For the latest documentation on Visual Studio 2017 RC, see Visual Studio 2017 RC Documentation.

Evaluate an expression and generate a debug report when the result is False (debug version only).



A scalar expression (including pointer expressions) that evaluates to nonzero (true) or 0 (false).

A wide string to display as part of the report.

The _ASSERT_EXPR, _ASSERT and _ASSERTE macros provide an application with a clean and simple mechanism for checking assumptions during the debugging process. They are very flexible because they do not need to be enclosed in #ifdef statements to prevent them from being called in a retail build of an application. This flexibility is achieved by using the _DEBUG macro. _ASSERT_EXPR, _ASSERT and _ASSERTE are only available when _DEBUG is defined at compile time. When _DEBUG is not defined, calls to these macros are removed during preprocessing.

_ASSERT_EXPR, _ASSERT and _ASSERTE evaluate their booleanExpression argument and when the result is false (0), they print a diagnostic message and call _CrtDbgReportW to generate a debug report. The _ASSERT macro prints a simple diagnostic message, _ASSERTE includes a string representation of the failed expression in the message, and _ASSERT_EXPR includes the message string in the diagnostic message. These macros do nothing when booleanExpression evaluates to nonzero.

_ASSERT_EXPR, _ASSERT and _ASSERTE invoke _CrtDbgReportW, which causes all output to be in wide characters. _ASSERTE properly prints Unicode characters in booleanExpression and _ASSERT_EXPR prints Unicode characters in message.

Because the _ASSERTE macro specifies the failed expression, and _ASSERT_EXPR lets you specify a message in the generated report, they enable users to identify the problem without referring to the application source code. However, a disadvantage exists in that every message printed by _ASSERT_EXPR and every expression evaluated by _ASSERTE is included in the output (debug version) file of your application as a string constant. Therefore, if a large number of calls are made to _ASSERT_EXPR or _ASSERTE, these expressions can greatly increase the size of your output file.

Unless you specify otherwise with the _CrtSetReportMode and _CrtSetReportFile functions, messages appear in a pop-up dialog box equivalent to setting:


_CrtDbgReportW generates the debug report and determines its destination or destinations, based on the current report mode or modes and file defined for the _CRT_ASSERT report type. By default, assertion failures and errors are directed to a debug message window. The _CrtSetReportMode and _CrtSetReportFile functions are used to define the destinations for each report type.

When the destination is a debug message window and the user clicks the Retry button, _CrtDbgReportW returns 1, causing the _ASSERT_EXPR, _ASSERT and _ASSERTE macros to start the debugger provided that just-in-time (JIT) debugging is enabled.

For more information about the reporting process, see the _CrtDbgReport, _CrtDbgReportW function. For more information about resolving assertion failures and using these macros as a debugging error handling mechanism, see Using Macros for Verification and Reporting.

In addition to the _ASSERT macros, the assert macro can be used to verify program logic. This macro is available in both the debug and release versions of the libraries. The _RPT, _RPTF debug macros are also available for generating a debug report, but they do not evaluate an expression. The _RPT macros generate a simple report. The _RPTF macros include the source file and line number where the report macro was called in the generated report. Wide character versions of these macros are available (_RPTWn, _RPTFWn). The wide character versions are identical to the narrow character versions except that wide character strings are used for all string parameters and output.

Although _ASSERT_EXPR, _ASSERT and _ASSERTE are macros and are available by including <crtdbg.h>, the application must link with a debug version of the C run-time library when _DEBUG is defined because these macros call other run-time functions.

MacroRequired header

In this program, calls are made to the _ASSERT and _ASSERTE macros to test the condition string1 == string2. If the condition fails, these macros print a diagnostic message. The _RPTn and _RPTFn group of macros is also exercised in this program, as an alternative to the printf function.

// crt_ASSERT_macro.c  
// compile with: /D_DEBUG /MTd /Od /Zi /link /verbose:lib /debug  
// This program uses the _ASSERT and _ASSERTE debugging macros.  
#include <stdio.h>  
#include <string.h>  
#include <malloc.h>  
#include <crtdbg.h>  
int main()  
   char *p1, *p2;  
   // The Reporting Mode and File must be specified  
   // before generating a debug report via an assert  
   // or report macro.  
   // This program sends all report types to STDOUT.  
   _CrtSetReportMode(_CRT_WARN, _CRTDBG_MODE_FILE);  
   _CrtSetReportFile(_CRT_WARN, _CRTDBG_FILE_STDOUT);  
   _CrtSetReportMode(_CRT_ERROR, _CRTDBG_MODE_FILE);  
   _CrtSetReportFile(_CRT_ERROR, _CRTDBG_FILE_STDOUT);  
   _CrtSetReportMode(_CRT_ASSERT, _CRTDBG_MODE_FILE);  
   // Allocate and assign the pointer variables.  
   p1 = (char *)malloc(10);  
   strcpy_s(p1, 10, "I am p1");  
   p2 = (char *)malloc(10);  
   strcpy_s(p2, 10, "I am p2");  
   // Use the report macros as a debugging  
   // warning mechanism, similar to printf.  
   // Use the assert macros to check if the   
   // p1 and p2 variables are equivalent.  
   // If the expression fails, _ASSERTE will  
   // include a string representation of the  
   // failed expression in the report.  
   // _ASSERT does not include the  
   // expression in the generated report.  
       "Use the assert macros to evaluate the expression p1 == p2.\n");  
   _RPTF2(_CRT_WARN, "\n Will _ASSERT find '%s' == '%s' ?\n", p1, p2);  
   _ASSERT(p1 == p2);  
   _RPTF2(_CRT_WARN, "\n\n Will _ASSERTE find '%s' == '%s' ?\n",  
          p1, p2);  
   _ASSERTE(p1 == p2);  
   _RPT2(_CRT_ERROR, "'%s' != '%s'\n", p1, p2);  
   return 0;  

Use the assert macros to evaluate the expression p1 == p2.  
crt_ASSERT_macro.c(54) :   
 Will _ASSERT find 'I am p1' == 'I am p2' ?  
crt_ASSERT_macro.c(55) : Assertion failed!  
crt_ASSERT_macro.c(58) :   
 Will _ASSERTE find 'I am p1' == 'I am p2' ?  
crt_ASSERT_macro.c(59) : Assertion failed: p1 == p2  
'I am p1' != 'I am p2'  


Debug Routines
assert Macro, _assert, _wassert