[llvm-dev] Clang++: No member name 'make_unique' in namespace 'std'

[Mehdi AMINI via llvm-dev]

On Thu, Feb 11, 2021 at 2:21 PM users users via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Dear LLVM Developers:
>
> 1.   Recently I built llvm/12.0 on IBM power8 using gcc/8.2.0. When I run
> clang++ with an example from
> https://en.cppreference.com/w/cpp/memory/unique_ptr/make_unique:
>
>  #include <iostream>
>
> #include <iomanip>#include <memory>
>  struct Vec3{
>     int x, y, z;
>
>     // following constructor is no longer needed since C++20
>     Vec3(int x = 0, int y = 0, int z = 0) noexcept : x(x), y(y), z(z) { }
>
>     friend std::ostream <http://en.cppreference.com/w/cpp/io/basic_ostream>& operator<<(std::ostream <http://en.cppreference.com/w/cpp/io/basic_ostream>& os, const Vec3& v) {
>         return os << "{ x=" << v.x << ", y=" << v.y << ", z=" << v.z << " }";
>     }};
>  int main(){
>     // Use the default constructor.
>     std::unique_ptr <http://en.cppreference.com/w/cpp/memory/unique_ptr><Vec3> v1 = std::make_unique<Vec3>();
>     // Use the constructor that matches these arguments
>     std::unique_ptr <http://en.cppreference.com/w/cpp/memory/unique_ptr><Vec3> v2 = std::make_unique<Vec3>(0,1,2);
>     // Create a unique_ptr to an array of 5 elements
>     std::unique_ptr <http://en.cppreference.com/w/cpp/memory/unique_ptr><Vec3[]> v3 = std::make_unique<Vec3[]>(5);
>
>     std::cout <http://en.cppreference.com/w/cpp/io/cout> << "make_unique<Vec3>():      " << *v1 << '\n'
>               << "make_unique<Vec3>(0,1,2): " << *v2 << '\n'
>               << "make_unique<Vec3[]>(5):   ";
>     for (int i = 0; i < 5; i++) {
>         std::cout <http://en.cppreference.com/w/cpp/io/cout> << std::setw <http://en.cppreference.com/w/cpp/io/manip/setw>(i ? 30 : 0) << v3[i] << '\n';
>     }}
>
>
> It failed with the following errors:
>     Error: no member named 'make_unique' in namespace 'std'
>        std::unique_ptr<Vec3> v1 = std::make_unique<Vec3>();
>     ... ...
>
> Any idea and suggestion about what is going on? or have I missed
> something? The command I used to compile the code above:
>      $ clang++ a.cpp
>

Maybe `clang++ -std=c++11 a.cpp` ?


>
> 2.  Comparing this llvm with my current gcc/8.2.0 on a project (openmp
> code running 1 thread), it showed that llvm is almost twice as slow as gcc
> (both compile with -O3) on my IBM power8 machine. Is it suppose to be with
> such slower performance than gcc?
>

It is very sensitive to how you built it, by default it'll be built in
debug mode. For best performance you would ideally you enable Release mode,
and bootstrap with LTO/PGO.

-- 
Mehdi



>
> Thank you very much for any advice!
>
> Best Regards,
> Shelton
>
>
>
>
>
>
>
>
>
>
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