C++ 提升 Shared_pointer NULL
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boost Shared_pointer NULL
提问by Yochai Timmer
I'm using reset()as a default value for my shared_pointer (equivalent to a NULL).
我将其reset()用作我的 shared_pointer （相当于 a NULL）的默认值。
But how do I check if the shared_pointer is NULL?
但是如何检查 shared_pointer 是否为NULL？
Will this return the right value ?
这会返回正确的值吗？
boost::shared_ptr<Blah> blah;
blah.reset()
if (blah == NULL)
{
//Does this check if the object was reset() ?
}
回答by Ralph
Use:
用：
if (!blah)
{
//This checks if the object was reset() or never initialized
}
回答by ymett
if blah == NULLwill work fine. Some people would prefer it over testing as a bool (if !blah) because it's more explicit. Others prefer the latter because it's shorter.
if blah == NULL会正常工作。有些人更喜欢将其作为 bool ( if !blah)进行测试，因为它更明确。其他人更喜欢后者，因为它更短。
回答by James McNellis
You can just test the pointer as a boolean: it will evaluate to trueif it is non-null and falseif it is null:
您可以将指针作为布尔值进行测试：它将评估true它是否为非空以及false是否为空：
if (!blah)
boost::shared_ptrand std::tr1::shared_ptrboth implement the safe-bool idiom and C++0x's std::shared_ptrimplements an explicit boolconversion operator. These allow a shared_ptrbe used as a boolean in certain circumstances, similar to how ordinary pointers can be used as a boolean.
boost::shared_ptr并且std::tr1::shared_ptr都实现了安全布尔习语，而 C++0xstd::shared_ptr实现了显式bool转换运算符。这些允许shared_ptr在某些情况下将 a用作布尔值，类似于普通指针如何用作布尔值。
回答by ildjarn
As shown in boost::shared_ptr<>'s documentation, there exists a boolean conversion operator:
如在所示boost::shared_ptr<>的文档中，存在一个布尔转换运算符：
explicit operator bool() const noexcept;
// or pre-C++11:
operator unspecified-bool-type() const; // never throws
So simply use the shared_ptr<>as though it were a bool:
所以简单地使用，shared_ptr<>就好像它是一个bool：
if (!blah) {
// this has the semantics you want
}
