In C++0x, there's a new language feature called initializer_list. In this post, we will see some hints about this helpful feature.
We have not a uniform initializing method in the previous C++ standards. For example:
int i = 0; int j(0); char a[6] = "C++0x"; // ={'C', '+', '+', '0', 'x'}; Student s("Bjarne", "Stroustrup");
or
vector<int> v(5); // 5 items v.push_back(8); v.push_back(1); //other 3 items are 0
There are different styles of initilizing which is confusing programmers. So, a uniform initializing method would be great: Thanks to C++0x.
First, we will see some quick usages of this feature to warm up:
vector<int> v = {8, 1, 7, 9}; //see the difference in previous vector initialization list<string> cities = {"Ankara", "Istanbul", "Izmir"}; int a[3][4] = { {0, 1, 2, 3}, {4, 5, 6, 7}, {8, 9, 10, 11} }; void f(initializer_list<int> args) //This function takes an initializer list as argument(an immutable sequence) { for (auto p = args.begin(); p != args.end(); ++p) cout << *p << "\n"; } f({1, 2}); // passing a list on the fly vector<double> v1 { 7 }; // ok: v1 has 1 element (with its value 7) v1 = {9}; // ok v1 now has 1 element (with its value 9). how can we do that without initializer_list? // we should first clear the vector and then push '9' to the vector vector<double> v2 = {9}; // ok: v2 has 1 element (with its value 9)
Of course this new feature is widely used in standard library of C++0x. Here are two samples taken from GCC implementation of C++0x:
* std::string class:
In GCC implementation there's a new constructor that makes use of initializer_list,
#ifdef __GXX_EXPERIMENTAL_CXX0X__ /** * @brief Construct string from an initializer list. * @param l std::initializer_list of characters. * @param a Allocator to use (default is default allocator). */ basic_string(initializer_list<_CharT> __l, const _Alloc& __a = _Alloc()); #endif // __GXX_EXPERIMENTAL_CXX0X__
Let's create a string object with this new style constructor,
std::string s({'C', '+', '+', '0', 'x'});
std::cout << s << "\n"; Output:
C++0x
* vector class:
Assignment operator in vector class shows us how to use initializer_list,
#ifdef __GXX_EXPERIMENTAL_CXX0X__ vector& operator=(initializer_list<value_type> __l) { this->assign(__l.begin(), __l.end()); return *this; } #endif // __GXX_EXPERIMENTAL_CXX0X__
When we assign an initializer_list to the vector object, this operator=() function is called:
v1 = {9};Analyzing the std::initializer_list implementation is a good work to understand the inner details.
Reference:
http://www2.research.att.com/~bs/C++0xFAQ.html#init-list
