explicit specifier
介紹
TL;DR: explicit 是控制顯性或隱性轉換
標上 explicit 就表示不接受隱式轉換或 copy-initialization， 可以套用在兩個地方：
constructor
conversion function
convertion function 指的是沒有明確的回傳型別的 member function， 而標上 explicit 後就必須使用顯示轉換， 例如：
struct X {
//implicit conversion
operator int() const { return 7; }
// explicit conversion
explicit operator int*() const { return nullptr; }
}
int main() {
X x;
int n = static_cast<int>(x); // OK: sets n to 7
int m = x; // OK: sets m to 7
int* p = static_cast<int*>(x); // OK: sets p to nullptr
// int* q = x; // Error: no implicit conversion
int (*pa)[3] = x; // OK
}
標上 explicit 的 constructor 則必須使用 direct-initialization， 不能用 copy-initialization， 相對的沒有標 explicit 的 constructor 又被稱為 converting constructor，
完整範例：
class A {
public:
A(int) { } // converting constructor
A(int, int) { } // converting constructor (C++11)
operator bool() const { return true; }
};
class B {
public:
explicit B(int) { }
explicit B(int, int) { }
explicit operator bool() const { return true; }
};
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
if (a1) {} // OK: A::operator bool()
bool na1 = a1; // OK: copy-initialization selects A::operator bool()
bool na2 = static_cast<bool>(a1); // OK: static_cast performs 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 does not consider B::B(int,int)
B b5 = (B)1; // OK: explicit cast performs static_cast
if (b2) {} // OK: B::operator bool()
// bool nb1 = b2; // error: copy-initialization does not consider B::operator bool()
bool nb2 = static_cast<bool>(b2); // OK: static_cast performs direct-initialization
}