How can I overload empty operator of struct?

Question

I want to overload a function to check if a struct object is empty.

Here is my struct definition:

struct Bit128 {
    unsigned __int64 H64;
    unsigned __int64 L64;

    bool operate(what should be here?)(const Bit128 other) {
        return H64 > 0 || L64 > 0;
    }
}

This is test code:

Bit128 bit128;
bit128.H64 = 0;
bit128.L64 = 0;
if (bit128)
    // error
bit128.L64 = 1
if (!bit128)
    // error

Look up `operator bool()`. As a member function that takes no arguments. — Peter, Mar 20 '17 at 12:48
[c] doesn't have methods in structs, or otherwise, please tag your questions accordingly from now on, thanks. — Antti Haapala -- Слава Україні, Mar 20 '17 at 12:48

score 4 · Answer 1 · answered Mar 20 '17 at 12:48

4

You want to overload the bool operator:

explicit operator bool() const {
 // ...

This operator doesn't have to be, but should be, a const method.

answered Mar 20 '17 at 12:48

Sam Varshavchik

114,536
5
94
148

Why `explicit` if the OP's use case has an implicit conversion desired? – AndyG Mar 20 '17 at 13:02
1

@AndyG Because `explicit operator bool` can be used in places where it can be [contextually converted to bool](http://stackoverflow.com/a/39995574/7571258). This is still more restrictive than implicit conversion so it should be the default way of declaring `operator bool`. – zett42 Mar 20 '17 at 13:14
@zett42: I see. Thanks! – AndyG Mar 20 '17 at 13:24

Pixelchemist · Accepted Answer · 2017-03-20T20:47:11.693

4

#include <cstdint>
struct Bit128 
{
    std::uint64_t H64;
    std::uint64_t L64;
    explicit operator bool () const {
        return H64 > 0u || L64 > 0u;
    }
};

edited Mar 20 '17 at 20:47

answered Mar 20 '17 at 12:48

Pixelchemist

24,090
7
47
71

This should be [`explicit operator bool()`](http://stackoverflow.com/q/6242768/7571258) unless OP doesn't have C++11 available in which case the `safe bool` idiom should be used, e. g. one of these [boost implementations](http://stackoverflow.com/a/11854479/7571258). – zett42 Mar 20 '17 at 19:44
@zett42: Would be safer, yes. – Pixelchemist Mar 20 '17 at 20:47

score 2 · Answer 3 · answered Mar 20 '17 at 12:48

There's no "empty" operator, but if you want the object to have a significance in boolean contexts (such as if-conditions), you want to overload the boolean conversion operator:

explicit operator bool() const {
  return H64 != 0 || L64 != 0;
}

Note that the explicit conversion operator requires C++11. Before that, you can use a non-explicit operator, but it comes with many downsides. Instead you'll want to google for the safe-bool idiom.

score 2 · Answer 4 · edited May 23 '17 at 12:00

2

The syntax you are looking for is explicit operator bool() const which is safe in c++11 and later

edited May 23 '17 at 12:00

Community

1
1

answered Mar 20 '17 at 12:49

Caleth

52,200
2
44
75

How can I overload empty operator of struct?

4 Answers4