C++ 带有 boost::shared_ptr 的 NULL 指针？
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NULL pointer with boost::shared_ptr?
提问by
What's the equivalent to the following:
什么相当于以下内容：
std::vector<Foo*> vec;
vec.push_back(NULL);
when dealing with boost::shared_ptr? Is it the following code?
什么时候处理boost::shared_ptr？是下面的代码吗？
std::vector< boost::shared_ptr<Foo> > vec;
vec.push_back(boost::shared_ptr<Foo>());
Note: I may push back a lot of such objects. Should I declare a global static nullPtrobject somewhere? That way only one of them would have to be constructed:
注意：我可能会推回很多这样的对象。我应该在nullPtr某处声明一个全局静态对象吗？这样一来，只需要构建其中一个：
boost::shared_ptr<Foo> nullPtr;
回答by j_random_hacker
Your suggestion (calling the shared_ptr<T>constructor with no argument) is correct. (Calling the constructor with the value 0 is equivalent.) I don't think that this would be any slower than calling vec.push_back()with a pre-existing shared_ptr<T>, since construction is required in both cases (either direct construction or copy-construction).
您的建议（shared_ptr<T>不带参数调用构造函数）是正确的。（使用值 0 调用构造函数是等效的。）我认为这不会比vec.push_back()使用预先存在的调用慢shared_ptr<T>，因为在这两种情况下都需要构造（直接构造或复制构造）。
But if you want "nicer" syntax, you could try the following code:
但是如果你想要“更好”的语法，你可以尝试以下代码：
class {
public:
template<typename T>
operator shared_ptr<T>() { return shared_ptr<T>(); }
} nullPtr;
This declares a single global object nullPtr, which enables the following natural syntax:
这声明了一个全局对象nullPtr，它启用了以下自然语法：
shared_ptr<int> pi(new int(42));
shared_ptr<SomeArbitraryType> psat(new SomeArbitraryType("foonly"));
...
pi = nullPtr;
psat = nullPtr;
Note that if you use this in multiple translation units (source files), you'll need to give the class a name (e.g. _shared_null_ptr_type), move the definition of the nullPtrobject to a separate .cpp file, and add externdeclarations in the header file where the class is defined.
请注意，如果您在多个翻译单元（源文件）中使用它，则需要为类命名（例如_shared_null_ptr_type），将nullPtr对象的定义移动到单独的 .cpp 文件中，并extern在头文件中添加声明，其中类已定义。
回答by Larry Gritz
Well, this is legal:
嗯，这是合法的：
shared_ptr<Foo> foo; /* don't assign */
And in this state, it doesn't point to anything. You can even test this property:
在这种状态下，它不指向任何东西。你甚至可以测试这个属性：
if (foo) {
// it points to something
} else {
// no it doesn't
}
So why not do this:
那么为什么不这样做：
std::vector < shared_ptr<Foo> > vec;
vec.push_back (shared_ptr<Foo>); // push an unassigned one
回答by bdonlan
In C++0x, you can simply convert from nullptrto std::shared_ptr:
在 C++0x 中，您可以简单地将 from 转换nullptr为std::shared_ptr：
std::vector< boost::shared_ptr<Foo> > vec;
vec.push_back(nullptr);
回答by m-sharp
You could declare a global nullPtrfor shared_ptr<Foo>. But if you pollute the global namespace, what would you call the global nullPtrfor shared_ptr<Bar>?
您可以nullPtr为shared_ptr<Foo>. 但是，如果你污染全局命名空间，你会调用全局nullPtr的shared_ptr<Bar>？
Typically I declare the null ptr as a static in the class of the pointer.
通常我在指针的类中将空 ptr 声明为静态。
#include <boost\shared_ptr.hpp>
class Foo; // forward decl
typedef boost::shared_ptr<Foo> FooPtr;
class Foo
{
public:
static FooPtr Null;
}
...
// define static in cpp file
FooPtr Foo::Null;
...
// use Foo Null
vec.push_back(Foo::Null);
That way each class has a static Null.
这样每个类都有一个静态空值。
回答by Robert S. Barnes
Here's something which I think is a bit simpler and works just fine
这是我认为有点简单并且工作正常的东西
( remember that typedefis your friend ):
（记住那typedef是你的朋友）：
#include <cstdlib>
#include <vector>
#include <iostream>
#include <boost/shared_ptr.hpp>
typedef boost::shared_ptr< std::vector<char> > CharVecHandle;
inline CharVecHandle newCharVec(std::vector<char>::size_type size) {
return CharVecHandle(new std::vector<char>(size));
}
inline CharVecHandle newCharVec(void) {
return CharVecHandle();
}
int main ( void )
{
CharVecHandle cvh = newCharVec();
if (cvh == NULL)
std::cout << "It's NULL" << std::endl;
else
std::cout << "It's not NULL" << std::endl;
std::vector< CharVecHandle > cvh_vec;
cvh_vec.push_back(newCharVec(64));
cvh_vec.push_back(newCharVec());
// or call the NULL constructor directly
cvh_vec.push_back(CharVecHandle());
return EXIT_SUCCESS;
}
回答by Christopher Dolan
Yes, declare a global static null pointer.
是的，声明一个全局静态空指针。
