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Showing content from https://timsong-cpp.github.io/cppwp/n4140/class.conv.fct below:

[class.conv.fct]

A member function of a class X having no parameters with a name of the form

conversion-function-id:
    operator conversion-type-id

conversion-type-id:
    type-specifier-seq conversion-declaratoropt

conversion-declarator:
    ptr-operator conversion-declaratoropt

specifies a conversion from X to the type specified by the conversion-type-id. Such functions are called conversion functions. No return type can be specified. If a conversion function is a member function, the type of the conversion function ([dcl.fct]) is “function taking no parameter returning conversion-type-id”. A conversion function is never used to convert a (possibly cv-qualified) object to the (possibly cv-qualified) same object type (or a reference to it), to a (possibly cv-qualified) base class of that type (or a reference to it), or to (possibly cv-qualified) void.118

[ Example:

struct X {
  operator int();
};

void f(X a) {
  int i = int(a);
  i = (int)a;
  i = a;
}

In all three cases the value assigned will be converted by X::operator int().  — end example ]

A conversion function may be explicit ([dcl.fct.spec]), in which case it is only considered as a user-defined conversion for direct-initialization ([dcl.init]). Otherwise, user-defined conversions are not restricted to use in assignments and initializations. [ Example:

class Y { };
struct Z {
  explicit operator Y() const;
};

void h(Z z) {
  Y y1(z);            Y y2 = z;           Y y3 = (Y)z;      }

void g(X a, X b) {
  int i = (a) ? 1+a : 0;
  int j = (a&&b) ? a+b : i;
  if (a) {
  }
}

 — end example ]

The conversion-type-id shall not represent a function type nor an array type. The conversion-type-id in a conversion-function-id is the longest possible sequence of conversion-declarators. [ Note: This prevents ambiguities between the declarator operator * and its expression counterparts. [ Example:

&ac.operator int*i;                                         

The * is the pointer declarator and not the multiplication operator.  — end example ]  — end note ]

Conversion functions are inherited.

Conversion functions can be virtual.

Conversion functions cannot be declared static.

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