In the following code, the code generator recognizes that the structure fields s.a and s.b are constants.

function y = mystruct()
s.a = 3;
s.b = 5;
y = zeros(s.a,s.b);

If a field of a structure is assigned inside a control flow construct, the code generator does not recognize that s.a and s.b are constant. Consider the following code:

function y = mystruct(x)
s.a = 3;
if x > 1
    s.b = 4;
else
    s.b = 5;
end
y = zeros(s.a,s.b);

If variable-sizing is enabled, y is treated as a variable-size array. If variable-sizing is disabled, y, the code generator reports an error.

You cannot assign mxArrays to structure elements; convert mxArrays to known types before code generation (see Working with mxArrays).

Global structure variables cannot contain handle objects or sparse arrays.