Why are structs stored on the stack while classes get stored on the heap(.NET)?

c# .net

I know that one of the differences between classes and structs is that struct instances get stored on stack and class instances(objects) are stored on the heap.

Since classes and structs are very similar. Does anybody know the difference for this particular distinction?

Solution 1:

(edited to cover points in comments)

To emphasise: there are differences and similarities between value-types and reference-types, but those differences have nothing to do with stack vs heap, and everything to do with copy-semantics vs reference-semantics. In particular, if we do:

Foo first = new Foo { Bar = 123 };
Foo second = first;

Then are "first" and "second" talking about the same copy of Foo? or different copies? It just so happens that the stack is a convenient and efficient way of handling value-types as variables. But that is an implementation detail.

(end edit)

Re the whole "value types go on the stack" thing... - value types don't always go on the stack;

if they are fields on a class
if they are boxed
if they are "captured variables"
if they are in an iterator block

then they go on the heap (the last two are actually just exotic examples of the first)

i.e.

class Foo {
    int i; // on the heap
}

static void Foo() {
    int i = 0; // on the heap due to capture
    // ...
    Action act = delegate {Console.WriteLine(i);};
}

static IEnumerable<int> Foo() {
    int i = 0; // on the heap to do iterator block
    //
    yield return i;
}

Additionally, Eric Lippert (as already noted) has an excellent blog entry on this subject

Solution 2:

It's useful in practice to be able to allocate memory on the stack for some purposes, since those allocations are very fast.

However, it's worth noting that there's no fundamental guarantee that all structs will be placed on the stack. Eric Lippert recently wrote an interesting blog entry on this topic.

Solution 3:

Every process has a data block consists of two different allocatable memory segment. These are stack and heap. Stack is mostly serving as the program flow manager and saves local variables, parameters and returning pointers (in a case of returning from the current working function).

Classes are very complex and mostly very large types compared to value types like structs (or basic types -- ints, chars, etc.) Since stack allocation should be specialized on the efficiency of program flow, it is not serving an optimal environment to keep large objects.

Therefore, to greet both of the expectations, this seperated architecture came along.

Solution 4:

How the compiler and run-time environment handle memory management has grown up over a long period of time. The stack memory v.s. heap memory allocation decision had a lot to do with what could be known at compile-time and what could be known at runtime. This was before managed run times.

In general, the compiler has very good control of what's on the stack, it gets to decide what is cleaned up and when based on calling conventions. The heap on the other hand, was more like the wild west. The compiler did not have good control of when things came and went. By placing function arguments on the stack, the compiler is able to make a scope -- that scope can be controlled over the lifetime of the call. This is a natural place to put value types, because they are easy to control as opposed to reference types that can hand out memory locations (pointers) to just about anyone they want.

Modern memory management changes a lot of this. The .NET runtime can take control of reference types and the managed heap through complex garbage collection and memory management algorithms. This is also a very, very deep subject.

I recommend you check out some texts on compilers -- I grew up on Aho, so I recommend that. You can also learn a lot about the subject by reading Gosling.

Solution 5:

In some languages, like C++, objects are also value types.

To find an example for the opposite is harder, but under classic Pascal union structs could only be instantiated on the heap. (normal structs could be static)

In short: this situation is a choice, not a hard law. Since C# (and Java before it) lack procedural underpinnings, one can ask themselves why it needs structures at all.

The reason it is there, is probably a combination of needing it for external interfaces and to have a performant and tight complex (container-) type. One that is faster than class. And then it is better to make it a value type.