std::ios_base::xalloc
static int xalloc(); | ? | ? |
|---|
返回一个唯一的%28程序范围%29索引值，该索引值可用于访问long还有一个void*的私有存储中的元素。std::ios_base打电话iword()和pword().呼吁xalloc不分配内存。
此函数是线程安全的；多个线程的并发访问不会导致数据竞争.。%28自C++14%29。
的私有静态数据成员。std::ios_base，就像通过执行return index++;，如果index是静态成员%28的名称，它可能是std::atomic以支持多线程的并发访问，或自C++14%29以来同步的%29%28。
参数
%280%29
返回值
作为pword/iword索引的唯一整数。
例
将基类pword存储用于派生流对象的运行时类型标识。
二次
#include <iostream>
template<class charT, class traits = std::char_traits<charT> >
class mystream : public std::basic_ostream<charT, traits>
{
public:
static const int xindex;
mystream(std::basic_ostream<charT, traits>& ostr) :
std::basic_ostream<charT, traits>(ostr.rdbuf())
{
this->pword(xindex) = this;
}
void myfn()
{
*this << "[special handling for mystream]";
}
};
// each specialization of mystream obtains a unique index from xalloc()
template<class charT, class traits>
const int mystream<charT, traits>::xindex = std::ios_base::xalloc();
// This I/O manipulator will be able to recognize ostreams that are mystreams
// by looking up the pointer stored in pword
template<class charT, class traits>
std::basic_ostream<charT,traits>& mymanip(std::basic_ostream<charT,traits>& os)
{
if (os.pword(mystream<charT,traits>::xindex) == &os)
static_cast<mystream<charT,traits>&>(os).myfn();
return os;
}
int main()
{
std::cout << "cout, narrow-character test " << mymanip << '\n';
mystream<char> myout(std::cout);
myout << "myout, narrow-character test " << mymanip << '\n';
std::wcout << "wcout, wide-character test " << mymanip << '\n';
mystream<wchar_t> mywout(std::wcout);
mywout << "mywout, wide-character test " << mymanip << '\n';
}二次
产出：
二次
cout, narrow-character test
myout, narrow-character test [special handling for mystream]
wcout, wide-character test
mywout, wide-character test [special handling for mystream]二次
另见
pword | resizes the private storage if necessary and access to the void* element at the given index (public member function) |
|---|---|
iword | resizes the private storage if necessary and access to the long element at the given index (public member function) |
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