How to solve type mismatch error when assigning variable to class in conditional?

Question

Roop 2024-7-9

0
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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/2135868-how-to-solve-type-mismatch-error-when-assigning-variable-to-class-in-conditional

回答： David Fink 2024-7-9

采纳的回答： David Fink

Example Code:

1 flag = true;

2 if flag

3 this.prop = TFlag();

4 else

5 this.prop = FFlag();

6 end

Error Code:

Type name mismatch: TFlag ~= FFlag.

In the diagnostic report, the error is shown in line 5, where this.prop cannot be assigned FFlag() because it is understood as a varible of class type TFlag().

Question:

It appears that this.prop is being intialized as a variable of class type TFlag regardless of the condition, and I am unable to run this conditional. It should also be noted that the constructor method of each of these classes returns a struct(), but they are different sizes. TFlag returns a struct of size 30, and FFlag returns a struct of size 10.

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Answer 1

David Fink 2024-7-9

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/2135868-how-to-solve-type-mismatch-error-when-assigning-variable-to-class-in-conditional#answer_1483483

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In code generation, since the generated code is statically typed, the type of this.prop must be determined at code generation time.

When processing the first assignment to this.prop, it assigns its type as TFlag. Then when processing the second assignment, it produces the type mismatch error you observed, as FFlag is a different class type than TFlag.

See Use Caution When Reassigning Variable Properties and Reassignment of Variable Properties.

It is possible to assign two distinct values with the same type, but different sizes. For example:

if runtimeCond
    s = struct('f', 0); % 1x1 struct, field f = 1x1 double
    this.prop = s;
else
    s2 = struct('f', {1, 2}); % 1x2 struct array, field f = 1x1 double
    this.prop = s2;
end