Function to print data type to debug console

Question

While searching for any kind of function in could help printing data to debug console, I've found this function here on StackOverflow that can print almost any kind of data:

template <typename Arg, typename... Args>
void doPrint(Arg&& arg, Args&&... args)
{
    std::stringstream out;

    out << std::forward<Arg>(arg);
    using expander = int[];
    (void)expander {
        0, (void(out << std::left << std::setw(20) << std::forward<Args>(args)), 0)...
    };

    OutputDebugStringA(out.str().c_str());
}

But it doesn't work when I try to pass a wstring to it, ex:

std::wstring color = L"blue";
doPrint("color: ", color);

I get this error:

Error    C2679    binary '<<': no operator found which takes a right-hand operand of type 'std::wstring' (or there is no acceptable conversion)

Is it possible to also support wstring?

I tried to change std::stringstream to std::wstringstream, but now I get error on OutputDebugStringA():

void OutputDebugStringA(LPCSTR)': cannot convert argument 1 from 'std::basic_string<wchar_t,std::char_traits<wchar_t>,std::allocator<wchar_t>>' to 'LPCSTR'

Remy Lebeau · Accepted Answer · 2022-08-12T01:30:48.900

By default, you can't print a std::wstring to a char-based std::ostream, such as std::stringstream, because std::basic_ostream<char> (aka std::ostream) does not have an overloaded operator<< defined for std::wstring (or const wchar_t*).

You could define your own operator<< for that purpose, which converts a std::wstring to a std::string/char* using WideCharToMultiByte() or equivalent conversion, and then print that instead, eg:

std::ostream& operator<<(std::ostream &os, const std::wstring &wstr)
{
    int len = WideCharToMultiByte(CP_ACP, 0, wstr.c_str(), wstr.size(), nullptr, 0, nullptr, nullptr);
    if (len > 0) {
        std::string str;
        str.resize(len);
        WideCharToMultiByte(CP_ACP, 0, wstr.c_str(), wstr.size(), str.data()/* or: &str[0] */, len, nullptr, nullptr);
        os << str;
    }
    return os;
}

template <typename Arg, typename... Args>
void doPrint(Arg&& arg, Args&&... args)
{
    std::stringstream out;

    out << std::forward<Arg>(arg);
    using expander = int[];
    (void)expander {
        0, (void(out << std::left << std::setw(20) << std::forward<Args>(args)), 0)...
    };

    OutputDebugStringA(out.str().c_str());
}

Online Demo

Otherwise, you need a wchar_t-based std::basic_ostream<wchar_t> (aka std::wostream) instead, such as std::wstringstream, using OutputDebugStringW() for the logging (unless you convert the output std::wstring to std::string to log with OutputDebugStringA()). Fortunately, std::basic_ostream<CharT> does have an overloaded operator<< for const char* strings (but not std::string) regardless of whether CharT is char or wchar_t, so std::wstringstream can print the pointer returned by std::string::c_str(), eg:

std::wostream& operator<<(std::wostream &os, const std::string &str)
{
    os << str.c_str();
    return os;
}

template <typename Arg, typename... Args>
void doPrint(Arg&& arg, Args&&... args)
{
    std::wostringstream out;

    out << std::forward<Arg>(arg);

    using expander = int[];
    (void)expander {
        0, (void(out << std::left << std::setw(20) << std::forward<Args>(args)), 0)...
    };

    OutputDebugStringW(out.str().c_str());
}

Online Demo

I'm getting some errors, In the first example: `argument of type "const char *" is incompatible with parameter of type "LPSTR"` and in the second example, `identifier "arg" is undefined` — Cesar, Aug 12 '22 at 01:19
The "incompatible" error is because you are using an version of C++ that predates C++17, where `std::string::data()` is overloaded to return a non-const `char*`. In which case, you can use `&str[0]` instead. The "identifier undefined" error was a typo on my part, I have fixed it. — Remy Lebeau, Aug 12 '22 at 01:29
Remy, how do I declare the template `doPrint` into the `.h` file to be able to access it in others `.cpp` files? I have tried adding into the `.h`: `template ` `void doPrint(Arg&& arg, Args&&... args);` but i got compile errors: `Error LNK2001 unresolved external symbol "void __cdecl doPrint(char const (&)[14],int &)" (??$doPrint@AEAY0O@$$` — Cesar, Aug 12 '22 at 21:57
@Katia see [Why can templates only be implemented in the header file?](https://stackoverflow.com/questions/495021/) — Remy Lebeau, Aug 12 '22 at 22:19
I read it but im more confuse than before, i tried now: `template struct test { void doPrint(Arg&& arg, Args&&... args); }` — Cesar, Aug 12 '22 at 22:33
@Katia you can't *declare* a template in a header file and then *define* it in a .cpp file. Templates don't work that way. That is why you are getting "unresolved external" errors and such. The template has to be *defined* inside the same translation unit that *declares* it. You *can* separate the declaration and definition, as long as they are compiled together in the same translation unit, ie they exist in the same header file, or the definition is in a separate file that is `#include`'d by the header file. Otherwise, the template should be *declared* and *defined* as an inline function. — Remy Lebeau, Aug 12 '22 at 22:52

Function to print data type to debug console

1 Answers1