这取决于实现.

该标准允许在将临时引用绑定到常量引用时进行复制.在您的情况下，VC++ 仅在隐式定义构造函数时执行复制.这是出乎意料的，但也是允许的.

The standard allows a copy to occur when binding a temporary to a const reference. In your case, VC++ performs a copy only when the constructor is implicitly defined. This is unexpected, but permitted.

C++1x 将解决这个问题.