inherit from std::string or operator overloading

Question

I want to have a class like below:

Class Test {
  Test();
  ~Test();
  ...
}

I want to be able to use following statement:

std::string str;
Test t;
str = t;

what should I do? should I override to_string? If yes it seems that it is not possible to inherit from std::string class. Or I have to override special operator? What about Pointer assignment? like below:

std::string str;
Test* t = new Test();
str = t;

TartanLlama · Accepted Answer · 2016-02-16T12:03:18.080

7

You can provide a user-defined conversion operator to std::string:

class Test {
  //...
public:
  operator std::string () const {
    return /*something*/;
  }
};

This will allow a Test object to be implicitly-converted to a std::string.

edited Feb 16 '16 at 12:03

answered Feb 16 '16 at 11:59

TartanLlama

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`Test::operator std::string() const` just my 2c – iksemyonov Feb 16 '16 at 12:01
what should I do If I want to assign pointer of Test class to std::string? – JGC Feb 16 '16 at 12:27
I'd recommend you don't. Or do this: `std::string tmp = my_test; doSomething(&tmp);` – TartanLlama Feb 16 '16 at 12:32
I need to. I did not get your last comment answer. I have a pointer of my Test Class and I need to assign it to a string. – JGC Feb 16 '16 at 12:36
http://stackoverflow.com/questions/35432898/assign-a-class-pointer-to-stdstring-in-c – JGC Feb 16 '16 at 12:37
@JGC Sorry, I misunderstood, I thought you meant assigning a `Test*` to a `std::string*`, not a `Test*` to a `std::string`. `str = *t;` should work fine if you have the above operator. – TartanLlama Feb 16 '16 at 12:38
@TartanLlama I can not add * before pointers because of reasons. Is it possible to do other thing? – JGC Feb 16 '16 at 12:49
@JGC Can you write a function to do the indirection for you? – TartanLlama Feb 16 '16 at 12:54
Thank you for your replies. I have solved it using above answer. – JGC Feb 16 '16 at 12:58

score 2 · Answer 2 · edited May 23 '17 at 10:28

Although in C++ it is possible, it is generally not advised to inherit from standard classes, see Why should one not derive from c++ std string class? for more info.

I am wondering, what is the function of the assignment of

str = t;

str is a std::string type, t is Test type. So what is the expected value of the assigment? No one can guess. I suggest to explicitly call a conversion method or an operator for code clarity.

This would make your example look like:

str = t.convertToString();

or the more standard way is to implement the stream operator, which makes

str << t;

(Note this example works if str is a stream type, if it is a string, you need further code, see C++ equivalent of java.toString?.)

But, if you really want it to work without a conversion method, you can override the string assignment operator of Test:

class Test {
public:
    operator std::string() const { return "Hi"; }
}

inherit from std::string or operator overloading

2 Answers2