std :: ostringstream困境

Question

I can do

std::ostringstream oss;
oss << 1;
oss.str();

so why can't I do:

((std::ostringstream()) << 1).str() ?

Thanks!

Answer 1

<<运算符返回基类型ostream ，而str成员函数仅存在于派生类型ostringstream 。