Question

I’d really like to be able to assign a std::string object from a DecoratedString object that I’m writing.

class DecoratedString
{
private:
    std::string m_String;
public:
     DecoratedString(const std::string& initvalue)
     : m_String(initvalue)
     {
     }

     const std::string& ToString() const
     {
         return m_String;
     }

     const std::string& operator=(const DecoratedString& rhs)
     {
         return rhs.ToString();
     }

}

I’ve written a unit test to make sure this works:

void DecoratedStringTest::testAssignmentToString()
{
    std::string expected("test");
    DecoratedString sut(expected);
    std::string actual;
    actual = sut;

    CPPUNIT_ASSERT_EQUAL(actual, sut.ToString());
}

However, the compiler says error: no match for 'operator=' in 'actual = sut'. It then lists the overloaded operator= options from the standard library.

Why isn’t the compiler finding the operator= I defined?

EDIT:

So I guess I need a conversion operator, not an assignment operator. Huge thanks to the people that saw what I was trying to do and explained what I should do instead.

Answer 1

The operator = you have defined is for assigning decorated strings to other decorated strings and returning an std::string from that assignment.

What you want is a member “conversion operator” that automatically converts a decorated string to an std::string whenever required, like this:

operator std::string const &() const { return ToString(); }

That will also convert a decorated string automatically to a std::string const & whenever one is needed (i.e. when comparing to an std::string, or passing a DecoratedString to a function which takes a std::string const &).