By default, make objects immutable

Reason

Immutable objects are easier to reason about, so make objects non-const only when there is a need to change their value. Prevents accidental or hard-to-notice change of value.

Example

for (const int i : c) cout << i << '\n';    // just reading: const

for (int i : c) cout << i << '\n';          // BAD: just reading

Exceptions

A local variable that is returned by value and is cheaper to move than copy should not be declared const because it can force an unnecessary copy.

std::vector<int> f(int i)
{
    std::vector<int> v{ i, i, i };  // const not needed
    return v;
}

Function parameters passed by value are rarely mutated, but also rarely declared const. To avoid confusion and lots of false positives, don't enforce this rule for function parameters.

void g(const int i) { ... }  // pedantic

Note that a function parameter is a local variable so changes to it are local.

Enforcement

and returned local variables)

• Flag non-const variables that are not modified (except for parameters to avoid many false positives