c++ - 如何避免 "(void)a"cast 导致副作用？

Question

我有这段代码，GCC 会打印“什么!？”。我怎样才能避免这种情况，所以 void Actor 只是有 C 意思“忽略孤独的 'a;'”？

#include <iostream>

struct A {
  template<typename T>
  operator T() { 
    std::cout << "what!?";
  }
};

int main() {
  A a;
  (void)a;
}

Answer 1

如您所见，这是 bug in gcc .标准如下:

c++11

12.3.2 Conversion functions [class.conv.fct]

(1) A conversion function is never used to convert a (possibly cv-qualified) object to [...] (possibly cv-qualified) void.
116) A conversion to void does not invoke any conversion function (5.2.9).

5.2.9 Static cast [expr.static.cast]

(6) Any expression can be explicitly converted to type cv void, in which case it becomes a discarded-value expression (Clause 5).

作为一种解决方法，您可以编写:

a, void();

重载 operator,(void) 是不可能的，因此从符合要求的实现中调用用户定义行为的可能性为零。