353. std::pair missing template assignment

Section: 22.3 [pairs] Status: Resolved Submitter: Martin Sebor Opened: 2001-12-02 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The class template std::pair defines a template ctor (20.2.2, p4) but no template assignment operator. This may lead to inefficient code since assigning an object of pair<C, D> to pair<A, B> where the types C and D are distinct from but convertible to A and B, respectively, results in a call to the template copy ctor to construct an unnamed temporary of type pair<A, B> followed by an ordinary (perhaps implicitly defined) assignment operator, instead of just a straight assignment.

Proposed resolution:

Add the following declaration to the definition of std::pair:

    template<class U, class V>
    pair& operator=(const pair<U, V> &p);

And also add a paragraph describing the effects of the function template to the end of 20.2.2:

    template<class U, class V>
    pair& operator=(const pair<U, V> &p);

Effects: first = p.first; second = p.second; Returns: *this

[Curaçao: There is no indication this is was anything other than a design decision, and thus NAD.  May be appropriate for a future standard.]

[ Pre Bellevue: It was recognized that this was taken care of by N1856, and thus moved from NAD Future to NAD EditorialResolved. ]