head File
#include <initializer_list>
template< class T >
class initializer_list;
std :: initializer_list type of object is an object of type const T access an array of lightweight proxy object.
std :: initializer_list objects are automatically constructed at these times:
- Initializing a list of objects braces initializer lists, where the corresponding constructor takes a parameter std :: initializer_list
- In brace initializer list is a right operand assignment, or the function call parameters, to correspond to the assignment operator / function takes parameters std :: initializer_list
- Binding brace initialization list to auto, included in the scope of the for loop
A pair of pointers or pointer initializer_list its length may be achieved. Copy a std :: initializer_list does not copy its underlying objects.
for example:
#include <iostream>
#include <vector>
#include <initializer_list>
template <class T>
struct S {
std::vector<T> v;
S(std::initializer_list<T> l) : v(l) {
std::cout << "constructed with a " << l.size() << "-element list\n";
}
void append(std::initializer_list<T> l) {
v.insert(v.end(), l.begin(), l.end());
}
std::pair<const T*, std::size_t> c_arr() const {
return {&v[0], v.size()}; // 在 return 语句中复制列表初始化
// 这不使用 std::initializer_list
}
};
template <typename T>
void templated_fn(T) {}
int main()
{
S<int> s = {1, 2, 3, 4, 5}; // 复制初始化
s.append({6, 7, 8}); // 函数调用中的列表初始化
std::cout << "The vector size is now " << 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}) // auto 的规则令此带范围 for 工作
std::cout << x << ' ';
std::cout << '\n';
auto al = {10, 11, 12}; // auto 的特殊规则
std::cout << "The list bound to auto has size() = " << al.size() << '\n';
// templated_fn({1, 2, 3}); // 编译错误！“ {1, 2, 3} ”不是表达式，
// 它无类型，故 T 无法推导
templated_fn<std::initializer_list<int>>({1, 2, 3}); // OK
templated_fn<std::vector<int>>({1, 2, 3}); // 也 OK
}
The output is:
constructed with a 5-element list
The vector size is now 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
