This rule is part of MISRA C++:2023.
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Rule 19.3.3 - The argument to a mixed-use macro parameter shall not be subject to further expansion
Category: Required
Analysis: Decidable,Single Translation Unit
Amplification
A mixed-use macro parameter is a macro parameter that is used both:
- As an operand to
# or ##; and
- Not as an operand to these operators.
This rule prohibits invoking a macro with a mixed-use macro parameter when the corresponding macro argument is itself subject to further
macro replacement.
Rationale
A macro parameter that is used as an operand of a # or ## operator is not expanded prior to being used, whilst the same
parameter appearing elsewhere in the replacement text is expanded. This causes a macro to exhibit different behavior depending on whether or not its
arguments are subject to macro replacement.
Example
In the macro SCALE, X is a mixed-use macro parameter.
#define SCALE( X ) ( ( X ) * X ## _scale )
int32_t speed;
int32_t speed_scale;
int32_t scaled_speed = SCALE( speed ); // Compliant - speed not expanded
// SCALE expands to
// ( ( speed ) * speed_scale )
#define AA BB
int32_t AA_scale = 1;
int32_t BB = 42;
int32_t BB_scale = 2;
int32_t scaled_AA = SCALE( AA ); // Non-compliant - AA is expanded further
// SCALE expands to ( (BB) * AA_scale )
// User might expect ( (BB) * BB_scale )
The rule does not apply to expansions of the following macro as the parameter Y is not a mixed-use macro parameter.
#define CC( Y ) ( var1 ## Y + var2 ## Y )
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