CODEWITHSUNDEEP
c++stringoptimizationtypesreference
Zack Lee
Zack Lee

Reputation: 3044

Is it good practice to make every const string be referenced?

Code A:

string a = "Hello World";

for (int i=0; i<1000000; ++i) 
     const string b = a;

Code B: 

string a = "Hello World";

for (int i=0; i<1000000; ++i) 
     const string &b = a;

If I run the 2 codes separately and measure CPU time, the Code B was about 2.5 times faster than the Code A.

Except for the primitive types such as char, int, float... I learned it is faster to get a reference to the original rather than copying it.

Although the difference is almost neglectable in most cases, can it be considered as a good practice to always make const string type(and other non-primitive types) be referenced?

Upvotes: 4

Views: 269

Answers (2)

Mark Ransom
Mark Ransom

Reputation: 308196

Yes, it is generally better to use a const reference for any object that is expensive to copy. But you must take care that the original object doesn't change when you don't expect it; in particular you need to make sure that the original object isn't destroyed, because that leads to undefined behavior. That isn't a problem in your toy example, but in the real world it's a big concern.

The best place to use a const reference is in function parameters. You know that the object passed into the function can't change or be destroyed while still inside the function.

Upvotes: 3

Ivaylo Valchev
Ivaylo Valchev

Reputation: 10425

There's a couple of things to take into account.

  1. Your use case. Do you want to copy or do you want to observe?
  2. Does the type perform deep-copy semantics (e.g. allocate dynamic memory on copy)?

If you wish to merely observe the object:
If the type of the object allocates dynamic memory on copy, then always pass by const reference.
Otherwise if the sizeof of the type is larger than sizeof(void*) then pass by const reference.
Otherwise pass by value.

If you wish to copy the object, then just pass by value.

Of course in a strange use case you may do something else, but this is the general guideline that I see is commonly followed.

There's also move semantics to take into account, but that's a different question.

Upvotes: 2

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