toString() const string&

I've a variable called val of Type Var.
Var Class have a tostring() methods that returns const string& type.
But When I compile it compiler is firing this Error on the following line.

Code:

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key+"="+val.toString()+"; at=";

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Code:

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error: passing ‘const Var’ as ‘this’ argument of ‘const std::string& Var::toString()’ discards qualifiers

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Code of toString() method

Code:

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const string& Var::toString(){
        return (const string&)__data;//__data is of type string (just string not a reference or a const)
}

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don't return a const type, that doesn't make sense here. Why do you return a reference and not just an object?

Const-correctness is something that applies to arguments of functions, not of their return types. Remember: try to avoid declaring variables as 'const' (like in a class).

Code:

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string Var::toString()
{
        return static_cast<string>(__data);
}

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A nice example why you should use C++ style casts : what does '(const string&)__data' do?

This function should be declared as const (because it returns data and doesn't modify it), otherwise you can't call it on constant instances. E.g:

Code:

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void foo(const Var& v)
{
    v.toString(); //discards qualifiers error, unless toString is a const method
}

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As to casts:
if you want to return a reference, you don't need a cast. This is how you normally make a reference to something (i doesn't have to be a reference for a reference to refer to it):

Code:

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    int i = 42;
    int& ref = i; //note: no cast!

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If __data is a std::string then casting it to std::string is meaningless.

I seem to remember that you had other rather meaningless casts in your code, such as:

Code:

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Var x = (Var)42;

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If it didn't already work without that cast (because Var has a non-explicit constructor that takes an integer argument), then the cast wouldn't do the right thing either. Normal ways to declare and initialize an instance:

Code:

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Var x(42);
Var x = 42; //if the constructor is not explicit
Var x = Var(42); //copy from a temporary

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@annon
exactly it fixed after I changed it to

Code:

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const string& toString() const{
  //
}

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Thanks