std::initializer_list - cppreference.com (original) (raw)

(not to be confused with member initializer list)

| | | | | --------------------------------------------- | | ------------- | | template< class T > class initializer_list; | | (since C++11) |

An object of type std::initializer_list<T> is a lightweight proxy object that provides access to an array of objects of type const T (that may be allocated in read-only memory).

A std::initializer_list object is automatically constructed when:

• a brace-enclosed initializer list is used to list-initialize an object, where the corresponding constructor accepts an std::initializer_list parameter,
• a brace-enclosed initializer list is used as the right operand of assignment or as a function call argument, and the corresponding assignment operator/function accepts an std::initializer_list parameter,
• a brace-enclosed initializer list is bound to auto, including in a ranged for loop.

std::initializer_list may be implemented as a pair of pointers or pointer and length. Copying a std::initializer_list does not copy the backing array of the corresponding initializer list.

The program is ill-formed if an explicit or partial specialization of std::initializer_list is declared.

Contents

• 1 Member types
• 2 Member functions
  • 2.1 Capacity
  • 2.2 Iterators
• 3 Non-member functions
  • 3.1 Free function templates overloaded for std::initializer_list
• 4 Notes
• 5 Example
• 6 Defect reports
• 7 See also

[edit] Member types

[edit] Member functions

[edit] Non-member functions

[edit] Notes

[edit] Example

#include #include #include #include   template struct S { std::vector v;   S(std::initializer_list l) : v(l) { std::cout << "constructed with a " << l.size() << "-element list\n"; }   void append(std::initializer_list l) { v.insert(v.end(), l.begin(), l.end()); }   std::pair<const T*, std::size_t> c_arr() const { return {&v[0], v.size()}; // copy list-initialization in return statement // this is NOT a use of std::initializer_list } };   template void templated_fn(T) {}   int main() { S s = {1, 2, 3, 4, 5}; // copy list-initialization s.append({6, 7, 8}); // list-initialization in function call   std::cout << "The vector now has " << s.c_arr().second << " ints:\n";
for (auto n : s.v) std::cout << n << ' '; std::cout << '\n';   std::cout << "Range-for over brace-init-list: \n"; for (int x : {-1, -2, -3}) // the rule for auto makes this ranged-for work std::cout << x << ' '; std::cout << '\n';   auto al = {10, 11, 12}; // special rule for auto std::cout << "The list bound to auto has size() = " << al.size() << '\n'; auto la = al; // a shallow-copy of top-level proxy object assert(la.begin() == al.begin()); // guaranteed: backing array is the same   std::initializer_list il{-3, -2, -1}; assert(il.begin()[2] == -1); // note the replacement for absent operator[] il = al; // shallow-copy assert(il.begin() == al.begin()); // guaranteed   // templated_fn({1, 2, 3}); // compiler error! "{1, 2, 3}" is not an expression, // it has no type, and so T cannot be deduced templated_fn<std::initializer_list>({1, 2, 3}); // OK templated_fn<std::vector>({1, 2, 3}); // also OK }

Output:

constructed with a 5-element list The vector now has 8 ints: 1 2 3 4 5 6 7 8 Range-for over brace-init-list: -1 -2 -3 The list bound to auto has size() = 3

[edit] Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

[edit] See also