Best Practices for Defining Variables for C/C++ Code Generation

Define Variables By Assignment Before Using Them

For C/C++ code generation, you should explicitly and unambiguously define the class, size, and complexity of variables before using them in operations or returning them as outputs. Define variables by assignment, but note that the assignment copies not only the value, but also the size, class, and complexity represented by that value to the new variable. For example:

Assignment: Defines:
a = 14.7;a as a real double scalar.
b = a;b with properties of a (real double scalar).
c = zeros(5,2);c as a real 5-by-2 array of doubles.
d = [1 2 3 4 5; 6 7 8 9 0];d as a real 5-by-2 array of doubles.
y = int16(3);y as a real 16-bit integer scalar.

Define properties this way so that the variable is defined on the required execution paths during C/C++ code generation (see Defining a Variable for Multiple Execution Paths).

The data that you assign to a variable can be a scalar, matrix, or structure. If your variable is a structure, define the properties of each field explicitly (see Defining Fields in a Structure).

Initializing the new variable to the value of the assigned data sometimes results in redundant copies in the generated code. To avoid redundant copies, you can define variables without initializing their values by using the coder.nullcopy construct as described in Eliminate Redundant Copies of Variables in Generated Code.

When you define variables, they are local by default; they do not persist between function calls. To make variables persistent, see Define and Initialize Persistent Variables.

Defining a Variable for Multiple Execution Paths

Consider the following MATLAB® code:

...
if c > 0
  x = 11;
end
% Later in your code ...
if c > 0
  use(x);
end
...

Here, x is assigned only if c > 0 and used only when c > 0. This code works in MATLAB, but generates a compilation error during code generation because it detects that x is undefined on some execution paths (when c <= 0),.

To make this code suitable for code generation, define x before using it:

x = 0;
...
if c > 0
  x = 11;
end
% Later in your code ...
if c > 0
  use(x);
end
...

Defining Fields in a Structure

Consider the following MATLAB code:

...
if c > 0 
  s.a = 11;
  disp(s);
else
  s.a = 12;
  s.b = 12;
end
% Try to use s
use(s);
...

Here, the first part of the if statement uses only the field a, and the else clause uses fields a and b. This code works in MATLAB, but generates a compilation error during C/C++ code generation because it detects a structure type mismatch. To prevent this error, do not add fields to a structure after you perform certain operations on the structure. For more information, see Structure Definition for Code Generation.

To make this code suitable for C/C++ code generation, define all fields of s before using it.

...
% Define all fields in structure s
s = struct(‘a',0, ‘b', 0);
if c > 0 
  s.a = 11;
  disp(s);
else
  s.a = 12;
  s.b = 12;
end
% Use s
use(s);
...

Use Caution When Reassigning Variables

In general, you should adhere to the "one variable/one type" rule for C/C++ code generation; that is, each variable must have a specific class, size and complexity. Generally, if you reassign variable properties after the initial assignment, you get a compilation error during code generation, but there are exceptions, as described in Reassignment of Variable Properties.

Use Type Cast Operators in Variable Definitions

By default, constants are of type double. To define variables of other types, you can use type cast operators in variable definitions. For example, the following code defines variable y as an integer:

...
x = 15; % x is of type double by default.
y = uint8(x); % y has the value of x, but cast to uint8.
...

Define Matrices Before Assigning Indexed Variables

When generating C/C++ code from MATLAB, you cannot grow a variable by writing into an element beyond its current size. Such indexing operations produce run-time errors. You must define the matrix first before assigning values to its elements.

For example, the following initial assignment is not allowed for code generation:

g(3,2) = 14.6; % Not allowed for creating g
               % OK for assigning value once created

For more information about indexing matrices, see Incompatibility with MATLAB in Matrix Indexing Operations for Code Generation.

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