What are assertions? and why would you use them?

Question

How are assertions done in c++? Example code is appreciated.

Answer 1

Asserts are a way of explicitly checking the assumptions that your code makes, which helps you track down lots of bugs by narrowing down what the possible problems could be. They are typically only evaluated in a special "debug" build of your application, so they won't slow down the final release version.

Let's say you wrote a function that took a pointer as an argument. There's a good chance that your code will assume that the pointer is non-NULL, so why not explicitly check that with an assertion? Here's how:

#include <assert.h>

void function(int* pointer_arg)
{
    assert(pointer_arg != NULL);

    ...
}

An important thing to note is that the expressions you assert must never have side effects, since they won't be present in the release build. So never do something like this:

assert(a++ == 5);

Some people also like to add little messages into their assertions to help give them meaning. Since a string always evaulates to true, you could write this:

assert((a == 5) && "a has the wrong value!!");

Answer 2

Assertion are boolean expressions which should typically always be true.

They are used to ensure what you expected is also what happens.

void some_function(int age)
{
     assert(age > 0);
}

You wrote the function to deal with ages, you also 'know' for sure you're always passing sensible arguments, then you use an assert. It's like saying "I know this can never go wrong, but if it does, I want to know", because, well, everyone makes mistakes.

So it's not to check for sensible user input, if there are scenario's where something can go wrong, don't use an assert. Do real checks and deal with the errors.

Asserts are typically only for debug builds, so don't put code with side effects in asserts.