在构造函数中初始化 <random> 类会导致段错误

Question

在下面的代码中，RandomCharSource 应该根据请求简单地 return 一个随机字符。它的构造函数正在初始化 mt19937、uniform_int_distribution<int> 和 random_device。但是，当我实例化我的对象时，出现段错误。

当我在下面的 bar() 函数中手动创建这些随机类时，它工作正常。

我做错了什么？这里有初始化顺序问题吗？我正在使用 GCC 4.7.3。

#include <random>
#include <iostream>

class RandomCharSource
{
public:
    explicit RandomCharSource() : _re{_rd()}, _dist{0, 255} {};
    inline char get_next_char() { return _dist(_re); };
private:
    std::mt19937 _re;
    std::uniform_int_distribution<int> _dist;
    std::random_device _rd;
};

void foo()
{
    RandomCharSource s;
    std::cout << s.get_next_char() << std::endl;
}

void bar()
{
    std::random_device _rd;
    std::mt19937 _re{_rd()};
    std::uniform_int_distribution<int> _dist{0,255};
    std::cout << (char)_dist(_re) << std::endl;
}

int main()
{
    bar(); // Works OK
    foo(); // Segfault

    return 0;
}

Answer 1

是因为你的初始化顺序

class RandomCharSource
{
public:
    explicit RandomCharSource() : _re{_rd()}, _dist{0, 255} {};
    inline char get_next_char() { return _dist(_re); };
private:
    std::mt19937 _re;
    std::uniform_int_distribution<int> _dist;
    std::random_device _rd;
};

您需要 _rd 在 _re 之前。成员按照它们在 class 中声明的顺序进行初始化。因此，当您尝试使用 _rd 初始化 _re 时，_rd 尚未初始化。

来自标准 §12.6.2.13（强调我的）

In a non-delegating constructor, initialization proceeds in the following order:

13.1 — First, and only for the constructor of the most derived class (1.8), virtual base classes are initialized in the order they appear on a depth-first left-to-right traversal of the directed acyclic graph of base classes, where “left-to-right” is the order of appearance of the base classes in the derived class base-specifier-list.
13.2 — Then, direct base classes are initialized in declaration order as they appear in the base-specifier-list (regardless of the order of the mem-initializers).
13.3 — Then, non-static data members are initialized in the order they were declared in the class definition (again regardless of the order of the mem-initializers).
13.4 — Finally, the compound-statement of the constructor body is executed.
[Note: The declaration order is mandated to ensure that base and member subobjects are destroyed in the reverse order of initialization. —end note]

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