placed_new.cc

#include <cstdlib>
#include <cstdint>
#include <cstring>
#include <iostream>
#include <memory>
#include <string>

class InlineStorage {
 public:
  // Custom delete operator to keep C++14 from using the global operator delete(void*, size_t),
  // which would result in the compiler error:
  // "exception cleanup for this placement new selects non-placement operator delete"
  static void operator delete(void* address) {
    std::cout << "Deleting " << address << std::endl;
    ::operator delete(address);
  }

 protected:
  static void* operator new(size_t object_size, size_t data_size) {
    void* p = ::operator new(object_size + data_size);
    std::cout << "Allocating " << (object_size + data_size) << " bytes --> " << p << std::endl;
    return p;
  }
};

class MyString : public InlineStorage {
 public:
  static MyString* create(const std::string& str) {
    return new (str.size()) MyString(str.data(), str.size());
  }

  std::string toString() const { return std::string(data_, size_); }
  size_t size() const { return size_; }
  const char* data() const { return data_; }

 private:
  MyString(const char* str, size_t size) : size_(size) {
    memcpy(data_, str, size);
  }

  size_t size_;
  char data_[];
};

int main() {
  std::unique_ptr<MyString> str(MyString::create("Hello, world!"));
  std::cout << "str=" << str->toString() << std::endl;
}