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How do I include a trace statement in a C/C++ macro?

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Strictly speaking, you cannot have a trace statement within a macro, because the WPP preprocessor runs before the C preprocessor. One solution is to run the C preprocessor twice, but there is an even simpler solution: define optional PRE and POST steps to the trace macros.

For example, you might want an "exit on failed" macro, such as

If (FAILED(HR)) {
     DoTraceMessage(ERROR,"We failed!");
     Goto done ;
}

In this case, using PRE and POST forms of the macro makes this possible.

Define the function

In the source file, define the function, for example:

FUNC:_EXIT_IF_EXP_FAILED{LEVEL=WSM_ERROR}(_EXIT_IF_EXP_FAILED_EXP,MSG,...)

Define the macros

In a header file, add the following definition directives. Put them after the WPP_CONTROL_GUIDS definition and before the #include statement for the trace message header file.

#define WPP_LEVEL__EXIT_IF_EXP_FAILED_EXP_PRE(LEVEL, HR) {HRESULT hr=S_OK ; if(FAILED(hr = HR)) {
#define WPP_LEVEL__EXIT_IF_EXP_FAILED_EXP_POST(LEVEL, HR) ; goto done; } }
#define WPP_LEVEL__EXIT_IF_EXP_FAILED_EXP_ENABLED(LEVEL, HR) WPP_LEVEL_ENABLED(LEVEL)
#define WPP_LEVEL__EXIT_IF_EXP_FAILED_EXP_LOGGER(LEVEL, HR) WPP_LEVEL_LOGGER(WSM_ERROR)

Add formatting

You can make the trace messages easier to read by including formatting data in the header file. This step is optional.

// MACRO: _EXIT_IF_EXP_FAILED
//
// begin_wpp config
// USEPREFIX (_EXIT_IF_EXP_FAILED,"%!STDPREFIX!");
// FUNC _EXIT_IF_EXP_FAILED{LEVEL=WSM_ERROR}(_EXIT_IF_EXP_FAILED_EXP,MSG,...);
// USESUFFIX (_EXIT_IF_EXP_FAILED," hr= %!HRESULT!", hr);
// end_wpp
#define WPP_LEVEL__EXIT_IF_EXP_FAILED_EXP_PRE(LEVEL, HR) {HRESULT hr=S_OK ; if(FAILED(hr = HR)) {
#define WPP_LEVEL__EXIT_IF_EXP_FAILED_EXP_POST(LEVEL, HR) ; goto done; } }
#define WPP_LEVEL__EXIT_IF_EXP_FAILED_EXP_ENABLED(TRACELEVEL, HR) WPP_LEVEL_ENABLED(TRACELEVEL)
#define WPP_LEVEL__EXIT_IF_EXP_FAILED_EXP_LOGGER(LEVEL, HR) WPP_LEVEL_LOGGER(WSM_ERROR)

In this example, the begin_wpp config and end_wpp statements identify the configuration data in the header file for WPP.

Also, to notify WPP that there is configuration data in the header file, add the -scan parameter to the RUN_WPP macro that invokes the WPP preprocessor. For example:

RUN_WPP -scan:trace.h

For a complete list of the optional parameters for RUN_WPP, see WPP Preprocessor.

Use the macros

In the source code, use the macros, such as in the following call:

_EXIT_IF_EXP_FAILED(hr,"it failed");