A function should perform a single logical operation

Reason

A function that performs a single operation is simpler to understand, test, and reuse.

Example

Consider:

void read_and_print()    // bad
{
    int x;
    cin >> x;
    // check for errors
    cout << x << "\n";
}

This is a monolith that is tied to a specific input and will never find another (different) use. Instead, break functions up into suitable logical parts and parameterize:

int read(istream& is)    // better
{
    int x;
    is >> x;
    // check for errors
    return x;
}

void print(ostream& os, int x)
{
    os << x << "\n";
}

These can now be combined where needed:

void read_and_print()
{
    auto x = read(cin);
    print(cout, x);
}

If there was a need, we could further templatize read() and print() on the data type, the I/O mechanism, the response to errors, etc. Example:

auto read = [](auto& input, auto& value)    // better
{
    input >> value;
    // check for errors
};

void print(auto& output, const auto& value)
{
    output << value << "\n";
}

Enforcement

• Consider functions with more than one "out" parameter suspicious. Use return values instead, including tuple for multiple return values.
• Consider "large" functions that don't fit on one editor screen suspicious. Consider factoring such a function into smaller well-named suboperations.
• Consider functions with 7 or more parameters suspicious.