std::map<Key,T,Compare,Allocator>::map
(1) | ||
map(); |
(直到 C++11) | |
map() : map(Compare()) {} |
(自 C++11 起) | |
explicit map( const Compare& comp, const Allocator& alloc = Allocator() ); |
(2) | |
explicit map( const Allocator& alloc ); |
(3) | (自 C++11 起) |
template< class InputIt > map( InputIt first, InputIt last, |
(4) | |
template< class InputIt > map( InputIt first, InputIt last, |
(5) | (自 C++14 起) |
map( const map& other ); |
(6) | |
map( const map& other, const Allocator& alloc ); |
(7) | (自 C++11 起) |
map( map&& other ); |
(8) | (自 C++11 起) |
map( map&& other, const Allocator& alloc ); |
(9) | (自 C++11 起) |
map( std::initializer_list<value_type> init, const Compare& comp = Compare(), |
(10) | (自 C++11 起) |
map( std::initializer_list<value_type> init, const Allocator& alloc ) |
(11) | (自 C++14 起) |
template< container-compatible-range<value_type> R > map( std::from_range_t, R&& rg, |
(12) | (自 C++23 起) |
template< container-compatible-range<value_type> R > map( std::from_range_t, R&& rg, |
(13) | (自 C++23 起) |
使用各种数据源构建新的容器,并可选地使用用户提供的分配器 alloc 或比较函数对象 comp。
如果没有提供 alloc,则分配器通过调用 std::allocator_traits<allocator_type>:: |
(自 C++11 起) |
在 类模板参数推导 期间,只有第一个参数有助于推导出容器的 |
(自 C++23 起) |
在 类模板参数推导 期间,只有第一个参数有助于推导出容器的 |
(自 C++23 起) |
内容 |
[编辑] 参数
alloc | - | 用于此容器所有内存分配的分配器 |
comp | - | 用于所有键比较的比较函数对象 |
first, last | - | 复制元素的范围 |
other | - | 另一个容器,用作源来初始化容器的元素 |
init | - | 初始化列表,用它来初始化容器的元素 |
rg | - | 一个 容器兼容范围,也就是说,一个 input_range ,其元素可转换为 value_type |
类型要求 | ||
-InputIt 必须满足 LegacyInputIterator 的要求。 | ||
-Compare 必须满足 Compare 的要求。 | ||
-Allocator 必须满足 Allocator 的要求。 |
[edit] 复杂度
[edit] 异常
对 Allocator::allocate
的调用可能会抛出异常。
[edit] 注释
容器移动构造(重载 (8,9))之后,对 other
的引用、指针和迭代器(除了结束迭代器)仍然有效,但它们引用现在位于 *this 中的元素。当前的标准通过 [container.reqmts]/67 中的笼统声明提供了这种保证,而更直接的保证正在通过 LWG issue 2321 考虑。
虽然在 C++23 之前没有正式要求,但一些实现已经在早期模式中将模板参数 Allocator
放入了 非推断上下文。
功能测试 宏 | 值 | Std | 功能 |
---|---|---|---|
__cpp_lib_containers_ranges |
202202L | (C++23) | 支持范围的 构造和插入;重载 (12,13) |
[edit] 示例
#include <iomanip> #include <iostream> #include <map> #include <string> template<typename Key, typename Value, typename Cmp> std::ostream& operator<<(std::ostream& os, std::map<Key, Value, Cmp> const& map) { os << "{ "; for (auto comma{map.size()}; auto const& p : map) os << '\'' << p.first << "' is " << p.second << (--comma ? ", " : " "); return os << "}\n"; } struct Point { double x, y; friend std::ostream& operator<<(std::ostream& os, Point pt) { return os << '(' << pt.x << ", " << pt.y << ')'; } }; struct PointCmp { bool operator()(const Point& lhs, const Point& rhs) const { return lhs.x < rhs.x; // NB: y is intentionally ignored } }; int main() { // (1) Default constructor std::map<std::string, int> map1; map1["something"] = 69; map1["anything"] = 199; map1["that thing"] = 50; std::cout << "map1 = " << map1; // (4) Range constructor std::map<std::string, int> iter(map1.find("anything"), map1.end()); std::cout << "\niter = " << iter; std::cout << "map1 = " << map1; // (6) Copy constructor std::map<std::string, int> copied(map1); std::cout << "\ncopied = " << copied; std::cout << "map1 = " << map1; // (8) Move constructor std::map<std::string, int> moved{std::move(map1)}; std::cout << "\nmoved = " << moved; std::cout << "map1 = " << map1; // (10) Initializer list constructor const std::map<std::string, int> init { {"this", 100}, {"can", 100}, {"be", 100}, {"const", 100} }; std::cout << "\ninit = " << init; std::cout << "\nCustom Key class option 1:\n"; // Use a comparison struct std::map<Point, double, PointCmp> mag = { {{5, -12}, 13}, {{3, 4}, 5}, {{-8, -15}, 17} }; std::cout << "mag = " << mag << '\n'; std::cout << "Custom Key class option 2:\n"; // Use a comparison lambda // This lambda sorts points according to their magnitudes, where // these magnitudes are taken from the local variable mag. auto cmpLambda = [&mag](const Point& lhs, const Point& rhs) { return mag[lhs] < mag[rhs]; }; // You could also use a lambda that is not dependent on local variables, like this: // auto cmpLambda = [](const Point& lhs, const Point& rhs){ return lhs.y < rhs.y; }; std::map<Point, double, decltype(cmpLambda)> magy(cmpLambda); // Various ways of inserting elements: magy.insert(std::pair<Point, double>({5, -12}, 13)); magy.insert({{3, 4}, 5}); magy.insert({Point{-8.0, -15.0}, 17}); std::cout << "magy = " << magy << '\n'; std::cout << "Construction from a range:\n"; using PS = std::pair<const std::string, int>; const auto rg = {PS{"one", 1}, {"one", 101}, {"two", 2}, {"three", 3}}; #if __cpp_lib_containers_ranges std::map<std::string, int> nums(std::from_range, rg); // overload (12) #else std::map<std::string, int> nums(rg.begin(), rg.end()); // fallback to (4) #endif std::cout << "nums = " << nums << '\n'; }
输出
map1 = { 'anything' is 199, 'something' is 69, 'that thing' is 50 } iter = { 'anything' is 199, 'something' is 69, 'that thing' is 50 } map1 = { 'anything' is 199, 'something' is 69, 'that thing' is 50 } copied = { 'anything' is 199, 'something' is 69, 'that thing' is 50 } map1 = { 'anything' is 199, 'something' is 69, 'that thing' is 50 } moved = { 'anything' is 199, 'something' is 69, 'that thing' is 50 } map1 = { } init = { 'be' is 100, 'can' is 100, 'const' is 100, 'this' is 100 } Custom Key class option 1: mag = { '(-8, -15)' is 17, '(3, 4)' is 5, '(5, -12)' is 13 } Custom Key class option 2: magy = { '(3, 4)' is 5, '(5, -12)' is 13, '(-8, -15)' is 17 } Construction from a range: nums = { 'one' is 1, 'three' is 3, 'two' is 2 }
[edit] 缺陷报告
以下更改行为的缺陷报告被追溯应用于之前发布的 C++ 标准。
DR | 应用于 | 已发布的行为 | 正确行为 |
---|---|---|---|
LWG 2076 | C++11 | 重载 (4) 有条件地要求 Key 和 T 为 CopyInsertable 到 *this |
不需要 |
LWG 2193 | C++11 | 默认构造函数是显式的 | 改为非显式的 |
[edit] 另请参阅
将值分配给容器 (公有成员函数) |