c++ - Conversion from Custom Classes to built in types

Question

For the sake of clarity Let my new class be:

class MyInt{ public: MyInt(int x){theInt = x /10;} int operator+(int x){return 10 * theInt + x;} private int theInt; };

let's say I want to be able to define:

MyInt Three(30); int thirty = Three;

But in order to get this result I'm writing:

MyInt Three(30); int thirty = Three + 0;

how can I get automatic conversion from my Custom class to a built-in type?

Remark: it's usually a bad idea to have both conversions implicit (here int->MyInt via the non-explicit constructor and MyInt->int via a conversion operator). (Consider for example std::string, for which there is an implicit conversion from const char* (converting constructor) but not to const char* (for this you need to call .c_str() or related member functions).) [Also, typo: private -> private:] — gx_
– gx_, Commented Aug 30, 2013 at 13:58

jrok · Accepted Answer · 2013-08-30 14:06:38Z

19

With a type conversion function:

class MyInt{ public: MyInt(int x){theInt = x /10;} int operator+(int x){return 10 * theInt + x;} operator int() const { return theInt; } // <-- private int theInt; };

edited Aug 30, 2013 at 14:06

answered Aug 30, 2013 at 13:50

jrok

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5 Comments

Zac Howland Over a year ago

With the added note that doing this should be transparent. That is, if you are converting to an int, the class should be a numeric class and not some custom class that does something totally unrelated.

gx_ Over a year ago

It should be const (but operator+ too, if not a non-member function), and should probably return 10 * theInt according to OP's weirdness.

jrok Over a year ago

@gx_ thanks, I agree about const. I'll let OP do the arithmetic :)

qkhhly Over a year ago

how about more complex types, like std::vector<int> or std::vector<std::tuple<int, long>>? how do i determine what operator i should write? can you give some examples?

jrok Over a year ago

@qkhhly You can't write conversion operators from standard library classes (vector, etc) if that's what you mean. From a custom class written by you to a vector? Well, nothing special about them. The name of the type conversion function is the target type, so for example operator std::vector<int>() { /* code here */ }

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