What is Type-safe?

Question

What does "type-safe" mean?

Solution 1

Type safety means that the compiler will validate types while compiling, and throw an error if you try to assign the wrong type to a variable.

Some simple examples:

// Fails, Trying to put an integer in a string
String one = 1;
// Also fails.
int foo = "bar";

This also applies to method arguments, since you are passing explicit types to them:

int AddTwoNumbers(int a, int b)
{
    return a + b;
}

If I tried to call that using:

int Sum = AddTwoNumbers(5, "5");

The compiler would throw an error, because I am passing a string ("5"), and it is expecting an integer.

In a loosely typed language, such as javascript, I can do the following:

function AddTwoNumbers(a, b)
{
    return a + b;
}

if I call it like this:

Sum = AddTwoNumbers(5, "5");

Javascript automaticly converts the 5 to a string, and returns "55". This is due to javascript using the + sign for string concatenation. To make it type-aware, you would need to do something like:

function AddTwoNumbers(a, b)
{
    return Number(a) + Number(b);
}

Or, possibly:

function AddOnlyTwoNumbers(a, b)
{
    if (isNaN(a) || isNaN(b))
        return false;
    return Number(a) + Number(b);
}

if I call it like this:

Sum = AddTwoNumbers(5, " dogs");

Javascript automatically converts the 5 to a string, and appends them, to return "5 dogs".

Not all dynamic languages are as forgiving as javascript (In fact a dynamic language does not implicity imply a loose typed language (see Python)), some of them will actually give you a runtime error on invalid type casting.

While its convenient, it opens you up to a lot of errors that can be easily missed, and only identified by testing the running program. Personally, I prefer to have my compiler tell me if I made that mistake.

Now, back to C#...

C# supports a language feature called covariance, this basically means that you can substitute a base type for a child type and not cause an error, for example:

 public class Foo : Bar
 {
 }

Here, I created a new class (Foo) that subclasses Bar. I can now create a method:

 void DoSomething(Bar myBar)

And call it using either a Foo, or a Bar as an argument, both will work without causing an error. This works because C# knows that any child class of Bar will implement the interface of Bar.

However, you cannot do the inverse:

void DoSomething(Foo myFoo)

In this situation, I cannot pass Bar to this method, because the compiler does not know that Bar implements Foo's interface. This is because a child class can (and usually will) be much different than the parent class.

Of course, now I've gone way off the deep end and beyond the scope of the original question, but its all good stuff to know :)

Solution 2

Type-safety should not be confused with static / dynamic typing or strong / weak typing.

A type-safe language is one where the only operations that one can execute on data are the ones that are condoned by the data's type. That is, if your data is of type X and X doesn't support operation y, then the language will not allow you to to execute y(X).

This definition doesn't set rules on when this is checked. It can be at compile time (static typing) or at runtime (dynamic typing), typically through exceptions. It can be a bit of both: some statically typed languages allow you to cast data from one type to another, and the validity of casts must be checked at runtime (imagine that you're trying to cast an Object to a Consumer - the compiler has no way of knowing whether it's acceptable or not).

Type-safety does not necessarily mean strongly typed, either - some languages are notoriously weakly typed, but still arguably type safe. Take Javascript, for example: its type system is as weak as they come, but still strictly defined. It allows automatic casting of data (say, strings to ints), but within well defined rules. There is to my knowledge no case where a Javascript program will behave in an undefined fashion, and if you're clever enough (I'm not), you should be able to predict what will happen when reading Javascript code.

An example of a type-unsafe programming language is C: reading / writing an array value outside of the array's bounds has an undefined behaviour by specification. It's impossible to predict what will happen. C is a language that has a type system, but is not type safe.

Solution 3

Type safety is not just a compile time constraint, but a run time constraint. I feel even after all this time, we can add further clarity to this.

There are 2 main issues related to type safety. Memory** and data type (with its corresponding operations).

Memory**

A char typically requires 1 byte per character, or 8 bits (depends on language, Java and C# store unicode chars which require 16 bits). An int requires 4 bytes, or 32 bits (usually).

Visually:

char: |-|-|-|-|-|-|-|-|

int : |-|-|-|-|-|-|-|-| |-|-|-|-|-|-|-|-| |-|-|-|-|-|-|-|-| |-|-|-|-|-|-|-|-|

A type safe language does not allow an int to be inserted into a char at run-time (this should throw some kind of class cast or out of memory exception). However, in a type unsafe language, you would overwrite existing data in 3 more adjacent bytes of memory.

int >> char:

|-|-|-|-|-|-|-|-| |?|?|?|?|?|?|?|?| |?|?|?|?|?|?|?|?| |?|?|?|?|?|?|?|?|

In the above case, the 3 bytes to the right are overwritten, so any pointers to that memory (say 3 consecutive chars) which expect to get a predictable char value will now have garbage. This causes undefined behavior in your program (or worse, possibly in other programs depending on how the OS allocates memory - very unlikely these days).

** While this first issue is not technically about data type, type safe languages address it inherently and it visually describes the issue to those unaware of how memory allocation "looks".

