Meaningful const in Function Declarations

meaningfulConst.md

void F(int);                     // 1: declaration of F(int)
void F(const int);               // 2: re-declaration of F(int)
void F(int) { /* ... */ }        // 3: definition of F(int)
void F(const int) { /* ... */ }  // 4: error: re-definition of F(int)

void F(const int* x);                  // 1
void F(const int& x);                  // 2
void F(std::unique_ptr<const int> x);  // 3
void F(int* x);                        // 4

void F(const int x);          // 1: declares F(int)
void F(int* const x);         // 2: declares F(int*)
void F(const int* const x);   // 3: declares F(const int*)

• const type-specifiers buried within a parameter type specification are significant and can be used to distinguish overloaded function declarations
• All of these uses of const are important and not ignored because they are part of the parameter type specification and are not top-level const qualifications that affect the parameter x itself.
• Only top-level, or outermost, const qualifications of the parameter type specification are ignored.

Rules of Thumb

• Never use top-level const on function parameters in declarations that are not definitions (and be careful not to copy/paste a meaningless const). It is meaningless and ignored by the compiler, it is visual noise, and it could mislead readers.
• Do use top-level const on function parameters in definitions at your (or your team’s) discretion. You might follow the same rationale as you would for when to declare a function-local variable const.