Are the days of passing const std::string & as a parameter over?

Question

I heard a recent talk by Herb Sutter who suggested that the reasons to pass std::vector and std::string by const & are largely gon

Answer 1

This highly depends on the compiler's implementation.

However, it also depends on what you use.

Lets consider next functions :

bool foo1( const std::string v )
{
  return v.empty();
}
bool foo2( const std::string & v )
{
  return v.empty();
}

These functions are implemented in a separate compilation unit in order to avoid inlining. Then :
1. If you pass a literal to these two functions, you will not see much difference in performances. In both cases, a string object has to be created
2. If you pass another std::string object, foo2 will outperform foo1, because foo1 will do a deep copy.

On my PC, using g++ 4.6.1, I got these results :

• variable by reference: 1000000000 iterations -> time elapsed: 2.25912 sec
• variable by value: 1000000000 iterations -> time elapsed: 27.2259 sec
• literal by reference: 100000000 iterations -> time elapsed: 9.10319 sec
• literal by value: 100000000 iterations -> time elapsed: 8.62659 sec