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C6220

warning C6220 - Implicit cast between semantically different integer types: comparing HRESULT to -1. Consider using SUCCEEDED or FAILED macro instead

This warning indicates that an HRESULT is being compared with an explicit, non-HRESULT value of -1, which is not a well-formed HRESULT. A failure in HRESULT (E_FAIL) is not represented by a -1. Therefore, an implicit cast of an HRESULT to an integer will generate an incorrect value and is likely to lead to the wrong result.

In most cases, this warning is caused by the code mistakenly expecting that a function that should return an HRESULT instead returns an integer, by using –1 as a failure value. The following code sample generates this warning:

#include <windows.h>

HRESULT f( )
{
  HRESULT hr;
  LPMALLOC pMalloc;
  
  hr = CoGetMalloc(1, &pMalloc);
  if (hr == -1)
  {
    // failure code ...
    return E_FAIL;
  }
  else
  {
    // success code ...
    return S_OK;
  }
}

It is best to use the SUCCEEDED or FAILED macro to test the value of an HRESULT. To correct this warning, use the following code:

#include <windows.h>

HRESULT f( )
{
  HRESULT hr;
  LPMALLOC pMalloc;
  
  hr = CoGetMalloc(1, &pMalloc);
  if (FAILED(hr))
  {
    // failure code ...
    return E_FAIL;
  }
  else
  {
    // success code ...
    return S_OK;
  }
}

For this warning, the SCODE type is equivalent to HRESULT.

Explicit comparison is appropriate to check for specific HRESULT values, such as, E_FAIL. Otherwise, use the SUCCEEDED or FAILED macros.

For more information, see SUCCEEDED Macro and FAILED Macro.

Note that the use of malloc and free (and related dynamic memory allocation APIs) have many pitfalls in terms of memory leaks and exceptions. To avoid these kinds of leaks and exception problems altogether, use the mechanisms that are provided by the C++ Standard Template Library (STL). These include shared_ptr, unique_ptr, and vector. For more information, see Smart Pointers (Modern C++) and Standard C++ Library Reference.

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