COleDateTime::SetDateTime and noexcept (code analysis)

Question

See this code:

COleDateTime CSpecialEventDlg::GetSpecialEventDate() noexcept
{
    COleDateTime    datEvent;

    if (datEvent.SetDateTime(m_datEvent.GetYear(),
            m_datEvent.GetMonth(),
            m_datEvent.GetDay(),
            0, 0, 0) != 0)
    {
        // Error

    }

    return datEvent;
}

My code analysis said I could add noexcept (which I did). But I decided to investigate a little more. I noticed that SetDateTime returns a value:

Zero if the value of this COleDateTime object was set successfully; otherwise, 1. This return value is based on the DateTimeStatus enumerated type. For more information, see the SetStatus member function.

For that latter function (SetStatus) it states:

The status parameter value is defined by the DateTimeStatus enumerated type, which is defined within the COleDateTime class. See COleDateTime::GetStatus for details.

Now, the documentation for GetStatus is documented to have a definition of:

DateTimeStatus GetStatus() const throw();

So, it throws an exception if there is an error. Therefore, I decided to look at the MFC source for SetDateTime:

inline int COleDateTime::SetDateTime(
    _In_ int nYear,
    _In_ int nMonth,
    _In_ int nDay,
    _In_ int nHour,
    _In_ int nMin,
    _In_ int nSec) throw()
{
    SYSTEMTIME st;
    ::ZeroMemory(&st, sizeof(SYSTEMTIME));

    st.wYear = WORD(nYear);
    st.wMonth = WORD(nMonth);
    st.wDay = WORD(nDay);
    st.wHour = WORD(nHour);
    st.wMinute = WORD(nMin);
    st.wSecond = WORD(nSec);

    m_status = ConvertSystemTimeToVariantTime(st) ? valid : invalid;
    return m_status;
}

It uses ConvertSystemTimeToVariantTime to set the status. That uses:

inline BOOL COleDateTime::ConvertSystemTimeToVariantTime(_In_ const SYSTEMTIME& systimeSrc)
{
    return AtlConvertSystemTimeToVariantTime(systimeSrc,&m_dt);
}

And that uses:

inline BOOL AtlConvertSystemTimeToVariantTime(
    _In_ const SYSTEMTIME& systimeSrc,
    _Out_ double* pVarDtTm)
{
    ATLENSURE(pVarDtTm!=NULL);
    //Convert using ::SystemTimeToVariantTime and store the result in pVarDtTm then
    //convert variant time back to system time and compare to original system time.
    BOOL ok = ::SystemTimeToVariantTime(const_cast<SYSTEMTIME*>(&systimeSrc), pVarDtTm);
    SYSTEMTIME sysTime;
    ::ZeroMemory(&sysTime, sizeof(SYSTEMTIME));

    ok = ok && ::VariantTimeToSystemTime(*pVarDtTm, &sysTime);
    ok = ok && (systimeSrc.wYear == sysTime.wYear &&
            systimeSrc.wMonth == sysTime.wMonth &&
            systimeSrc.wDay == sysTime.wDay &&
            systimeSrc.wHour == sysTime.wHour &&
            systimeSrc.wMinute == sysTime.wMinute &&
            systimeSrc.wSecond == sysTime.wSecond);

    return ok;
}

At this point I have got lost. In short, I am assuming that SetDateTime does not throw an exception.

---

I understand this much, if I decide to make a call to GetStatus inside the if clause then we do have a potential for exception to be thrown.

Answer 1

All three MFC functions for which you have shown the source code (COleDateTime::SetDateTime, COleDateTime::ConvertSystemTimeToVariantTime and AtlConvertSystemTimeToVariantTime) have – according to their declarations – the potential to throw exceptions (because they have no specification to the contrary, such as noexcept).

However, that doesn't mean that they will (or are even likely to). Digging a bit further into the MFC source code, the only place I can see, in those three functions, where an exception could be thrown is in the ATLENSURE(pVarDtTm!=NULL); line (in the third function).

The ATLENSURE macro is defined as follows:

#define ATLENSURE(expr) ATLENSURE_THROW(expr, E_FAIL)

And ATLENSURE_THROW is, in turn, defined thus:

#define ATLENSURE_THROW(expr, hr)          \
do {                                       \
    int __atl_condVal=!!(expr);            \
    ATLASSUME(__atl_condVal);              \
    if(!(__atl_condVal)) AtlThrow(hr);     \
} __pragma(warning(suppress:4127)) while (0)

So, it would seem that, in your code, an exception will be thrown if expr (in the above snippets) is null (the double-bang, !! pseudo-operator makes any non-zero value into 1 and leaves a zero as zero). That expr is the result of the pVarDtTm!=NULL expression, which can only be 0 (false) if the &m_dt argument in the call in your second MFC source excerpt is itself NULL – and, as the address of a member of the class object through which it is being called, that seems improbable (if not impossible).

---

Another issue you have is that you appear to misunderstand what the throw() specification in the DateTimeStatus GetStatus() const throw(); declaration means. As described here, it is actually (since C++17) an alias for noexcept (or noexcept(true), to be more precise). To specify that a function can throw any type of exception, the throw(...) or noexcept(false) specifications should be used (or use no except/throw specification at all).

So, your last sentence is not really true:

I understand this much, if I decide to make a call to GetStatus inside the if clause then we do have a potential for exception to be thrown.

Because the GetStatus() function is specified explicitly as non-throwing, and you already have a call to the SetDateTime member function, which (as described above) can throw an exception (but won't, in your code).