Programming Tutorials

assert() example program in C++

By: Charles in C++ Tutorials on 2007-09-17  

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.

Rather than depending on the compiler-provided assert(), you are free to write your own assert() macro. Listing below provides a simple assert() macro and shows its use.

A simple assert() macro.

1:     //ASSERTS
2:     #define DEBUG
3:     #include <iostream.h>
4:
5:     #ifndef DEBUG
6:        #define ASSERT(x)
7:     #else
8:        #define ASSERT(x) \
9:                 if (! (x)) \
10:                { \
11:                   cout << "ERROR!! Assert " << #x << " failed\n"; \
12:                   cout << " on line " << __LINE__  << "\n"; \
13:                   cout << " in file " << __FILE__ << "\n";  \
14:                }
15:    #endif
16:
17:
18:    int main()
19:    {
20:       int x = 5;
21:       cout << "First assert: \n";
22:       ASSERT(x==5);
23:       cout << "\nSecond assert: \n";
24:       ASSERT(x != 5);
25:       cout << "\nDone.\n";
26:     return 0;
27: }

Output: First assert:

Second assert:
ERROR!! Assert x !=5 failed
 on line 24
 in file test1704.cpp
Done.

Analysis: On line 2, the term DEBUG is defined. Typically, this would be done from the command line (or the IDE) at compile time, so you can turn this on and off at will. On lines 8-14, the assert() macro is defined. Typically, this would be done in a header file, and that header (ASSERT.HPP) would be included in all your implementation files.

On line 5, the term DEBUG is tested. If it is not defined, assert() is defined to create no code at all. If DEBUG is defined, the functionality defined on lines 8-14 is applied.

The assert() itself is one long statement, split across seven source code lines, as far as the precompiler is concerned. On line 9, the value passed in as a parameter is tested; if it evaluates FALSE, the statements on lines 11-13 are invoked, printing an error message. If the value passed in evaluates TRUE, no action is taken.