Lesson 12.2. Properties of structures in C

Assignment of structures

It is possible to assign a structure as a whole:

structure_A = structure_B ;

The above code copies all fields from structure_B into structure_A.

Comparison of structures

It is not possible to compare two structures with the == operator.

// DOES NOT COMPARE STRUCTURES
if (structure_A == structure_B)...

To test if two structures are identical, each field must be tested individually.

Scope of structures

The scope of the structures is similar to the variables:

local : within a function, it is only accessible from this function.
global : outside a function, it is accessible to all the part of the code that follows its declaration.

Size of a structure

It is possible to know the size of a structure in memory with the sizeof() function. Be careful, the size of a structure is not always equal to the sum of the size of its fields. Without going into details, memory alignment allows to increase the execution speed.

Exercises

Exercise 1

Create a vector structure that contains 3 double precision fields x, y and z. Create a variable v of type vector and display the size

of the structure vector ;
of the variable v ;
of each field of the structure.

Here is the expected output:

Size of structure vector : 24 bytes
Size of variable v : 24 bytes
Size of v.x : 8 bytes
Size of v.y : 8 bytes
Size of v.z : 8 bytes

Do the exercise

Solution

Exercise 2

The following circle structure is provided:

struct circle {
  int cx,cy;      // Center
  double radius;  // Radius
};

Declare a variable C1 for a circle centered in (2,4) and with a radius of 2.5. Copy the circle into a new variable C2 previously declared.

Check that the copy has been done correctly (C1 equals C2) and that the variables C1andC2` have two distinct addresses:

Circle C1 with radius 2.50 and center (2,4)
Circle C2 with radius 2.50 and center (2,4)
The structures are identical
Address of C1 : 0x7fff25306248
Address of C2 : 0x7fff25306238

Do the exercise

Solution

Quiz

To copy a variable of type structure in another ...

You can use the structure assignment operator (A<=B)

we can use the standard assignment operator (A=B)

you can copy the fields one by one

none of the other answers

Check

To check that variables of type structure are identical ...

you can use the strcmp() function (compare structure)

we can use the standard comparison operator (A==B)

we can compare the fields one by one

none of the other answers

Check Bravo! The == operator does not work on structures, you have to compare the fields individually. However, in this case, a minimum of 16 bytes is required.

What is the size of the structure below?

// Structure circle
struct circle {
  int cx,cy;      // Center
  double radius;  // Radius
};

0 bytes

16 bytes

32 bytes

We can't know