Item 20: Prefer Pass-by-Reference-to-const to Pass-by-Value

Because it’s (most likely) more efficient and avoids the slicing problem.

void foo(const Bar& bar);

• Pass-by-value needs 1 copy constructor and 1 destructor call (usually less efficient)
  • But built-in types are more efficient to pass it by value
• References are usually implemented as pointers
• A small object doesn’t mean its copy constructor/destructor is fast
• Compilers usually treat objects (even small one with only one integer member) different from a built-in type, and refuse to store it in register
• Exception: STL iterator (which is modelled after pointer types) and STL function object types should be passed by value