c++之map賦值

對map賦值有下面4種方法：

// 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));
 

stack overflow上大神是這樣分析的：

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.

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.

• 參考：

https://stackoverflow.com/questions/14218042/most-efficient-way-to-assign-values-to-maps