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In debug mode (when the _DEBUG symbol is defined), DEBUG_ONLY evaluates its argument.

DEBUG_ONLY(expression )

In a release build, DEBUG_ONLY does not evaluate its argument. This is useful when you have code that should be executed only in debug builds.

The DEBUG_ONLY macro is equivalent to surrounding expression with #ifdef _DEBUG and #endif.

void ExampleFunc(char* p, int size, char fill)
   char* q;               // working copy of pointer 
   VERIFY(q = p);         // copy buffer pointer and validate
   ASSERT(size >= 100);   // make sure buffer is at least 100 bytes
   ASSERT(isalpha(fill)); // make sure fill character is alphabetic
   // if fill character is invalid, substitute 'X' so we can continue
   // debugging after the preceding ASSERT fails.
   DEBUG_ONLY(fill = (isalpha(fill)) ? fill : 'X');

Header: afx.h