std::seed_seq 编译错误（GCC 错误？）

Question

我有以下代码行，最新版本的 GCC 无法编译。我很确定它甚至没有发出警告消息。

seed_seq.cpp：

void func()
{
    std::mt19937_64 engine;
    std::hash<std::thread::id> hasher;
    uint64_t rdSeed, threadID, now;

    try
    {
        std::random_device rd;

        if (rd.entropy())
            rdSeed = rd();
        else
            rdSeed = 0;
    }
    catch (std::exception &)
    {
        rdSeed = 0;
    }

    threadID = hasher(std::this_thread::get_id());

    now = std::chrono::system_clock::now().time_since_epoch().count();

    engine.seed(std::seed_seq{ rdSeed, threadID, now });
}

命令：gcc -std=c++11 -c seed_seq.cpp -o seed_seq.o

无法编译的GCC版本：

• gcc (GCC) 6.3.1 20161221（红帽 6.3.1-1）
• gcc (Ubuntu 5.4.0-6ubuntu1~16.04.4) 5.4.0 20160609

VS2013 Update 5 没有抱怨。确定是 GCC 的错？

我不记得有效的 GCC 版本。对不起。

错误：

seed_seq.cpp: In function ‘void func()’:
seed_seq.cpp:28:19: error: invalid initialization of non-const reference of type ‘std::seed_seq&’ from an rvalue of type ‘std::seed_seq’
  engine.seed(std::seed_seq{ rdSeed, threadID, now });
                   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from /usr/include/c++/6.3.1/random:51:0,
                 from seed_seq.cpp:2:
/usr/include/c++/6.3.1/bits/random.tcc:353:7: note:   initializing argument 1 of ‘typename std::enable_if<std::is_class<_Sseq>::value>::type std::mersenne_twister_engine<_UIntType, __w, __n, __m, __r, __a, __u, __d, __s, __b, __t, __c, __l, __f>::seed(_Sseq&) [with _Sseq = std::seed_seq; _UIntType = long unsigned int; long unsigned int __w = 64ul; long unsigned int __n = 312ul; long unsigned int __m = 156ul; long unsigned int __r = 31ul; _UIntType __a = 13043109905998158313ul; long unsigned int __u = 29ul; _UIntType __d = 6148914691236517205ul; long unsigned int __s = 17ul; _UIntType __b = 8202884508482404352ul; long unsigned int __t = 37ul; _UIntType __c = 18444473444759240704ul; long unsigned int __l = 43ul; _UIntType __f = 6364136223846793005ul; typename std::enable_if<std::is_class<_Sseq>::value>::type = void]’
       mersenne_twister_engine<_UIntType, __w, __n, __m, __r, __a, __u, __d,
       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
          __s, __b, __t, __c, __l, __f>::
          ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

更新

在系统的 dnf.log 中找到了这个。不确定哪个是我在项目中使用的...

Nov 01 18:17:41 DEBUG ---> Package gcc-gdb-plugin.x86_64 6.2.1-2.fc24 will be installed
Nov 01 18:17:42 DEBUG ---> Package gcc.x86_64 5.3.1-6.fc23 will be upgraded
Nov 01 18:17:42 DEBUG ---> Package gcc.x86_64 6.2.1-2.fc24 will be an upgrade
Nov 01 18:17:42 DEBUG ---> Package gcc-c++.x86_64 5.3.1-6.fc23 will be upgraded
Nov 01 18:17:42 DEBUG ---> Package gcc-c++.x86_64 6.2.1-2.fc24 will be an upgrade
Nov 01 18:17:42 DEBUG ---> Package libgcc.x86_64 5.3.1-6.fc23 will be upgraded
Nov 01 18:17:42 DEBUG ---> Package libgcc.x86_64 6.2.1-2.fc24 will be an upgrade                                                  
Nov 02 11:03:25 DEBUG ---> Package gcc-debuginfo.x86_64 6.2.1-2.fc24 will be installed
Nov 02 11:03:25 DEBUG ---> Package gcc-base-debuginfo.x86_64 6.2.1-2.fc24 will be installed

Jan 04 09:59:34 DEBUG ---> Package gcc.x86_64 6.2.1-2.fc24 will be upgraded
Jan 04 09:59:34 DEBUG ---> Package gcc.x86_64 6.3.1-1.fc24 will be an upgrade
Jan 04 09:59:34 DEBUG ---> Package gcc-gdb-plugin.x86_64 6.2.1-2.fc24 will be upgraded
Jan 04 09:59:34 DEBUG ---> Package gcc-gdb-plugin.x86_64 6.3.1-1.fc24 will be an upgrade
Jan 04 09:59:34 DEBUG ---> Package gcc-c++.x86_64 6.2.1-2.fc24 will be upgraded
Jan 04 09:59:34 DEBUG ---> Package gcc-c++.x86_64 6.3.1-1.fc24 will be an upgrade
Jan 04 09:59:34 DEBUG ---> Package libgcc.x86_64 6.2.1-2.fc24 will be upgraded
Jan 04 09:59:34 DEBUG ---> Package libgcc.x86_64 6.3.1-1.fc24 will be an upgrade

Answer 1

很明显，您传递给engine.seed() 的参数是问题所在。
建议你看看

seed()

让我们简要地看看原因。
如您所见，有两种方法可以调用seed()，由以下定义：

void seed( result_type value = default_seed );

和

void seed( Sseq& seq );

如果您担心 gcc 中存在错误，那么我认为您不应该担心。
事实上，Clang 3.9.1（只是提到一个版本）也给出了类似的错误：

Start
prog.cc:32:12: error: no matching member function for call to 'seed'
    engine.seed(std::seed_seq{ rdSeed, threadID, now });
    ~~~~~~~^~~~
/opt/wandbox/clang-3.9.1/include/c++/v1/random:2116:10: note: candidate function not viable: no known conversion from 'std::seed_seq' to 'result_type' (aka 'unsigned long') for 1st argument
    void seed(result_type __sd = default_seed);
         ^
/opt/wandbox/clang-3.9.1/include/c++/v1/random:2124:9: note: candidate function [with _Sseq = std::__1::seed_seq] not viable: expects an l-value for 1st argument
        seed(_Sseq& __q)
        ^
1 error generated.
1
Finish

并准确指出问题所在。
我发现 gcc 和 clang 不太可能有相同的错误，但一切皆有可能。
要了解如何将seed() 与std::seed_seq 结合使用，您应该查看以下seed()，它完全有一个类似的示例。
正如您已经提到的那样，问题可以解决

std::seed_seq seq{ rdSeed, threadID, now };
engine.seed(seq);

或（这是冗长的方式，我可能不会这样做）：

std::seed_seq seq{ rdSeed, threadID, now };
std::seed_seq& pr = seq;
engine.seed(pr);

要了解更多关于这种情况/错误的信息，我建议您阅读以下 SO 帖子：
invalid initialization of non-const reference of type 'int&' from a temporary of type 'int'

正如@Ashe 人类在 cmets 中指出的那样，VS2013 让它编译，因为它是默认启用的语言扩展：它应该发出警告让人们知道。您可以在此处阅读更多信息：
Non-const reference bound to temporary, Visual Studio bug?