CODEWITHSUNDEEP
c++
polmonroig
polmonroig

Reputation: 1017

Convert string to custom class

Is there a way to convert a string to a custom class, for example if I have a class named Numb, but I want to declared it as a string with the = operator, can I overload it?

class Numb{
      std::string x;
};

int main(){
    Numb n = "32";
   //Creates a Numb and makes x = "32"
}

Upvotes: 2

Views: 645

Answers (3)

Jack Meagher
Jack Meagher

Reputation: 306

You want to construct Numbs from a string literal. String literals are indistinguishable from type const char * String literals have type const char [N], which we can take as an argument by writing a function that accepts const char *.

To define a converting constructor with this behavior, just write a signature like that of a copy constructor, but instead of expecting an argument of the same type, expect an argument of type const char *. Its signature would look like Myclass(const char *);

Alternatively, you can copy or move construct from strings, but that would require doing Numb n = std::string { "32" }; or similar, to convert the string constant to a std::string.

Here is some sample code, in which main() returns 3. Here we also demonstrate what to do with the value: if we instead did Num n2 = std::string { "TRAP" };, the code would return 1. If we did Num n2 = std::string { "ANYTHING OTHER THAN TRAP" }; it would return 2.

#include <string>

struct Num {
    Num()
      : _val(2) {}

    Num(const std::string & str) {
        if (str == "TRAP") {
            _val = 1;
        } else {
            _val = 2;
        }
    }

    Num(const char * s) {
        _val = 3;
    }

    int _val;    
};

int main(void) {
    // Num n = std::string { "TRAP" }; // returns 1
    // Num n = std::string { "NOTTRAP" }; // returns 2
    Num n = "TRAP";
    return n._val;
}

https://godbolt.org/g/Lqwdiw

EDIT: Fix a mistake re the type system, take the string arg as & not &&, simplify example, update compiler explorer link.

Upvotes: 2

Sid S
Sid S

Reputation: 6125

If you make x a public member, you can assign to it like this:

class Numb
{
    public:
        std::string x;
};

int main()
{
    Numb n{ "32" };
    Numb o = { "33" };
    n = { "34" };
    o.x = "35";
}

Upvotes: 1

ehargitt
ehargitt

Reputation: 91

Yes, you can using converting constructors. Something like:

struct A {
  A(std::string);
  // A is a struct, so str is public
  std::string str;
};

// implementation for converting constructor
A::A(std::string s) {
  str = s;
}

int main() {
  A my_a = std::string("hello");
  std::cout << my_a.str << '\n';
}

Sometimes you might not want this behavior. You can mark the constructor as explicit to disable it.

Upvotes: 2

Related Questions