14_struct.md

name: main

.aim[Systems: Constructing Data]

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template: main

??? better gh repo setup source code now available: https://github.com/mks65/dwsource, link on site

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template: main

Structs

• A custom data type that is a collection of values.

--

• The following line creates a variable, s, who's type is an anonymous struct:
• struct { int a; char x; } s;

--

• struct { int a; char x; } is the full type of s, it is syntactically identical to int or float ...

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• We use the . operator to access a value inside a struct
• s.a = 10;
• s.x = ‘@‘;

--

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• Here is an example of creating and using a struct:

int main() {
    struct {int a; char x;} s0;

    s0.a = 51;
    s0.x = '%';

    printf("s0: %d\t%c\n", s0.a, s0.x);

    return 0;
 }

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template: main

Structs

• It is preferable to prototype your structs, which will make it easier to create and work with multiple variables of the same struct type.

• struct foo { int a; char x; };

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• Note that since we are not creating a variable, there is no name between the } and the ; at the end.

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• After creating a prototye for a struct, you can declare new variables of that type like so:
• struct foo s;
• You still must include the word struct.

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• It is typically better practice to prototype structs outside of any particular function.

• Struct prototypes are most commonly found in .h files.

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• Here is an example of creating and using a struct with a prototype:

struct foo {int a; char x;};

int main() {

    struct foo s0;
    struct foo s1;

    s0.a = 51;
    s0.x = '%';

    s1 = s0;
    printf("s0: %d\t%c\n", s0.a, s0.x);
    printf("s1: %d\t%c\n", s1.a, s1.x);

    return 0;
 }

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template: main

Pointers and Structs

• You can make pointers to structs like pointers to primitaves.
• struct foo *p = &s;

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• One very important note, . takes precedence over *.

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• This means that *p.x is the same as *(p.x) which is almost certainly NOT what you want. (This will look for x inside p and de-reference that result).

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• To access a value in a struct via a pointer you need to do: (*p).x, that is, de-reference first, then get x.
• In C, p->x is syntactic shorthand for (*p).x

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template: main

Stack memory vs Heap memory

• Every program can have its own stack and heap.

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• Stack memory
  • Stores all normally declared variables (including pointers and structs), arrays and function calls.

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• Functions are pushed onto the stack in the order they are called, and popped off when completed.

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• When a function is popped off the stack, the stack memory associated with it is released.

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• Heap memory
  • Stores dynamically allocated memory.

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• Data will remain in the heap until it is manually released. (or the program terminates)