c++ - 当构造函数抛出异常时删除运算符段错误

Question

最好从代码开始，以了解这一点

#include "Hello1.h"
#include "Hello2.h"

int main(int argc, char ** argv)
{
    // Hello1 and Hello2 are derevied classes of Hello
    // And there constructor throws an exception
    Hello * h;

    try
    {
        if (argv[1][0]=='1')
            h = new Hello1;
        else
            h = new Hello2;
    }
    catch (std::exception ex) { /*print error*/ }

    delete h;
}

如果 Hello1 和 Hello2 抛出异常，它是 segfault。但是如果我添加

Hello h = NULL;

有效!!!

Hello 是一个带有抛出异常的构造函数的类

我能想到的就是构造函数中的异常从内存中删除对象!为什么在哪里谁...解释!请。

Answer 1

However If I add Hello h = NULL; it works!!! Why where who...Explain! Please.

那是因为当指针为null 时，operator delete 什么都不做。它应该什么也不做，这是标准行为。 C++11 标准第 3.7.4.2 段规定:

[...] The value of the first argument supplied to a deallocation function may be a null pointer value; if so, and if the deallocation function is one supplied in the standard library, the call has no effect. [...]

另一方面，如果它不是null，operator delete 将尝试删除hello 指向的对象，并且由于指针未初始化(因为构造抛出并且控制在分配给 hello 之前转移到异常处理程序)，您将得到未定义的行为。

根据第 5.3.5/2 段:

[...] In the first alternative (delete object), the value of the operand of delete may be a null pointer value, a pointer to a non-array object created by a previous new-expression, or a pointer to a subobject (1.8) representing a base class of such an object (Clause 10). If not, the behavior is undefined. [...]