CODEWITHSUNDEEP
c++
crommy
crommy

Reputation: 493

What's the difference in these overloading of << operator

So I'd like some clarification over overloading the print operator <<

now this is how I have learned to do it:

1) ostream& operator<<(ostream& os, const Vector& v1)

in this example we've got some Vector class that holds 3 integers.

I saw somebody try this instead:

2) ostream& operator<<(ostream& os, Vector& v1)

so the difference is v1 is just a reference and not a const reference. The compiler then couldn't print any Vector objects that weren't Lvalues for example he also overloaded the * operator to be able to do things such as v1 * 2, simple vector times a scalar. for example :

std::cout << v1 << std::end - this works (v1 is a vector) std::cout << v1 * 2 << std::end - this does not work

However when we changed the signature to be by value:

3) ostream& operator<<(ostream& os, Vector v1)

his print was able to work... and I'm left wondering why this all happened?

Why couldn't the compiler recognize overload (2) for v1 * 2 but why sending it by value to (3) did work

Upvotes: 0

Views: 82

Answers (1)

Jans
Jans

Reputation: 11250

v1 * 2 would return a temporary, the second overload can't be used because non-const lvalue references don't bind to temporaries.

The third overload is used copying the value returned by v1 * 2

Upvotes: 0

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