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 index notation
   Foo["Bar"] = 12345;

   // 2) Assignment using member function insert() and STL pair
   Foo.insert(std::pair<string,int>("Bar", 12345));

   // 3) Assignment using member function insert() and "value_type()"
   Foo.insert(map<string,int>::value_type("Bar", 12345));

   // 4) Assignment using member function insert() and "make_pair()"
   Foo.insert(std::make_pair("Bar", 12345));

(I know I could benchmark and check compiler output, but this question arose now and the only thing I have close at hand is my mobile phone… hehe)

Answer 1

First, there are semantic differences between [] and insert:

• [] will replace the old value (if any)
• insert will ignore the new value (if an old value is already present)

therefore comparing the two is useless in general.

Regarding the inserts versions:

• std::map<std::string, int>::value_type is std::pair<std::string const, int> so no (important) difference between 3 and 4
• std::make_pair("Bar", 12345) is cheaper than std::pair<std::string, int>("Bar", 12345) because the std::string type is a full fledged class with operations to do on copy and "Bar" is just a string literal (thus just a pointer copy); however since at the end you do need to create the std::string… it will depend on the quality of your compiler

In general, I would recommend:

• [] for updating
• insert(std::make_pair()) for ignoring duplicates

std::make_pair is not only shorter, it also respects the DRY guideline: Don’t Repeat Yourself.

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For completeness though, the fastest (and easiest) would be emplace (C++11 enabled):

map.emplace("Bar", 12345);

Its behavior is that of insert, but it constructs the new element in-place.