C++ Assert (): Assertion Handling In C++ With Examples

This C++ Assert Tutorial Sheds Light on Assertions in C++ which are Statements to Test the Assumptions in the Program Made by the Programmer:

In a C++ program, we usually make assumptions in a program like an array index should be greater than zero.

When these assumptions come true, the program runs the fine but when these assumptions become false, the program does not end normally.

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Assertion handling in C++

Assertions In C++

An assert is a statement in C++ which tests for a condition like the one explained above. If the condition is true, the program continues normally and if the condition is false, the program is terminated and an error message is displayed.

We can provide an assertion using an assert preprocessor macro.

With the integer myInt, we can declare an assertion saying myInt should be > 0 in the following way.

assert (myInt > 0);

Suppose the value of myInt specified is -1, hence the assertion fails when the compiler encounters the above statement as myInt value is -1. Once assertion fails, a message stating invalid assertion is issued along with the program name and line number and the program is terminated.

A general prototype of assert is as follows:

assert (condition) where condition=> expression of scalar type

An assert is a preprocessor macro that is used to evaluate a conditional expression. If the conditional expression evaluates false, then the program is terminated after displaying the error message. The error message typically consists of the failed conditional expression, name of the code file and the line number of the assert.

Thus we get to know where the problem occurred as well as what is the problem that occurred in the code. Hence using assertions makes debugging more efficient.

The C++ header <cassert> contains the assert functionality. We mostly use the assert functionality in the code to check if the parameters passed to a function are valid, to check the return value of a function or to check the array bounds among other things.

Basic Example of C++ Assertion.

#include <iostream>     
#include <cassert>  
using namespace std;
void display_number(int* myInt) {
  assert (myInt!=NULL);
  cout<<"myInt contains value" << " = "<<*myInt<<endl;
}
int main ()
{
  int myptr=5;
  int * second_ptr = NULL;
  int * third_ptr = NULL;
  second_ptr=&myptr;
  display_number (second_ptr);
  display_number (third_ptr);
  return 0;
}

Output:

C++ assertion

In the above program, we have used an assert call that contains the expression (myInt!=NULL) in the display_number function. In the main function first, we pass a pointer variable second_ptr that contains the address of variable myptr. When this call is made, the assert is true. Hence program execution is normal and the value is displayed.

In the second call to display_number, we pass the null pointer thereby making assert false. Thus when the second call is made, as assertion failed message is displayed as shown in the output.

Disabling Assertion With NDEBUG

When we use assertions they are checked at runtime. Assertions make debugging efficient, but care should be taken on not to include assertions in the release build of the application. This is because we know that when we release an application, we do it only when we are sure that the application is tested thoroughly.

So we need to disable all the assertions when we release the software. We can disable assertions in a program by using NDEBUG macro. Using NDEBUG macro in a program disables all calls to assert.

We can include a line given below in the program to disable all assert statements.

#define NDEBUG

Following C++ programs shows how the program behaves when NDEBUG is commented as well as when NDEBUG is active.

#1) NDEBUG specified but commented.

#include <iostream>
// uncomment to disable assert()
//#define NDEBUG
#include <cassert>
using namespace std;
int main()
{
    assert(2+2==3+1);
    cout << "Expression valid...Execution continues.\n";
    assert(2+2==1+1);
    cout << "Asset disabled...execution continuous with invalid expression\n";
}

Output:

NDEBUG specified but commented.

In this program, we have specified the #define NDEBUG statement but is commented. This means that the assert statement is active. Thus when the program is executed, the second call to assert returns false and an error message is flashed and the program is aborted.

#2) NDEBUG is active.

#include <iostream>
// uncomment: assert() disabled
#define NDEBUG
#include <cassert>
using namespace std;
int main()
{
    assert(2+2==3+1);
    cout << "Expression valid...Execution continues.\n";
    assert(2+2==1+1);
    cout << "Assert disabled...execution continuous with invalid expression\n";
}

Output:

NDEBUG is active.

In this program, we uncommented the NDEBUG macro. Now when we execute the program, the assert statements are no more active. Hence the program continues its normal execution even when the second condition in the assert statement is false.

Thus by uncommenting the line #define NDEBUG, we have disabled the assert statements in the program.

Assert And static_assert

The assert that we have seen so far is executed at run time. C++ supports yet another form of assert known as the static_assert and it performs compile-time assertion checking. It is present since C++11.

A static_assert has the following general syntax.

static_assert (bool_constexpr, message)

Here bool_constexpr => cContextually converted constant expression of type bool.

Message => String that will appear as an error message if bool_constexpr is false.

So if the bool_constexpr evaluates to true, the program proceeds normally. If bool_constexpr evaluates to false, then a compiler error is issued.

The below program shows the usage of static_assert in a C++ program.

#include <iostream>
#include <cassert>
using namespace std;
int main()
{
    assert(2+2==3+1);
    static_assert(2+2==3+1, "2+2 = 3+1");
    cout << "Expression valid...Execution continues.\n";
    assert(2+2==1+1);
    static_assert(2+2==1+1, "2+2 != 1+1");
    cout << "Assert disabled...execution continuous with invalid expression\n";
}

Output:

static_assert - output

In the above program, we have provided static_assert with an expression and a message. When it fails, a compiler error is issued as shown in the output.

Frequently Asked Questions

Q #1) What is Assert in C++?

Answer: An assert in C++ is a predefined macro using which we can test certain assumptions that are set in the program. When the conditional expression in an assert statement is set to true, the program continues normally. But when the expression is false, an error message is issued and the program is terminated.

Q #2) What is static_assert?

Answer: Static_assert is evaluated at compile time as against the assert () statement that is evaluated at run time.

Static_assert has been incorporated in C++ from C++11 onwards. It takes the conditional expression and a message to be displayed as arguments. When the condition evaluates to false, a compiler error is issued and the message displayed. The program is then terminated.

Q #3) What is the purpose of assert () macro?

Answer: Assert () macro is used to test the conditions or assumptions that should not occur in a program. For example, the array index should always be > 0. Another assumption can be 2+2 == 3+1.

So using assert () we can test such assumptions and as long as they evaluate to true, our program runs normally. When they are false, the program is terminated.

Conclusion

In this tutorial, we have seen the working of assert () statements in C++. The assert () statement is defined in the header <cassert>. We can disable the assert using NDEBUG macro. Developers should be careful that assert cannot be used in the production code as it is expected that the production code is tested thoroughly and is bug-free.

Apart from the assert () statement C++11 also supports static_assert () that is evaluated at compile time. When static_asset () evaluates to false, a compiler error is issued and the program gets terminated.

Assertions are a way to test the assumptions in the program and by evaluating the conditional expressions inside assertions, we can test the program thoroughly and debug becomes more efficient.

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