template pass by value or const reference or…?

问题

I can write a templated function this way

template<class T> void f(T x) {...}

or this way

template<class T> void f(T const& x) {...}

I guess that the second option can be more optimal as it explicitly avoids a copy, but I suspect that it can also fail for some specific types T (eg functors?). So, when should use the first option, and when to use the second? There are also this boost::call_traits<T>::param_type and boost::reference_wrapper that were in the answers to my previous question, but people don't use them everywhere, do they? Is there a rule of thumb for this? Thanks.

回答1:

Is there a rule of thumb for this?

The same general rules for when to use pass by reference vs. pass by value apply.

If you expect T always to be a numeric type or a type that is very cheap to copy, then you can take the argument by value. If you are going to make a copy of the argument into a local variable in the function anyway, then you should take it by value to help the compiler elide copies that don't really need to be made.

Otherwise, take the argument by reference. In the case of types that are cheap to copy, it may be more expensive but for other types it will be faster. If you find this is a performance hotspot, you can overload the function for different types of arguments and do the right thing for each of them.

回答2:

I suspect that it can also fail for some specific types

Pass by reference-to-const is the only passing mechanism that "never" fails. It does not pose any requirements on T, it accepts both lvalues and rvalues as arguments, and it allows implicit conversions.

回答3:

Thou shalt not wake the dead, but head a similar problem and here's some example code that shows how to use C++11s type traits to deduce whether a parameter should be passed by value or reference:

#include <iostream>
#include <type_traits>

template<typename key_type>
class example
{
    using parameter_type = typename std::conditional<std::is_fundamental<key_type>::value, key_type, key_type&>::type;

 public:
  void function(parameter_type param)
  {
      if (std::is_reference<parameter_type>::value)
      {
          std::cout << "passed by reference" << std::endl;
      } else {
          std::cout << "passed by value" << std::endl;
      }
  }
};

struct non_fundamental_type
{
    int one;
    char * two;
};

int main()
{
  int one = 1;
  non_fundamental_type nft;

  example<int>().function(one);
  example<non_fundamental_type>().function(nft);

  return 0;
}

Hope it helps others with a similar issue.