为什么这个复合语句作为由大括号和括号括起来的语句序列似乎不是有效的语句表达式

Question

为什么这个复合语句作为由大括号（在 GNU C++ 中）和括号内括起来的语句序列似乎不是有效的语句表达式。

// 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 function 'std::basic_ios<char>::basic_ios(const std::basic_ios<char>&)'
In file included from /usr/include/c++/4.9/ios:44:0,
                 from /usr/include/c++/4.9/ostream:38,
                 from /usr/include/c++/4.9/iostream:39,
                 from 2:
/usr/include/c++/4.9/bits/basic_ios.h:66:11: note: 'std::basic_ios<char>::basic_ios(const std::basic_ios<char>&)' is implicitly deleted because the default definition would be ill-formed:
     class basic_ios : public ios_base
           ^
In file included from /usr/include/c++/4.9/ios:42:0,
                 from /usr/include/c++/4.9/ostream:38,
                 from /usr/include/c++/4.9/iostream:39,
                 from 2:
/usr/include/c++/4.9/bits/ios_base.h:786:5: error: 'std::ios_base::ios_base(const std::ios_base&)' is private
     ios_base(const ios_base&);
     ^
In file included from /usr/include/c++/4.9/ios:44:0,
                 from /usr/include/c++/4.9/ostream:38,
                 from /usr/include/c++/4.9/iostream:39,
                 from 2:
/usr/include/c++/4.9/bits/basic_ios.h:66:11: error: within this context
     class basic_ios : public ios_base
           ^

我在What's this C++ syntax that puts a brace-surrounded block where an expression is expected?找到了一个关于“语句表达式”的好答案

Answer 1

来自reference：

复合语句中的最后一件事应该是一个表达式，后跟一个分号；此子表达式的值用作整个构造的值。

和

在G++中，语句表达式的结果值会经过数组和函数指针的衰减，并按值返回给封闭的表达式。

这意味着在表达式中：

({cout << "I'm a C++ program";}); 

对象std::cout由值返回。这会调用 std::basic_ostream 的复制构造函数，该构造函数已被删除，因此出现错误。