Question

有没有办法可以访问shared_ptr：

e.g。

# Maximum number of representations
# double.base ^ double.max.exp
base <- mpfr(2, 2048)
max.exp <- mpfr( 1024, 2048 )

# This is where the big part of the 1.79... comes from
base^max.exp

## 1 'mpfr' number of precision  2048   bits 
## [1] 179769313486231590772930519078902473361797697894230657273430081157732675805500963132708477322407536021120113879871393357658789768814416622492847430639474124377767893424865485276302219601246094119453082952085005768838150682342462881473913110540827237163350510684586298239947245938479716304835356329624224137216

# Smallest definitive unit.
# Find the largest negative integer...
neg.ulp.digits <- -64; while( ( 1 - 2^neg.ulp.digits ) == 1 ) 
  neg.ulp.digits <<- neg.ulp.digits + 1

neg.ulp.digits

## [1] -53

# It makes a real small number...
neg.eps <- base^neg.ulp.digits

neg.eps

## 1 'mpfr' number of precision  2048   bits 
## [1] 1.11022302462515654042363166809082031250000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000e-16

# Largest difinitive floating point number less than 1
# times the number of representations
xmax <- (1-neg.eps) * base^max.exp

xmax

## 1 'mpfr' number of precision  2048   bits 
## [1] 179769313486231570814527423731704356798070567525844996598917476803157260780028538760589558632766878171540458953514382464234321326889464182768467546703537516986049910576551282076245490090389328944075868508455133942304583236903222948165808559332123348274797826204144723168738177180919299881250404026184124858368

identical( asNumeric(xmax), .Machine$double.xmax )

## [1] TRUE

当我运行程序时，执行bar后会得到一个segafult。我确实理解了seg-fault的原因。

我的最终目标是使用弱指针然后回调。

由于

Answer 1

y的生命周期由共享指针管理，或者当它在终止时创建的堆栈帧时超出范围。下定决心并坚持下去。

如果此代码可能以某种方式工作，它会导致y两次，一次在main结束时超出范围，一次在最后一次shared_ptr消失时消失。 / p>

也许你想要：

int main()
{
   std::shared_ptr<Y> y = std::make_shared<Y>();
   y->foo();
}

此处，实例在其最后shared_ptr消失时被销毁。这可能发生在main的末尾，但如果foo松开shared_ptr的副本，它可以延长对象的生命周期。它可以松开一个弱指针，也许在全局对象的析构函数中告诉对象已经消失。

弱指针的要点是能够将它提升为强指针，这可以延长对象的生命周期。如果没有办法动态管理对象的生命周期，这将无法工作。

访问此指针的shared_ptr

1 个答案: