Avoid "magic constants"; use symbolic constants

Reason

Unnamed constants embedded in expressions are easily overlooked and often hard to understand:

Example

for (int m = 1; m <= 12; ++m)   // don't: magic constant 12
    cout << month[m] << '\n';

No, we don't all know that there are 12 months, numbered 1..12, in a year. Better:

// months are indexed 1..12
constexpr int first_month = 1;
constexpr int last_month = 12;

for (int m = first_month; m <= last_month; ++m)   // better
    cout << month[m] << '\n';

Better still, don't expose constants:

for (auto m : month)
    cout << m << '\n';

Enforcement

Flag literals in code. Give a pass to 0, 1, nullptr, \n, "", and others on a positive list.