Using variadic parameters in MFC / C++20 application

Question

I have a MFC application, but it is configured to build with the C++ 20 standard. Where useful I try to use the newer constructs.

Today, I decided to try and implement some variadic functions. This is what I have so far:

    void CBaseCsvImporter::LogInfo(const CString& msg)
    {
        m_dlgLog.AddLogEntry(msg, true);
    }

    void CBaseCsvImporter::LogSkip(const CString& msg)
    {
        m_dlgLog.AddLogEntry(msg, true);
        ++m_skippedCount;
    }

    void CBaseCsvImporter::LogError(const CString& msg)
    {
        m_dlgLog.AddLogEntry(msg, true);
        m_dlgLog.SetErrorMode(true);
        ++m_errorCount;
    }

    void CBaseCsvImporter::LogInfo(const wchar_t* fmt, ...)
    {
        CString msg;
        va_list args;
        va_start(args, fmt);
        msg.FormatV(fmt, args);
        va_end(args);

        LogInfo(msg); // calls the existing CString overload
    }

    void CBaseCsvImporter::LogSkip(const wchar_t* fmt, ...)
    {
        CString msg;
        va_list args;
        va_start(args, fmt);
        msg.FormatV(fmt, args);
        va_end(args);

        LogSkip(msg);
    }

    void CBaseCsvImporter::LogError(const wchar_t* fmt, ...)
    {
        CString msg;
        va_list args;
        va_start(args, fmt);
        msg.FormatV(fmt, args);
        va_end(args);

        LogError(msg);
    }

Understandably the Visual Studio IDE started showing code analysis warnings, discouraging me from using this approach:

I decided to ask Copilot / ChatGPT about this and it prosed using these functions in the header instead:

template<typename... Args>
void LogInfo(std::wstring_view fmt, Args&&... args)
{
    std::wstring msg = std::vformat(fmt, std::make_wformat_args(args...));
    LogInfo(CString(msg.c_str()));
}

AI says that I would need to adjust my code to call it like this:

LogSkip(L"Skipped line {}: duplicate publisher '{}'", csvLine, name);

Eventually the error messages will come from the string table as a resource. I would appreciate guidance on the best way forward to make use of the new way of doing the variadic functiosn, but keeping in mind that ultimately I use CString.

Answer 1

Which errors do you mean?

In order to make this work:

CString s1 = L"text1";
const wchar_t* s2 = L"text2";
std::wstring s3 = L"text3";

LogInfo( L"Info {}, {}, {}", s1, s2, s3 );

this class can be added to header file (according to https://en.cppreference.com/cpp/utility/format/formatter):

template< >
struct std::formatter<CString, wchar_t> 
{
    template<class ParseContext>
    constexpr ParseContext::iterator parse( ParseContext& ctx )
    {
        return ctx.end( );
    }

    template<class FmtContext>
    FmtContext::iterator format( CString s, FmtContext& ctx ) const
    {
        std::wostringstream out;

		out << LPCWSTR(s);

        return std::ranges::copy( std::move( out ).str( ), ctx.out( ) ).out;
    }
};

Also add:

#include <string>
#include <string_view>
#include <sstream>
#include <format>