This rule is part of MISRA C++:2023.
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Rule 8.9.1 - The built-in relational operators >, >=, < and <= shall not be
applied to objects of pointer type, except where they point to elements of the same array
[expr.rel] Unspecified 4
Category: Required
Analysis: Undecidable,System
Amplification
Uses of std::less, std::less_equal, std::greater, std::greater_equal are permitted as
specializations for any pointer type yield a strict total order.
Notes:
- A pointer to one beyond the last element of an array is considered to point to an element of that array.
- A pointer to an object that is not an array is treated as if it were a pointer to the first element of an array with a single element.
Rationale
Attempting to make comparisons between unrelated pointers may result in surprising or unspecified behaviour.
Example
void f1()
{
int32_t a1[ 10 ];
int32_t a2[ 10 ];
int32_t * p1 = a1;
if ( p1 < a1 ) {} // Compliant
if ( p1 < std::end( a1 ) ) {} // Compliant - right operand is one beyond
if ( p1 < a2 ) {} // Non-compliant
if ( std::less<>{}( p1, a2 ) ) { } // Compliant
}
struct S
{
int32_t m1;
int32_t m2;
};
void f2()
{
S x { };
if ( &x.m1 <= &x.m2 ) {} // Non-compliant - m1 and m2 are not array elements
}
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