参考文档
user-defined conversion function - cppreference.com
The Safe Bool Idiom - 知乎
一般形式为operator type() const, 比如:
1operator int() const;
2operator bool() const;
3operator AA() const;
自定义类型转换
1struct To
2{
3 To() = default;
4 To(const struct From&) {} // converting constructor
5};
6
7struct From
8{
9 operator To() const {return To();} // conversion function
10};
11
12int main()
13{
14 From f;
15 To t1(f); // direct-initialization: calls the constructor
16 // Note: if converting constructor is not available, implicit copy constructor
17 // will be selected, and conversion function will be called to prepare its argument
18
19// To t2 = f; // copy-initialization: ambiguous
20 // Note: if conversion function is from a non-const type, e.g.
21 // From::operator To();, it will be selected instead of the ctor in this case
22
23 To t3 = static_cast<To>(f); // direct-initialization: calls the constructor
24 const To& r = f; // reference-initialization: ambiguous
25}
为什么operator bool()需要用explicit修饰?
c++ - Why does declaring an operator bool() const member overload the [] operator? - Stack Overflow
The operator is coming from the built-in subscript operator which treats expressions A[B] as *(A + B).
This results in the evaluation of *(1 + "wut") => 'u', which then causes the if condition to pass, as 'u' is a non-zero value.
Declare your member as explicit operator bool() to prevent your type from being implicitly converted to other integral types.
1#include <iostream>
2using namespace std;
3
4struct Test {
5 operator bool() const {
6 return true;
7 }
8};
9
10int main(int argc, char** argv) {
11 Test test;
12
13 if (test)
14 cout << "Object test is true\n";
15
16 if (test["wut"])
17 std::cout << "Success (test[\"wut\"])\n";
18}
19
20// Output:
21// Object test is true
22// Success (test["wut"])
一个operator bool()的坑
c++ - Why is my “explicit operator bool()” not called? - Stack Overflow