Data Type

The more subtle and direct type issue is where two data types use the same memory allocation. Take a int vs an unsigned int. Both are 32 bits. (Just as easily could be a char[4] and an int, but the more common issue is uint vs. int).

|-|-|-|-|-|-|-|-| |-|-|-|-|-|-|-|-| |-|-|-|-|-|-|-|-| |-|-|-|-|-|-|-|-|

|-|-|-|-|-|-|-|-| |-|-|-|-|-|-|-|-| |-|-|-|-|-|-|-|-| |-|-|-|-|-|-|-|-|

A type unsafe language allows the programmer to reference a properly allocated span of 32 bits, but when the value of a unsigned int is read into the space of an int (or vice versa), we again have undefined behavior. Imagine the problems this could cause in a banking program:

"Dude! I overdrafted $30 and now I have $65,506 left!!"

...'course, banking programs use much larger data types. ;) LOL!

As others have already pointed out, the next issue is computational operations on types. That has already been sufficiently covered.

Speed vs Safety

Most programmers today never need to worry about such things unless they are using something like C or C++. Both of these languages allow programmers to easily violate type safety at run time (direct memory referencing) despite the compilers' best efforts to minimize the risk. HOWEVER, this is not all bad.

One reason these languages are so computationally fast is they are not burdened by verifying type compatibility during run time operations like, for example, Java. They assume the developer is a good rational being who won't add a string and an int together and for that, the developer is rewarded with speed/efficiency.

Solution 4

Many answers here conflate type-safety with static-typing and dynamic-typing. A dynamically typed language (like smalltalk) can be type-safe as well.

A short answer: a language is considered type-safe if no operation leads to undefined behavior. Many consider the requirement of explicit type conversions necessary for a language to be strictly typed, as automatic conversions can sometimes leads to well defined but unexpected/unintuitive behaviors.

Solution 5

A programming language that is 'type-safe' means following things:

1. You can't read from uninitialized variables
2. You can't index arrays beyond their bounds
3. You can't perform unchecked type casts

View more solutions

Comments

[Basil Bourque]

en.wikipedia.org/wiki/Type_safety

[Peter Ramos]

Personally, I hate the Convert.To notation, why don't you just use safe cast? Its only less function call on the callstack as well.

[villasenor]

Liberal arts major would say an explanation :) You're also conflating static typing and dynamic typing.

[mipadi]

Python variables are typed (strongly typed, in fact). Try doing this, for example: "str" + 1. You'll get an error. However, the types are checked at runtime, rather than compile time.

[rapidfyre]

Liberal arts "majors", not "major".

[Nicolas Rinaudo]

I feel that this answer is wrong: type safety is not necessarily enforced at compile time. I understand that Scheme, for instance, is considered type safe, but is dynamically checked (type safety is enforced at runtime). This is mostly paraphrasing the introduction to Types and Programming Languages, by Benjamin C. Pierce.

[VasiliNovikov]

Wait, your definition of type-safety does not have a single word "type" :D if no operation leads to undefined behavior.

[VasiliNovikov]

Also, I would disagree to such a definition. I think type-safety means exactly 1. the existence of types 2. the knowledge of them to the compiler, and appropriate checks of course.

[IS4]

What you describe is called polymorphism, not covariance. Covariance is used in generics.

[Code Abominator]

@NicolasRinaudo note that the gap between dynamic languages and static is being eroded by dynamic compilation and precompilation for "interpreted" languages, and by reflection in "compiled" languages. Reflection allows runtime duck typing, for example, so a compiled language can say "hey, this has a Quack() method, I'll call that and see what happens". Pascal-like languages also often have (optional) runtime overflow checking, leading to those "compiler" errors happening at runtime "cannot fit integer supplied into 8 bit destination {core dump}".

[golopot]

Do you mean memory safety?

[ARK]

what are other examples of type-unsafe languages? What do you mean by "writing an array value outside of the array's bounds has an undefined behaviour by specification. It's impossible to predict what will happen". Like Javascript, it will return undefined right? Or really anything can happen. Can you give example of this?

[Nicolas Rinaudo]

@AkshayrajKore sure. Arrays are memory pointers, so by writing out of bounds, you might be overwriting another program’s data - which can do nothing, crash the program, cause it to erase your hard drive - it’s undefined and depends on who’s reading that bit of memory and how it will react to it.

[ilstam]

@Nicolas Rinaudo That is not correct. You should read about virtual memory. Each process has its own virtual address space so a process cannot "overwrite another program's data" in such way.

[Nicolas Rinaudo]

You're correct - this should have read you might be overwriting another part of your program's memory - up to and including, I believe, the program itself?

[ilstam]

@NicolasRinaudo The code segment of the program is mapped read-only in the virtual address space. So if you tried to write to it that would cause a segmentation fault and your program would crash. As well if you tried to write to unmapped memory that would cause a page fault and again crash. However, if you are unlucky you might just overwrite data from the process's stack or heap (like other variables or other stuff). In that case you probably wouldn't crash immediately which is even worse because you won't notice the bug until (hopefully) later!

[dantebarba]

Your example references to a concept called "strongly typed" which is not the same as type safety. Type safety is when a language can detect type errors on execution or compile time. Python for example is weakly typed and type safe. This answer should be flagged as it's very misleading.

[dantebarba]

Excellent answer. Just to add, Python is another well known example of type-safe language that is dynamically typed.

[Prateek93a]

It is true that ensuring Type Safety puts constraints on Speed. But it is really important that Type Safety is ensured given that C/C++ code is more susceptible to BufferOverflow attacks and other related attacks. Threats of such attacks are reduced by ensuring Type Safety.