warning C6031 return value ignored in macro expansion

Question

I'm using following code to format HRESULT to message and write the message to file only if HRESULT is an error.

The code compiles and works fine, except that I'm getting following compiler warning:

Warning C6031 Return value ignored: 'wcsrchr'.

I don't want to disable warning but to resolve it, but I'm not able to figure out how? Here is a minimum compilable code:

// compile with: /Wall
#include <Windows.h>
#include <cwchar>       // std::wcsrchr
#include <comdef.h>     // _com_error
#include <iostream>     // std::cin

// Show only file name instead of full path wide version
#define __FILENAME__ (std::wcsrchr(TEXT(__FILE__), L'\\') ? std::wcsrchr(TEXT(__FILE__), L'\\') + 1 : TEXT(__FILE__))

// Writes a sprintf-formatted string to the logging file.
#define TRACE(...) DebugLogTrace(__VA_ARGS__)

// Log HRESULTs if failed.
#define LOG_IF_FAILED(file_name, line, hr) if constexpr (FAILED(hr)) \
    { TRACE((TEXT("%s %i %s"), file_name, line, _com_error(hr).ErrorMessage())); }

// Writes a sprintf-formatted string to the logging file.
void DebugLogTrace(PCTSTR format_string, ...) noexcept
{
    // implementation not important
}

int main()
{
      // generate example failure
      LOG_IF_FAILED(__FILENAME__, __LINE__, E_FAIL);

      std::cin.get();
      return 0;
}

Example file output for E_FAIL error code:

7:50:11 Unspecified error

Nitpcik: `__FILENAME__` [is a reserved identifier](https://stackoverflow.com/questions/228783/what-are-the-rules-about-using-an-underscore-in-a-c-identifier). You are tangoing with undefined behavior by defining it. — StoryTeller - Unslander Monica, Oct 03 '19 at 06:40
@L.F. `LOG_IF_FAILED((void)__FILENAME__, __LINE__, E_FAIL);` results in `C4548` I see now what is the problem! I should reimplement my LOG_IF_FAILED macro because output is wrong, it excludes filename :) — metablaster, Oct 03 '19 at 06:47

score 1 · Accepted Answer · answered Oct 03 '19 at 06:47

I was able to get the warning to go away by changing:

#define LOG_IF_FAILED(file_name, line, hr) if constexpr (FAILED(hr)) \
    { TRACE((TEXT("%s %i %s"), file_name, line, _com_error(hr).ErrorMessage())); }

to

#define LOG_IF_FAILED(file_name, line, hr) if constexpr (FAILED(hr)) \
    { TRACE(TEXT("%s %i %s"), file_name, line, _com_error(hr).ErrorMessage()); }

That is: { TRACE((..., ..., ..., ...)); } to { TRACE(..., ..., ..., ...); }

But I have to admit, that I don't know if there are some other unintended result of removing the extra parenthesis.

Yes that did the trick! file output now prints entry string, thanks a lot! — metablaster, Oct 03 '19 at 06:51

warning C6031 return value ignored in macro expansion

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