CODEWITHSUNDEEP
c++referenceconstants
SmileyCraft
SmileyCraft

Reputation: 257

Can c++ 'pass by reference if possible'?

I have a method with a parameter that is read-only.

template <typename T>
void foo(const T t) {...}

This definition has the benefit that foo(2) is syntactically correct. However, it has the downside that applying foo on a big object makes it copy the object. Since I only read from the argument, this is just a waste of time and memory.

The standard solution is to make the parameter t a const reference. However, this has the downside that foo(2) is not syntactically correct anymore. I would have to do int i = 2; foo(i); which I find unfortunate.

Is there a way to get the best of both worlds? So such that foo(2) is syntactically correct and applying foo on an object makes it pass by reference?

EDIT: Apparently a const reference is already the best of both worlds. Now I would like to know why this is. Why is foo(2) syntactically correct? How can 2 become a reference here?

Upvotes: 1

Views: 208

Answers (1)

Yakk - Adam Nevraumont
Yakk - Adam Nevraumont

Reputation: 275405

C++ will materialize temporaries when needed, and you can bind a temporary to a const&, but not a &.

So foo_value(2) creates an int with value 2 called t as a parameter. foo_cref(2) creates an anonymous temporary, then binds the int const& t parameter to it, where foo_value and foo_cref are the obvious variants of foo you talk about in the question.

Upvotes: 1

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