c++ - clang:从互斥锁初始化锁引用

Question

这个程序是clang编译的:

#include <mutex>

int main() {
    std::mutex mtx;
    const std::lock_guard<std::mutex>& lock(mtx);
    return 0;
}

其他主要编译器拒绝它(我已经尝试过 gcc、msvc 和 icc)。这是来自 gcc 的错误消息:

error: invalid initialization of reference of type ‘const 
       std::lock_guard<std::mutex>&’ from expression of type ‘std::mutex’

其他人给出了类似的错误。

clang是对还是错？这可以用一个不涉及库类的更简单的例子来重现吗？我试过了，但没有用。

编辑这似乎是最小的复制:

struct A {};

struct X
{
    explicit X(A&) {};
};

int main()
{
    A a;
    const X& x(a);
}

有趣的是，用 int 代替 A 确实会触发 clang 中的错误消息(这就是我最初无法重现的原因)。

Answer 1

我没有C++标准的相关章节；我只能引用CppReference on Converting Constructors现在(强调我的):

A constructor that is not declared with the specifier explicit and which can be called with a single parameter (until C++11) is called a converting constructor.

Unlike explicit constructors, which are only considered during direct initialization (which includes explicit conversions such as static_cast), converting constructors are also considered during copy initialization, as part of user-defined conversion sequence.

所以:

struct A {};

struct X
{
    explicit X(A const &) {};
};

int main()
{
    A a;
    const X& x1(A());                 // OK, direct init (no A object after init)
    const X& x3(a);                   // NOK, copy init
}