为什么这个复合语句作为由大括号括起来（在 GNU C++ 中）和括号内的语句序列似乎不是有效的 Statement 表达式。
// my second program in C++
#include <iostream>
using namespace std;
int main ()
{
cout << "Hello World! ";
({cout << "I'm a C++ program";});
}
编译器输出：
In function 'int main()':
8:32: error: use of deleted function 'std::basic_ostream<char>::basic_ostream(const std::basic_ostream<char>&)'
In file included from /usr/include/c++/4.9/iostream:39:0,
from 2:
/usr/include/c++/4.9/ostream:58:11: note: 'std::basic_ostream<char>::basic_ostream(const std::basic_ostream<char>&)' is implicitly deleted because the default definition would be ill-formed:
class basic_ostream : virtual public basic_ios<_CharT, _Traits>
^
/usr/include/c++/4.9/ostream:58:11: error: use of deleted …不应该std::unique_ptr防止出现这种错误的可能性吗？
#include <iostream>
#include <vector>
#include <memory>
struct B {
int b;
};
int main()
{
std::vector<std::unique_ptr<B>> v; // unique_ptr can be stored in a container
B* p = new B;
v.emplace_back(p);
std::cout << "p:" <<p <<"\n";
std::cout << "v[0]:"<<v[0].get() << "\n";
v.emplace_back(p);
std::cout << "p:" <<p <<"\n";
std::cout << "v[1]:"<<v[1].get() << "\n";
}
检测到双重释放时的错误信息：
*** Error in `./a.out': double free or corruption (fasttop): 0x0000000001094c20 ***
======= Backtrace: =========
/lib/x86_64-linux-gnu/libc.so.6(+0x777e5)[0x7f3da200b7e5]
...
======= Memory map: ========
00400000-00402000 r-xp 00000000 …