Question

I just learned about typedef. Suppose I have an instance:

private:
  typedef std::string int doubles abc;

when I make an accessors to instance abc:

returnType get(){...}

what should I put in the returnType? is it abc or the data type? thx

Answer 1

I’m not sure what you wanted to do here. Your typedef is invalid syntax, and “doubles” is no C++ type (but probably just a typo). Examples for valid syntax would be:

typedef std::string abc;
typedef int foo; 
...

You can then use the typedef in function signatures, just like other types:

abc getValue();

Although you should put the typedef in the public part of your class if you want to use it in public member functions. And you should be aware that code outside of your class will always have to prefix the typedef with the name of your class, unless it is typedef‘d again.

class SomeClass {
public:
    typedef std::string foo;
};

// Somewhere outside SomeClass
SomeClass::foo bar = ...
typedef SomeClass::foo localFoo;
localFoo fooBar = ...;

Apart from that: typedef does not provide any way to have a variable represent more than one type. C++ is a statically typed language, so this is not directly possible. You can, however:

• Use polymorphic classes with a common interface, instances of derived classes can then be treated like instances of the base class, but do different things
• Use a union and some discriminator to store what type it is currently storing
• Use void* and casting – Not recommended!
• Use something like boost::variant, boost::any etc. like suggested by others

Edit: Finally, on your use of the term “instance”: It is usally used to refer to an instance of a class, i.e. a particular object belonging to a class. What you mean is a “member variable”.