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+---- Thread: Rule 7.0.6 - why the requirement of "id-expression"? (/showthread.php?tid=1720)
Rule 7.0.6 - why the requirement of "id-expression"? - cgpzs - 09-01-2025
Rule 7.0.6 provides the following amplification for assigning numerical types:
2. The source and target shall have types of the same type category, signedness, the source size
shall be smaller than the target size, and the source shall be an id-expression;
Question: why must the source be an "id-expression"?
Consider this example:
Code:
struct Foo
{
float bar;
};
float a;
Foo f;
double d1 = a; // Compliant, since it's an id-expression
double d2 = f.bar; // Non-compliant, since it's NOT an id-expression (it's a member-expression)This is inconsistent. The fact that we are using a member-expression instead of an id-expression does not have any impact on how the conversion is made, it's equally safe.
Would it make sense to remove the "id-expression" part of the rule? Otherwise, can you elaborate on why is it needed, what problems does it guard against?
Thanks!
RE: Rule 7.0.6 - why the requirement of "id-expression"? - misra cpp - 10-01-2025
The argument for 'id-expression' is that we wanted to keep the rule simple, whilst preventing i1 = s1 + s2; (a widening assignment).
We tried to enumerate the possible cases which were safe, but this got very complex, so we went to a simple case that was safe.
We will consider the rule in future to allow other safe cases, such as your example of class member access ("member-expression")