Example
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When applied to a single-argument constructor, prevents that constructor from being used to perform implicit conversions.
class MyVector { public: explicit MyVector(uint64_t size); }; MyVector v1(100); // ok uint64_t len1 = 100; MyVector v2{len1}; // ok, len1 is uint64_t int len2 = 100; MyVector v3{len2}; // ill-formed, implicit conversion from int to uint64_tSince C++11 introduced initializer lists, in C++11 and later,
explicitcan be applied to a constructor with any number of arguments, with the same meaning as in the single-argument case.struct S { explicit S(int x, int y); }; S f() { return {12, 34}; // ill-formed return S{12, 34}; // ok }
C++11
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When applied to a conversion function, prevents that conversion function from being used to perform implicit conversions.
class C { const int x; public: C(int x) : x(x) {} explicit operator int() { return x; } }; C c(42); int x = c; // ill-formed int y = static_cast<int>(c); // ok; explicit conversion
