template and const

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2 total messages Started by Lars Farm Thu, 28 Mar 1996 17:31

#3853

Author: Lars Farm
Date: Thu, 28 Mar 1996 17:31

47 lines
2112 bytes

Is "pair<T1,T2>" valid as function argument where the 
declaration expects "const pair<const T1,T2>&"?

This comes up in the standard library map<K,V>::insert and make_pair

    template <class T1, class T2>
    struct pair {
      T1 first;
      T2 second;
      pair();
      pair(const T1& x, const T2& y);
    };

    template <class T1, class T2> 
    pair<T1,T2> make_pair(const T1&, const T2&);

    template <class Key, class T,
              class Compare = less<Key>,
              class Allocator = allocator>
    class map {
      ...
      typedef pair<const Key, T> value_type;
      ...
      pair<iterator, bool> insert(const value_type& x);
      ...
    };

void f( string a, string b )
{
    map<string,string> m;
    ...
    m.insert( make_pair(a,b) ); // valid c++?

make_pair yields a temporary of type pair<string,string>. temporaries are
const. insert() wants a const pair<const string,string>. So they should
both be a pair of constant strings, right? but the compilers I have access
to disagrees.

--
Lars Farm, lars.farm@nts.mh.se
---
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Re: template and const

#3860

Author: Nathan Myers, http://www.cantrip...
Date: Fri, 29 Mar 1996 16:09

32 lines
1945 bytes

Lars Farm <lars.farm@nts.mh.se> wrote:
> 
> Is "pair<T1,T2>" valid as function argument where the
> declaration expects "const pair<const T1,T2>&"?

[It would allow: ]
> void f( string a, string b )
> {
>     map<string,string> m;
>     ...
>     m.insert( make_pair(a,b) ); // valid c++?  [yes, now.]

The definition of the template pair<> was changed at the March '96
meeting specifically to allow this. 

pair<> now has a template constructor.  Of course that is of little
use to you (for now) if you (like most of us) don't yet have a compiler
that supports member templates.  In the meantime you might like to 
write a little template function, make_map_pair<>().

If we had type deduction from constructor arguments, even make_pair<>()
would be unnecessary.   Next time, next time.

Nathan Myers
ncm@cantrip.org  http://www.cantrip.org/

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