在讲解高级初始化技术之前，先看一个前置背景，使用下面的类来方便地说明何时调用它的特殊成员函数。这样，我们就能看到额外的临时对象。
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struct MyType { MyType() { std::cout << "MyType default\n"; } explicit MyType(std::string str) : str_(std::move(str)) { std::cout << std::format("MyType {}\n", str_); } ~MyType() { std::cout << std::format("~MyType {}\n", str_); } MyType(const MyType& other) : str_(other.str_) { std::cout << std::format("MyType copy {}\n", str_); } MyType(MyType&& other) noexcept : str_(std::move(other.str_)) { std::cout << std::format("MyType move {}\n", str_); } MyType& operator=(const MyType& other) { if (this != &other) str_ = other.str_; std::cout << std::format("MyType = {}\n", str_); return *this; } MyType& operator=(MyType&& other) noexcept { if (this != &other) str_ = std::move(other.str_); std::cout << std::format("MyType = move {}\n", str_); return *this; } std::string str_; };
现在可以从相对简单但基本的元素开始。
二、reserve 结合 emplace_back
C++中的vector是一种可以根据需要增长的动态数组。但是，每次向量增长超过其当前容量时，它可能需要重新分配内存，这代价是昂贵的。为了优化这一点，可以使用reserve()方法结合emplace_back()。
reserve方法不改变vector的大小，但确保vector有足够的已分配内存来存储指定数量的元素。通过提前预留空间，可以防止在向向量添加元素时进行多次重新分配。
示例：
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#include <iostream> #include <string> #include <vector> #include <format> struct MyType { MyType() { std::cout << "MyType default\n"; } explicit MyType(std::string str) : str_(std::move(str)) { std::cout << std::format("MyType {}\n", str_); } ~MyType() { std::cout << std::format("~MyType {}\n", str_); } MyType(const MyType& other) : str_(other.str_) { std::cout << std::format("MyType copy {}\n", str_); } MyType(MyType&& other) noexcept : str_(std::move(other.str_)) { std::cout << std::format("MyType move {}\n", str_); } MyType& operator=(const MyType& other) { if (this != &other) str_ = other.str_; std::cout << std::format("MyType = {}\n", str_); return *this; } MyType& operator=(MyType&& other) noexcept { if (this != &other) str_ = std::move(other.str_); std::cout << std::format("MyType = move {}\n", str_); return *this; } std::string str_; }; int main() { { std::cout << "--- push_back\n"; std::vector<MyType> vec; vec.push_back(MyType("First")); std::cout << std::format("capacity: {}\n", vec.capacity()); vec.push_back(MyType("Second")); } { std::cout << "--- emplace_back\n"; std::vector<MyType> vec; vec.emplace_back("First"); std::cout << std::format("capacity: {}\n", vec.capacity()); vec.emplace_back("Second"); } { std::cout << "--- reserve() + emplace_\n"; std::vector<MyType> vec; vec.reserve(2); // Reserve space for 2 elements vec.emplace_back("First"); vec.emplace_back("Second"); } }
输出：
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--- push_back MyType First MyType move First ~MyType capacity: 1 MyType Second MyType move Second MyType move First ~MyType ~MyType ~MyType First ~MyType Second --- emplace_back MyType First capacity: 1 MyType Second MyType move First ~MyType ~MyType First ~MyType Second --- reserve() + emplace_ MyType First MyType Second ~MyType First ~MyType Second
在这个例子中，可以看到三种插入技术之间的比较：
push_back()。emplace_back()。reserve()结合emplace_back。
第一种情况下，必须将临时对象传递给push_back，并移动它们来初始化vector的元素。但是还有一个重新分配，因为当添加第二个元素时，vector必须增长。
相对而言，emplace_back()技术更好，更容易编写，因为没有创建临时对象。
但是，第三种选择是最有效的，因为可以预先预留空间，然后在适当的地方创建元素。
通过使用reserve和emplace_back，可以确保vector在添加元素到预留容量时不需要重新分配内存。这种组合是优化性能的一种强大方式，特别是在向vector`中添加多个元素时。
三、C++ 20的constinit
constinit是一个强大的工具，用于强制常量初始化，特别是对于静态或线程局部变量。这个关键字在C++ 20中引入，它解决了C++中一个长期存在的难题：静态初始化顺序的问题。通过确保变量在编译时初始化，constinit提供了一个更可预测和更安全的初始化过程。
在其核心，constinit保证它所限定的变量在编译时被初始化。这对全局变量或静态变量尤其有益，可以确保它们不受动态初始化顺序问题的影响。
示例：
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#include <array> // 编译期初始化 constexpr int compute(int v) { return v*v*v; } constinit int global = compute(10); // 这个将不再起作用: // constinit int another = global; int main() { // 但允许以后更改…… global = 100; // global is not constant! // std::array<int, global> arr; }
在上面的代码中，全局变量是在编译时使用compute函数初始化的。然而，与const或constexpr不同，constinit不会使变量不可变。也就是说，虽然它的初始值是在编译时设置的，但可以在运行时对其进行修改，如main函数所示。此外，由于constinit变量不是constexpr，因此不能使用它初始化另一个constinit对象(如其他的int)。
四、Lambda表达式和初始化
C++ 14对Lambda捕获进行了重大更新，引入了在Lambda捕获子句中直接初始化新数据成员的能力。这个特性称为带有初始化器的捕获或广义Lambda捕获，它在使用Lambda时为我们提供了更大的灵活性和精度。
传统上，Lambda表达式可以从其封闭范围捕获变量。在C++ 14中，现在可以直接在捕获子句中创建和初始化新的数据成员，使Lambdas更加通用。
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