When should the explicit key word be used in the constructor?

Answer 1

Whenever the constructor's arguments must not be implicitly cast. For instance, if a constructor expects a float and you pass a double, the double will be implicitly cast to a float which may result in data loss due to narrowing. By declaring the constructor explicit, you must explicitly cast the double to a float, making it clear to the reader that narrowing is an acceptable consequence.

struct A {

explicit A (float);

};

void g ()

{

float f {42.0f};

double d {42.0};

A a (f); // ok -- explicit float

A b (d); // error -- implicit cast from double to float is not permitted

A c (static_cast<float> (d)); // ok -- explicit cast, narrowing is permitted

}

Note that all explicit casts should be done using named casts rather than C-style casts. This ensures the code stands out like the sore-thumb that it is because explicit casts are often a sign of poor design. When you finally come to your senses, named casts are much easier to locate and rectify.