转换构造函数
来自 cppreference.cn
未用说明符 explicit 声明且能以单个参数调用的构造函数(直至 C++11) 称为转换构造函数。
与仅在直接初始化（包括显式转换，如 static_cast）期间考虑的显式构造函数不同，转换构造函数也会在复制初始化期间被考虑，作为用户定义转换序列的一部分。
据说转换构造函数指定了从其参数类型（如果有）到其类类型的隐式转换。请注意，非显式用户定义转换函数也指定了隐式转换。
隐式声明和用户定义的非显式复制构造函数和移动构造函数都是转换构造函数。
[编辑] 示例
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struct A { A() { } // converting constructor (since C++11) A(int) { } // converting constructor A(int, int) { } // converting constructor (since C++11) }; struct B { explicit B() { } explicit B(int) { } explicit B(int, int) { } }; int main() { A a1 = 1; // OK: copy-initialization selects A::A(int) A a2(2); // OK: direct-initialization selects A::A(int) A a3{4, 5}; // OK: direct-list-initialization selects A::A(int, int) A a4 = {4, 5}; // OK: copy-list-initialization selects A::A(int, int) A a5 = (A)1; // OK: explicit cast performs static_cast, direct-initialization // B b1 = 1; // error: copy-initialization does not consider B::B(int) B b2(2); // OK: direct-initialization selects B::B(int) B b3{4, 5}; // OK: direct-list-initialization selects B::B(int, int) // B b4 = {4, 5}; // error: copy-list-initialization selected an explicit constructor // B::B(int, int) B b5 = (B)1; // OK: explicit cast performs static_cast, direct-initialization B b6; // OK, default-initialization B b7{}; // OK, direct-list-initialization // B b8 = {}; // error: copy-list-initialization selected an explicit constructor // B::B() [](...){}(a1, a4, a4, a5, b5); // may suppress "unused variable" warnings }
