c++ - boost::ref 和常规引用之间的区别

Question

boost::ref(i) 和 & i 有什么区别？在什么情况下我们不能使用常规引用而必须使用 boost::ref 代替？如果可能，请包括示例。

Answer 1

来自Boost.Ref Documentation :

The purpose of boost::reference_wrapper is to contain a reference to an object of type T. It is primarily used to "feed" references to function templates (algorithms) that take their parameter by value.

注意:boost::reference_wrapper 和 std::reference_wrapper(至少是 Boost 1.52)之间的一个重要区别是 std::reference_wrapper 的能力 完美包装函数对象。

这会启用如下代码:

// functor that counts how often it was applied
struct counting_plus {
  counting_plus() : applications(0) {}
  int applications;

  int operator()(const int& x, const int& y) 
  { ++applications; return x + y; }
};

std::vector<int> x = {1, 2, 3}, y = {1, 2, 3}, result;
counting_plus f;
std::transform(begin(x), end(x), begin(y), 
               std::back_inserter(result), std::ref(f));
std::cout << "counting_plus has been applied " << f.applications 
          << " times." << '\n';