Question

I was reading through the std::algorithm documentation at cppreference.com and I noticed a C++17 tag on a lot of cool things I haven't used yet. What got my attention most was the new execution policies. What I gathered from reading about them is that I can make any for_each loop I want multi-threaded just by specifying an execution policy.

For example, I have a program which outputs an image with a 2D graphic on it.

int main(){
    std::for_each(
        img.buffer().begin(),
        img.buffer().end(),
        renderer(
            {-0.5, 0.0, 2.666, 2.0, M_PI / 2.0, 0.0},
            img,
            16
        )
    );
    fout << img;
}

If I want to make this program multi-threaded I should be able to do it with one line.

int main(){
    std::for_each(
        std::execution::par_unseq, // c++17 feature
        img.buffer().begin(),
        img.buffer().end(),
        renderer(
            {-0.5, 0.0, 2.666, 2.0, M_PI / 2.0, 0.0},
            img,
            16
        )
    );
    fout << img;
}

However when I first tried this (with g++ -std=c++17) I got an error telling me that ‘std::execution’ has not been declared, so I tried adding #include <execution> but it says execution: No such file or directory. I've also tried #include<experimental/algorithm> instead of #include<algorithm> but I get the same result. How do I use this new feature?

Answer 1

c++17尚未最终确定。各种编译器还没有完全实现它。

-std=c++17表示＆＃34;给我所有的C ++ 17你已经完成＆＃34;，而不是＆＃34;是一个完全有效的C ++ 17编译器＆＃34;。

此时，编译器和/或标准库不支持此功能。几周/几个月/年后再回来看看。

没有普遍接受＆＃34;如果你完全支持它，请给我C ++ 17，否则给我一个错误＆＃34; flag你可以传递给编译器;部分原因是它几乎没有实际用途。如果他们提供的C ++ 17子集足够，那么你就赢了。如果你需要一个完全兼容的编译器，特定版本的编译器不知道它们是否有错误，所以你无论如何都不能信任该标志，并且必须针对编译器版本进行测试。如果您已经知道哪些版本的编译器具有足够有效的C ++ 17，那么您就不需要一个标记来告诉您。

Answer 2

据我所知cppreference 此功能在文档P0024R2中定义，并且在任何编译器中都不支持。

Answer 3

如果您正在使用g ++，那么您可以尝试使用非标准扩展程序：

https://gcc.gnu.org/onlinedocs/libstdc++/manual/parallel_mode.html

Answer 4

对于Microsoft编译器：请参阅C++17 Progress in VS 2017 15.5 and 15.6，您将在其中找到：

Status  Std   Paper   Title
Partial C++17 P0024R2 Parallel Algorithms

对于GCC，正如Fanael在其comment中所写，请参阅表1.5。您可以在https://gcc.gnu.org/onlinedocs/libstdc++/manual/status.html#status.iso.2017找到

的C ++ 2017实施状态

Library Feature                             Proposal    Status
The Parallelism TS Should be Standardized   P0024R2     No

How do I use the new C++17 execution policies?

4 个答案: