that auto_boolean doesn't work under broken compilers like VC6 or eVC4.

How it works

The implementation of boolean_cast is as follows:

template< class TargetT, class SourceT >
TargetT boolean_cast(SourceT b)
{
  typedef boolean_converter<TargetT, SourceT> converter_t;
  return converter_t::convert(b);
}

boolean_cast delegates its task to boolean_converter class template. The real task is performed by the template. Let's look at the implementation:

// primary template
template< class TargetT, class SourceT >
struct boolean_converter;

// full specialization
template< >
struct boolean_converter<bool, BOOL>
{
  static bool convert(BOOL b)
  {
    return b ? true : false;
  }
}

The task is sent to "full class template specialization" depending on two types, TargetT and SourceT.

Next, let's look into auto_boolean. It is somewhat technical:

template< class SourceT >
struct auto_boolean_type
{
  explicit auto_boolean_type(SourceT b) : m_b(b)
  { }

  // conversion operator
  template< class TargetT >
  operator TargetT() const
  { return boolean_cast<TargetT>(m_b); }

private:
  SourceT m_b;
};

template< class SourceT >
auto_boolean_type<SourceT> auto_boolean(SourceT b)
{
  return auto_boolean_type<SourceT>(b);
}

It is interesting that auto_boolean returns not a type but auto_boolean_type. The compilers must convert auto_boolean_type to the target type. They find and call the "conversion operator" and at last boolean_cast is automatically invoked instead of you invoking it.

Points of interest

reinterpret_cast and static_cast get a lot of use in most programs. It can be time consuming to determine what each cast is doing after you haven't looked at your code for several months, and a domain-specific cast such as boolean_cast can help you reduce this time.

Tested under

• Microsoft Visual C++ .NET version 7.1
• Embedded Visual C++ 4.0 SP4
• Test Drive Comeau C++ Online (Parsing only)

References

• P-Stade
• Boost.NumericCast

History

• 5^th December, 2005: Initial version uploaded.
• 6^th December, 2005: Document updated.