202605181622placement-new

你是否被面试官问起：“假如我有一块申请好的内存，如何在这块内存上创建我的对象？”

Bro，我经验少，实际工作中压根没用过呀。但答案是可以的，这就是placement new!

形如，

#include <iostream>
#include <new> // Required for placement new
 
struct MyClass {
    int data;
    MyClass(int d) : data(d) { std::cout << "Constructor called!\n"; }
    ~MyClass() { std::cout << "Destructor called!\n"; }
};
 
int main() {
    // 1. Pre-allocate raw memory (no constructor is called yet)
    char buffer[sizeof(MyClass)];
 
    // 2. Use placement new to construct the object inside the buffer
    MyClass* objPtr = new(buffer) MyClass(42);
 
    // 3. Manually call the destructor when done (critical step!)
    objPtr->~MyClass();
 
    return 0;
}