C++ › Casting & Conversions

What is the explicit keyword and why use it?

Intermediate

Answer

explicit prevents a constructor or conversion operator from being used for implicit conversions, requiring direct/explicit invocation. It avoids surprising conversions—like a class accidentally constructed from an int in a function call—that cause ambiguity or bugs. Best practice marks single-argument constructors explicit unless implicit conversion is genuinely desired.

struct Meters { explicit Meters(double); };
void f(Meters);
// f(3.0);       // error, good
f(Meters(3.0));  // explicit, clear

Real-world example explicit on a Distance(double) constructor prevents a raw double from silently becoming a Distance.

Common follow-ups: Which conversions does it block? | When would you allow implicit conversion?

OOP & Classes Casting & Conversions Casting & Conversions