Item 51 new和delete的規則

● 全域new/delete重載

void * operator new(std::size_t size) throw(std::bad_alloc)<br />{<br />using namespace std;<br />if (size == 0) {// C++規定：零申請也要給一個位元組<br />size = 1;<br />}<br />void* mem = NULL;<br />while (true) {<br />mem = malloc(size);<br />if (mem) {<br /> return mem;// 退出死迴圈的條件1：申請成功<br />}<br />new_handler globalHandler = set_new_handler(0);<br />set_new_handler(globalHandler);<br />if (globalHandler)<br />(*globalHandler)();// 條件2：讓new_handler處理<br />else<br />throw std::bad_alloc();// 條件3：異常<br />}<br />}<br />void operator delete(void* mem) throw()<br />{<br />if (mem == 0) return;// C++規定，對NULL指標delete應該是安全的<br />free(mem);<br />}

 

● 某個類的new重載

class Base {<br />public:<br />static void * operator new(std::size_t size) throw(std::bad_alloc);<br />static void * operator new[](std::size_t size) throw(std::bad_alloc);<br />static void operator delete(void *mem, std::size_t size) throw();<br />...<br />};<br />void * Base::operator new(std::size_t size) throw(std::bad_alloc)<br />{<br />if (size != sizeof(Base)) // 1. Base是個freestanding類，尺寸永不為零！--C++規定<br />return ::operator new(size); // 2. 如果是衍生類別調用，那麼交給全域new去處理<br />...<br />}<br />void * Base::operator new[](std::size_t size) throw(std::bad_alloc)<br />{<br />return malloc(size); // 3. 只需申請記憶體，不能處理那些尚不存在的對象，類型也未知<br />}<br />void Base::operator delete(void *mem, std::size_t size) throw()<br />{<br />if (mem == 0) return;<br />if (size != sizeof(Base)) { // 4. 檢查是否來自衍生類別的調用<br />::operator delete(mem);<br />return;<br />}<br />free(mem);<br />}<br />class Derived: public Base // 5. 沒有重載new的衍生類別<br />{...} // 6. 必須有虛擬dtor，否則delete調用Base::operator delete時size會傳錯<br />Derived *p = new Derived; // 6. 調用 Base::operator new