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Rule 3-9-1 - Printable Version

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Rule 3-9-1 - pmhill - 30-12-2011

The rule states that the types for a function declaration and definition must be token-for-token identical. How literally should this rule be taken. I have two cases where this rule seems to conflict with normal or good practice.

1. In a function definition, a 'const' qualifier is often included for parameters passed by value as a promise that the implementation will not modify these parameters. This qualifier cannot be included in the declaration. For example:

T low_bits_mask(unsigned n);

T low_bits_mask(const unsigned n) {
return ~((~static_cast(0))

Re: Rule 3-9-1 - misra cpp - 05-10-2015

In the examples quoted, there are not two instances of the same function.

Rule 3-9-1 relates to the case where the same function is declared in two translation units and these two declarations must obey the 'One Definition Rule'.

Token-for-token compliance is the easiest way to ensure this.