C++ alternatives to Java static blocks [duplicate]

Question

This question already has answers here:

What's the C++ idiom equivalent to the Java static block? (7 answers)

Closed 5 years ago.

I am writing a date class where I want a static map to map "Jan" to 1 and so on. I am wondering how I can initialize that static map. This is what I am currently doing, but I just feel that the extra if statement is inelegant compared with static block in Java. I understand the compilation of C++ program is much more complicated, but I still wonder whether a better solution exists.

class date {
    static map<string, int> month_map;
    int month;
    int year;
public:
    class input_format_exception {};
    date(const string&);
    bool operator< (const date&) const;
    string tostring() const;
};

map<string, int> date::month_map = map<string,int>();

date::date(const string& s) {
    static bool first = true;
    if (first)  {
        first = false;
        month_map["Jan"] = 1;
        month_map["Feb"] = 2;
        month_map["Mar"] = 3;
        month_map["Apr"] = 4;
        month_map["May"] = 5;
        month_map["Jun"] = 6;
        month_map["Jul"] = 7;
        month_map["Aug"] = 8;
        month_map["Sep"] = 9;
        month_map["Oct"] = 10;
        month_map["Nov"] = 11;
        month_map["Dec"] = 12;
    }   
    // the rest code.
}

// the rest code.

Answer 1

For non-c++11 systems: how about using a helper function and make month_map a static const member of date as it looks like you never wan't to change the association of the month's name with it's number, do you? This way month_map is initialized in your cpp-File and not in your constructor where it just messes things up. (Maybe you'll have several constructors in the future then you'll have to write a lot of boilerplate code)

const std::map<string, int> createMonthMap()
{
   std::map<string, int> result;

   // do init stuff

   return result;
}

const std::map<string, int> date::month_map(createMonthMap());

Answer 2

In C++11 you can use initializer lists:

map<string, int> date::month_map = { {"Jan", 1},
                                     {"Feb", 2}
                                     // and so on
                                   };

In C++03 I believe you're stuck with what you are currently doing.

Answer 3

You can "implement" a static block feature in C++, even pre-C++11. See my detailed answer here; it will let you do simply

#include "static_block.hpp"

static_block {
    month_map["Jan"] = 1;
    month_map["Feb"] = 2;
    month_map["Mar"] = 3;
    month_map["Apr"] = 4;
    month_map["May"] = 5;
    month_map["Jun"] = 6;
    month_map["Jul"] = 7;
    month_map["Aug"] = 8;
    month_map["Sep"] = 9;
    month_map["Oct"] = 10;
    month_map["Nov"] = 11;
    month_map["Dec"] = 12;
}   

However, it is much better to use initializer lists, so if you have a C++11 compiler, do use those like @syam's answer suggests.