What is the assert function

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I ve been studying OpenCV tutorials and came across the assert function  what does it do

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The assert   function can diagnose program bugs  In C  it is defined in  lt assert h gt   and in C   it is defined in  lt cassert gt   Its prototype is  void assert int expression     The argument expression can be anything you want to test--a variable or any C expression  If expression evaluates to TRUE  assert   does nothing  If expression evaluates to FALSE  assert   displays an error message on stderr and aborts program execution   How do you use assert    It is most frequently used to track down program bugs  which are distinct from compilation errors   A bug doesn t prevent a program from compiling  but it causes it to give incorrect results or to run improperly  locking up  for example   For instance  a financial-analysis program you re writing might occasionally give incorrect answers  You suspect that the problem is caused by the variable interest rate taking on a negative value  which should never happen  To check this  place the statement  assert interest rate    0   at locations in the program where interest rate is used  If the variable ever does become negative  the assert   macro alerts you  You can then examine the relevant code to locate the cause of the problem   To see how assert   works  run the sample program below  If you enter a nonzero value  the program displays the value and terminates normally  If you enter zero  the assert   macro forces abnormal program termination  The exact error message you see will depend on your compiler  but here s a typical example   Assertion failed  x  file list19 3 c  line 13 Note that  in order for assert   to work  your program must be compiled in debug mode  Refer to your compiler documentation for information on enabling debug mode  as explained in a moment   When you later compile the final version in release mode  the assert   macros are disabled    int x    printf   nEnter an integer value       scanf   d    amp x     assert x  gt   0     printf  You entered  d  n   x    return 0     Enter an integer value  10  You entered 10   Enter an integer value  -1  Error Message  Abnormal program termination  Your error message might differ  depending on your system and compiler  but the general idea is the same

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Stuff like  raises exception  and  halts execution  might be true for most compilers  but not for all   BTW  are there assert statements that really throw exceptions    Here s an interesting  slightly different meaning of assert used by c6x and other TI compilers  upon seeing certain assert statements  these compilers use the information in that statement to perform certain optimizations  Wicked   Example in C   int dot product short  x  short  y  short z      int sum   0   int i     assert     int  x   amp  0x3      0      assert     int  y   amp  0x3      0       for  i   0   i  lt  z     i       sum    x  i     y  i      return sum      This tells de compiler the arrays are aligned on 32-bits boundaries  so the compiler can generate specific instructions made for that kind of alignment

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Take a look at  assert   example program in C       Many compilers offer an assert     macro  The assert   macro returns TRUE   if its parameter evaluates TRUE and   takes some kind of action if it   evaluates FALSE  Many compilers will   abort the program on an assert   that   fails  others will throw an exception      One powerful feature of the assert     macro is that the preprocessor   collapses it into no code at all if   DEBUG is not defined  It is a great   help during development  and when the   final product ships there is no   performance penalty nor increase in   the size of the executable version of   the program    Eg   include  lt stdio h gt   include  lt assert h gt   void analyze  char    int    int main void       char  string    ABC      int length   3      analyze string  length      printf  The string  s is not null or empty               and has length  d  n   string  length      void analyze char  string  int length       assert string    NULL          cannot be NULL       assert  string      0          cannot be empty       assert length  gt  0              must be positive                          Output should be similar to                     The string ABC is not null or empty  and has length 3

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The assert computer statement is analogous to the statement make sure in English

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In addition  you can use it to check if the dynamic allocation was successful   Code example   int    p  p   new int    5           Dynamic array  size 5  of pointers to int for  int i   0  i  lt  5    i        p i    new int 3      Each i ptr  is now pointing to a dynamic                           array  size 3  of actual int values    assert  p                 Check the dynamic allocation    Similar to   if  p    NULL        cout  lt  lt   dynamic allocation failed   lt  lt  endl      exit 1

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C  11 N3337 standard draft  http   www open-std org jtc1 sc22 wg21 docs papers 2012 n3337 pdf  19 3 Assertions     1 The header  lt cassert   described in  Table 42   provides a macro for documenting C    program assertions   and a mechanism for disabling the assertion checks       2 The contents are the same as the Standard C library header  lt assert h     C99 N1256 standard draft  http   www open-std org JTC1 SC22 WG14 www docs n1256 pdf  7 2 Diagnostics  lt assert h      1 The header  lt assert h gt  defines the assert macro and refers to another macro  NDEBUG which is not defined by  lt assert h gt   If NDEBUG is defined as a macro name at the   point in the source file where  lt assert h  is included  the assert macro is defined simply as    define assert ignore    void 0        The assert macro is redefined according to the current state of NDEBUG each time that    lt assert h gt  is included       2    The assert macro shall be implemented as a macro  not as an actual function  If the   macro definition is suppressed in order to access an actual function  the behavior is   undefined    7 2 1 Program diagnostics  7 2 1 1 The assert macro     Synopsis      1    include  lt assert h gt  void assert scalar expression         Description      2   The assert macro puts diagnostic tests into programs  it expands to a void expression    When it is executed  if expression  which shall have a scalar type  is false  that is    compares equal to 0   the assert macro writes information about the particular call that   failed  including the text of the argument  the name of the source file  the source line   number  and the name of the enclosing function     the latter are respectively the values of   the preprocessing macros   FILE   and   LINE   and of the identifier     func    on the standard error stream in an implementation-defined format  165  It   then calls the abort function       Returns      3   The assert macro returns no value

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It is a function that will halt program execution if the value it has evaluated is false  Usually it is surrounded by a macro so that it is not compiled into the resultant binary when compiled with release settings    It is designed to be used for testing the assumptions you have made  For example   void strcpy char  dest  char  src         pointers shouldn t be null     assert dest  null       assert src  null          copy string     while  dest      src         The ideal you want is that you can make an error in your program  like calling a function with invalid arguments  and you hit an assert before it segfaults  or fails to work as expected

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assert will terminate the program  usually with a message quoting the assert statement  if its argument turns out to be false  It s commonly used during debugging to make the program fail more obviously if an unexpected condition occurs   For example   assert length  gt   0       die if length is negative    You can also add a more informative message to be displayed if it fails like so   assert length  gt   0  amp  amp   Whoops  length can t possibly be negative   didn t we just check 10 lines ago   Tell jsmith      Or else like this   assert   Length can t possibly be negative  Tell jsmith   length  gt   0      When you re doing a release  non-debug  build  you can also remove the overhead of evaluating assert statements by defining the NDEBUG macro  usually with a compiler switch  The corollary of this is that your program should never rely on the assert macro running      BAD assert x         GOOD assert x       x        Watch out  Depends on the function  assert foo         Here s a safer way  int ret   foo    assert ret     From the combination of the program calling abort   and not being guaranteed to do anything  asserts should only be used to test things that the developer has assumed rather than  for example  the user entering a number rather than a letter  which should be handled by other means

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There are three main reasons for using the assert   function over the normal if else and printf   assert   function is mainly used in the debugging phase  it is tedious to write if else with a printf statement everytime you want to test a condition  which might not even make its way in the final code  In large software deployments   assert comes very handy where you can make the compiler ignore the assert statements using the NDEBUG macro defined before linking the header file for assert   function  assert   comes handy when you are designing a function or some code and want to get an idea as to what limits the code will and not work and finally include an if else for evaluating it basically playing with assumptions

[c++] What is the "assert" function?

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