Operator overloading << error

Question

I get the compiler error

no match for 'operator<<' in 'std::cout << VertexPriority(2, 4u)'

In the main class referred to this operator overloading, but I can't undestand where the error is.

Here there is the operator overloading line, I implemented it inside the class definition.

std::ostream& operator<<(std::ostream& out) const { return out << "Vertex: " << this->vertex << ", Priority: " << this->priority; }

vertex and priority are integer and unsigner integer.

In the main class I'm trying to doing this:

std::cout << VertexPriority(2, 3) << std::endl;

You don't define insertion operators like that, unless it is your intention to insert an ostream in to your object (which I can all-but-guarantee it is not). See the section on common operator overloading in this answer. — WhozCraig
– WhozCraig, Commented Nov 3, 2013 at 10:40
See the linked article in my prior comment or click here — WhozCraig
– WhozCraig, Commented Nov 3, 2013 at 10:42

Paul Draper · Accepted Answer · 2013-11-03 10:47:21Z

2

Define it like this:

class VertexPriority {
    ...

    friend std::ostream& operator<< (std::ostream& out, const VertexPriority& vp);
};

std::ostream& operator<< (std::ostream& out, const VertexPriority& vp) {
    return out << "Vertex: " << vp.vertex << ", Priority: " << vp.priority;
}

The friend keyword is necessary if VertexPriority::vertex or VertexPriority::priority are not public.

For more help, read this tutorial: http://www.learncpp.com/cpp-tutorial/93-overloading-the-io-operators/

edited Nov 3, 2013 at 10:47

answered Nov 3, 2013 at 10:41

Paul Draper

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3 Comments

giacomotb Over a year ago

ok, but now the error is [Linker error] undefined reference to `operator<<(std::ostream&, VertexPriority&)' I changed a bit your code because it doesn't let me use pointers(vp->priority) so I used the already implemented getter (vp.getVertex)

giacomotb Over a year ago

Fixed, I don't know why but I deleted a 'const' before VertexPriority in the function declaration

Paul Draper Over a year ago

If you used getVertex and that was not declared const, then you would need to.

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