Compile error "No global operator found" for templated overloaded operator

Question

I've defined a template class and overloaded operators. When compiling, I get the following error message:

error C2677: binary '+=' : no global operator found which takes type 'Class' (or there is no acceptable conversion)

Here is the relevant code of the class:

template<int foo>
class Class
{
private:
    int value;
public:
    template<int other_foo>
    friend class Class;

    // Constructors and copy constructors for several data type 
    // including other possibilities of "foo"; all tested.

    Class& operator+=(const Class& x)
    {
        value += x.value;
        return *this;
    }
    template<class T>
    Class& operator+=(const T& x)
    {
        this += Class(x);
        return *this;
    }
};

If I create two objects of, e.g., Class<3>; the operator += works fine and does the right thing.

However, if I have an object of Class<3> and one of Class<2>, I get the above error which points at the line where "+=" is defined for T [the constructor for different value of foo works fine and is also tested].

What am I doing wrong? How can I resolve this error? The operator is defined, just a few lines above.

Mike Seymour · Accepted Answer · 2015-05-07 13:16:33Z

3

Assuming that the necessary constructor does indeed exist and work correctly, the error in the code you've posted is

this += Class(x);

which tries to modify the value of the immutable pointer this. It should be

*this += Class(x);

answered May 7, 2015 at 13:16

Mike Seymour

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1 Comment

Markus Klein Over a year ago

Thanks a lot! That was the problem.

rodrigo · Accepted Answer · 2015-05-07 13:19:24Z

1

I think that there are two problems, both in this line:

this += Class(x);

One: the object added to should be *this instead of this, because the latter is a pointer, not the object itself.

Two: There is no conversion constructor from T to Class. That is, you cannot convert from Class<3> to Class<2> so the Class(x) will not compile. The solution would be to add it:

template<int other> Class(const Class<other> &o)
{}

answered May 7, 2015 at 13:19

rodrigo

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1 Comment

Markus Klein Over a year ago

Thanks! It now works. Thanks for the comment with the constructor from T to Class; this was already done but not included in my code snippet to not have a long code above.

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