Constness of parameters in function definitions and declarations

[eggrobin]

It is of no concern to the caller whether a non-reference parameter is const or not. This matters in the function definition (it has to be non-const when, e.g., moving, or using the parameter as a local variable, and should be const when that is not required), but is useless clutter in the declaration; the compiler does not check consistency between the declaration and the definition, and even if consistency is maintained, this is an implementation detail.

Decision.

Definition (const pointer declarator).

A ptr-declarator is const pointer declarator if it is of any of the following forms:

• * [attribute-specifier-seq] [cv-qualifier-seq] declarator-id [attribute-specifier-seq], where the cv-qualifier-seq contains const;
• ptr-operator ptr-declarator, where the ptr-declarator is a const pointer declarator;
• a noptr-declarator of any of the following forms:
  • noptr-declarator [[constant-expression]][attribute-specifier-seq], where the noptr-declarator is a const pointer declarator;
  • noptr-declarator parameters-and-qualifiers, where the noptr-declarator is a const pointer declarator;
  • ( ptr-declarator ), where the ptr-declarator is a const pointer declarator.

Definition (const pointer abstract declarator).

A ptr-abstract-declarator is const pointer abstract declarator if it is of any of the following forms:

• * [attribute-specifier-seq] [cv-qualifier-seq], where the cv-qualifier-seq does contains const;
• ptr-operator ptr-abstract-declarator, where the ptr-abstract-declarator is a const pointer abstract declarator;
• a noptr-abstract-declarator of any of the following forms:
  • noptr-abstract-declarator [[constant-expression]][attribute-specifier-seq], where the noptr-abstract-declarator is a const pointer abstract declarator;
  • noptr-abstract-declarator parameters-and-qualifiers, where the noptr-abstract-declarator is a const pointer abstract declarator;
  • ( ptr-abtract-declarator ), where the ptr-abstract-declarator is a const pointer abstract declarator.

In the parameter-declaration-list of a function declarator that is not the declarator of a function-definition,

• for parameter-declarations with a declarator which is a ptr-declarator, that ptr-declarator shall not be a const pointer declarator; further, if the declarator is of the form declarator-id [attribute-specifier-seq], the decl-specifier-seq of the parameter-declaration shall not contain const as one of its decl_specifiers.
• for parameter-declarations with an abstract-declarator which is a ptr-abstract-declarator, that ptr-abstract-declarator shall not be a const pointer abstract declarator
• for parameter-declarations which do not have a declarator or an abstract-declarator, the decl-specifier-seq of the parameter-declaration shall not contain const as one of its decl_specifiers.

In other words,

void foo(int x);  // OK.
void foo(int);  // OK (though see the styleguide recommendations on
                // naming parameters in declarations).
void foo(int const);  // No.
void foo(int const x);  // No.
void foo(int const* x);  // OK.
void foo(int const& x);  // OK.
void foo(int* const x);  // No.
void foo(int (* const x)());  // No. Really?
void foo(int (*x)());  // You may want a type alias, but OK.
void foo(int const x[5]);  // OK.
void foo(int const (* const x)[5]);  // No!
void foo(int const (* const x())());  // Just say no.
void foo(auto x() -> int);  // OK.

In the function definitions, const as much as possible.