[cfe-dev] Explicit template arguments in std::make_pair

[Csaba Raduly via cfe-dev]

This reminds me of STL's presentation "Don't help the compiler":

https://www.youtube.com/watch?v=AKtHxKJRwp4

Csaba

On Thu, Nov 16, 2017 at 6:39 PM, Roger Ferrer Ibanez via cfe-dev
<cfe-dev at lists.llvm.org> wrote:
> Hi Krzysztof,
>
> The reason is that in C++03 make_pair looked like this
>
>     template <class T1, class T2>
>     pair<T1, T2> make_pair(T1 x, T2 y);
>
> while in C++11 onwards looks like this
>
>      template <class T1, class T2>
>      pair<V1, V2> make_pair(T1&& x, T2&& y)
>
> where in libcxx (similarly for libstdc++) V1 and V2 are implemented like
>
>      pair<typename __make_pair_return<_T1>::type, typename __make_pair_return<_T2>::type>
>
> because the C++ standard dictates a more sophisticated (compile-time computed) type for V1 and V2.
>
> This implies that if you define T1 and T2 in the template-argument list of the function call, you are always going to get rvalue references in the parameter types (in your example 'int&&' and 'unsigned int&&' respectively).
> Conversely, if you let template-argument deduction to infer T1 and T2, it will infer
>
>   std::pair<int, unsigned int> std::make_pair<int, unsigned int &>(int &&, unsigned int &);
>
> because of the C++11 rules about type deduction involving parameter types of the form Type&&.
>
> I'm not an expert in this area but this behaviour looks to me as expected and matches the C++ view of generic programming in which you should not constraint (in this case using template-arguments) if not needed.
>
> Hope this answers your question.
>
> Kind regards,
> Roger
>
>> -----Original Message-----
>> From: cfe-dev [mailto:cfe-dev-bounces at lists.llvm.org] On Behalf Of
>> Krzysztof Parzyszek via cfe-dev
>> Sent: 16 November 2017 17:21
>> To: Clang Dev
>> Subject: [cfe-dev] Explicit template arguments in std::make_pair
>>
>> Hi,
>>
>> This simple program fails to compile with -std=c++11, but compiles ok
>> with c++03. This seems consistent between libc++/libstdc++. Is this due
>> to a change in the standard? What is the reason behind it?
>>
>>
>>
>> #include <utility>
>>
>> std::pair<int,unsigned> good(unsigned L) {
>>    return std::make_pair(0, L);
>> }
>>
>> std::pair<int,unsigned> bad(unsigned L) {
>>    return std::make_pair<int,unsigned>(0, L);
>> }
>>
>>
>> clang++ -c -std=c++11 pp.cpp -stdlib=libc++
>> pp.cpp:8:10: error: no matching function for call to 'make_pair'
>>    return std::make_pair<int,unsigned>(0, L);
>>           ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
>> /w/c/org/bin/../include/c++/v1/utility:639:1: note: candidate function not
>>        viable: no known conversion from 'unsigned int' to 'unsigned int
>> &&' for
>>        2nd argument
>> make_pair(_T1&& __t1, _T2&& __t2)
>> ^
>> 1 error generated.
>>
>>
>> -Krzysztof
>>
>> --
>> Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum,
>> hosted by The Linux Foundation
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