By default, declare single-argument constructors explicit

Reason

To avoid unintended conversions.

Example, bad

class String {
public:
    String(int);   // BAD
    // ...
};

String s = 10;   // surprise: string of size 10

Exception

If you really want an implicit conversion from the constructor argument type to the class type, don't use explicit:

class Complex {
public:
    Complex(double d);   // OK: we want a conversion from d to {d, 0}
    // ...
};

Complex z = 10.7;   // unsurprising conversion

See also: Discussion of implicit conversions

Note

Copy and move constructors should not be made explicit because they do not perform conversions. Explicit copy/move constructors make passing and returning by value difficult.

Enforcement

(Simple) Single-argument constructors should be declared explicit. Good single argument non-explicit constructors are rare in most code bases. Warn for all that are not on a "positive list".