tr1 :: bind&通过引用，这是真的期望的行为吗？ [英] tr1::bind & pass by reference, is this behavior really expected?

问题描述

给定以下测试代码

#include <iostream>
#include <tr1/functional>
using namespace std;

struct cl { 
  cl(){ cout << "  cl()\n"; } 
  cl(const cl& from){ cout << "  cl()[copy]\n"; } 
  ~cl(){ cout << "  ~cl()\n";}
};
void  f1(const cl& data){}
void  f2(const cl* pData){}

int main(int c, char** a)
{
  cout << "enter:\n";
  cl data;
  cout << "ref:\n";
  tr1::bind(&f1, data);
  cout << "ptr:\n";
  tr1::bind(&f2, &data);
  cout << "exit:\n";
  return 0;
}

我得到以下输出：

$ g++ tr1BindDtorTest.cpp && ./a.out
enter:
  cl()
ref:
  cl()[copy]
  cl()[copy]
  cl()[copy]
  ~cl()
  ~cl()
  ~cl()
ptr:
exit:
  ~cl()

当我创建一个绑定时，引用我的类/ struct对象被多次创建和销毁。

When I create a binding involving references to my class/struct objects are created and destroyed multiple times.

相同的精确测试，但指针没有这样的对象

Same exact test but with pointers there are no such objects

我不明白为什么行为会不同通过值&

I can't see why the behaviour will be different between pass by value & reference, I always thought of reference as syntactic sugar for pointer, and so reasoned that the behaviours should be identical.

任何人都可以解释一下？

Anyone care to explain?

[g ++ 4.4.6 linux& 4.2.1 on macos]

[g++ 4.4.6 linux & 4.2.1 on macos]

推荐答案

而不是这样：

tr1::bind(&f1, data);

您需要：

tr1::bind(&f1, tr1::ref(data));

Boost有同样的事情：boost :: ref（）必须在boost :: bind ）如果你想绑定的函数对象存储对数据的引用。否则，数据将总是被复制到由bind（）生成的绑定函数对象中。

Boost has the same thing: boost::ref() must be used inside boost::bind() if you want the bound function object to store a reference to the data. Otherwise, the data will always be copied into the bound function object produced by bind().

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