Most efficient way to assign values to maps

Question

Which way to assign values to a map is most efficient? Or are they all optimized to the same code (on most modern compilers)?

   // 1) Assignment using array ind         


        

Answer 1

Your first possibility: Foo["Bar"] = 12345; has somewhat different semantics than the others -- it'll insert a new object if none exists (like the others) but overwrite the current contents if none exists (where the others using insert will fail if that key is already present).

As far as speed goes, it has the potential to be slower than the others. When you're inserting a new object, it has create a pair with the specified key and a default-constructed value_type, then assign the correct value_type afterwards. The others all construct the pair the correct key and value and insert that object. Being fair, however, my experience is that the difference is rarely significant (with older compilers, it was more significant, but with newer ones pretty minimal).

The next two are equivalent to each other. You're just using two different ways to name the same type. By run-time, there's no difference between them at all.

The fourth uses a template function (make_pair) that theoretically could involve an extra level of function call. I'd be quite surprised to see a real difference from this though, except (possibly) if you were careful to ensure that the compiler did absolutely no optimization (especially inlining) at all.

Bottom line: The first will often be a little slower than the rest (but not always and not by much). The other three will almost always be equal (as in: normally expect any reasonable compiler to produce identical code for all three) even though there's theoretical justification for the fourth being slower.