Notes

Thoughts, write-ups, and things worth remembering.

explicit

#cpp#keywords Updated Sep 16, 2026

Blocks a single-argument constructor from being used for an implicit conversion.

class Meters {
public:
    Meters(float value) : value(value) {}   // NOT explicit
};

void travel(Meters distance);
travel(5.0f);   // compiles! 5.0f silently becomes a Meters - probably not intended

class Meters {
public:
    explicit Meters(float value) : value(value) {}
};

travel(5.0f);           // error now - forces you to be explicit about it
travel(Meters(5.0f));   // this is what you actually meant

Default assumption: use it

Unless a type is specifically meant to be implicitly convertible from another (rare — usually only a lightweight wrapper meant to feel transparent), mark single-argument constructors explicit. Costs nothing, blocks an entire category of “wait, how did that convert?” bugs.

API design guidelines

Notes

Default assumption: use it

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