CODEWITHSUNDEEP
c++templatesoperator-overloading
sigvardsen
sigvardsen

Reputation: 1531

Overloading assignment operator for template class

I am trying to overload assignment operators for two different template classes but with the same template type:

template <class DataType> class Foo
{
    public: Foo<data_type>& operator=(Bar<data_type> const &bar);
};
template <class DataType> class Bar
{
    public: Bar<data_type>& operator=(Foo<data_type> const &foo);
};

However when I try:

Foo<int> a;
Bar<int> b = a;

I get the error:

No viable conversion from 'Foo< int >' to 'Bar< int >'.

How do I achieve this?

Upvotes: 0

Views: 1562

Answers (2)

Andy Prowl
Andy Prowl

Reputation: 126412

When you write:

Bar<int> b = a; // ERROR! No viable user-defined conversion sequence

You are copy-initializing object b from object a. This is not the same as assigning object a to an already constructed object b, in spite of the = symbol being used.

With copy-initialization, the compiler has to look for a user-defined conversion sequence that can convert a into an object of type Bar<int>, from which b could be eventually copy-constructed or move-constructed.

Assignment, on the other hand, would work:

Foo<int> a;
Bar<int> b;
b = a; // OK

Upvotes: 1

fredoverflow
fredoverflow

Reputation: 263058

Conversions are done via copy constructors, not assignment operators. So you want to implement:

Bar(const Foo<data_type>& foo);

Upvotes: 1

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