Why is this an issue?
Const correctness is an important tool for type safety. It allows for catching coding errors at compile time and it documents the code for
maintainers.
Correctly const-qualifying pointers can be tricky because the indirection they add can also be const.
For a pointer X * ptr, const can be written in three different places:
-
const X * ptr and X const * ptr are identical and mean that the X object ptr points to cannot be
changed.
-
X * const ptr means that the pointer cannot be changed to point to a different X object.
In a function signature, the first const X * ptr (or its equivalent X const * ptr) is the one that will bring
type-safety. It protects against changing the value pointed at.
void externalFunction(int * a, const int * b);
void myfunc() {
int a = 1;
int b = 2;
externalFunction(&a, &b);
// a can now have any value
// We know that b is still '2'
}
This rule detects when a pointer or reference parameter could be made const
void myfunc ( int * param1, // object is modified
const int * param2,
int * param3, // Noncompliant
int * const param4) // Noncompliant: const doesn't qualify what is pointed at.
{
*param1 = *param2 + *param3 + *param4;
}
void increment (int & value,
int & increment) // Noncompliant
{
value += increment;
}
When adding all possible const qualifications, we get:
void myfunc ( int * param1, // object is modified
const int * param2,
const int * param3,
const int * param4)
{
*param1 = *param2 + *param3 + *param4;
}
void increment (int & value,
const int & increment)
{
value += increment;
}
Resources
Articles & blog posts
External coding guidelines
- MISRA C:2004, 16.7 - A pointer parameter in a function prototype should be declared as pointer to const if the pointer is not used to modify
the addressed object.
- MISRA C++:2008, 7-1-2 - A pointer or reference parameter in a function shall be declared as pointer to const or reference to const if the
corresponding object is not modified.
- MISRA C:2012, 8.13 - A pointer should point to a const-qualified type whenever possible
