istream_iteratorandistreambuf_iteratorshould support literal
sentinel values. The default constructor is frequently used to
terminate ranges,and could easily be a literal value for
istreambuf_iterator,andistream_iteratorwhen iterating value
types. [Rest omitted]
这对我来说并没有什么意义.有人能告诉我一个他们的意思的例子吗？
N3308是另一篇提到但并不解释问题的论文：
Some of the
istream_iterator<T>constructors are required to be
constexprifTis a literal type. The intention is to allow
existing implementation technique of storing a type ofTinline to
continue to work. [libstdc++ does this,_Tp _M_value] However,it
actually rules out this technique: the default and copy constructors
ofTneed not be markedconstexpr,and if they are not,the
istream_iterator<T>constructors could not be instantiated as
constexpr.
以上解释了这个琐碎的拷贝构造函数和析构函数,但是并不是为什么默认构造函数被标记为constexpr.
此外,在线GCC 5.2.0的测试中,我复制了libstdc的实现.唯一的改变是我从istream_iterator()中删除了constexpr.在这两种情况下,组件是相同的.
解决方法
// istream_iterator example
#include <iostream> // std::cin,std::cout
#include <iterator> // std::istream_iterator
int main () {
double value1,value2;
std::cout << "Please,insert two values: ";
std::istream_iterator<double> eos; // end-of-stream iterator
std::istream_iterator<double> iit (std::cin); // stdin iterator
if (iit!=eos) value1=*iit;
++iit;
if (iit!=eos) value2=*iit;
std::cout << value1 << "*" << value2 << "=" << (value1*value2) << '\n';
return 0;
}
http://www.cplusplus.com/reference/iterator/istream_iterator/istream_iterator/
声明这个一个constexpr允许编译器将创建流终端迭代器的调用折叠成常量,而不是每次调用一个函数.否则可能会在循环的每次迭代中这样做.
