c++referenceconstantstemporarytemporary-objects

How to prevent temporary materialization for const reference parameters


I need to write a class whose constructor takes a constant reference to a object and stores it locally.

In order to avoid most common mistakes I can foresee, I'd like to only accept references to non-temporary (ie: references to lvalues).

How can I write a function that takes constant references to non-temporary only?


Of course even a non-temporary could go out of scope and thus break my class behavior, but I believe that by disallowing temporary references I will avoid most mistakes.


Solution

  • If you are going to store a reference and need to use it after the constructor has completed, it's probably best for the constructor to take a pointer:

    struct C {
        C(const X* p) : p_(p) { }
    
        const X* p_;
    };
    

    This way, it's pretty much guaranteed that you won't have a pointer to a temporary (unless X does something really dumb, like overloading the unary & to return this).

    If the constructor takes a pointer, it's also clearer to users of the class that they need to pay attention to the lifetime of the X object they pass to the constructor.