Effective C++ 52. Write placement delete if you write placement new-优快云博客

本文探讨了C++中自定义new和delete运算符的方法，并展示了如何通过这些运算符进行内存分配和释放，包括如何处理异常情况及如何在派生类中隐藏全局和继承的new运算符。

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I.
if memory allocation succeeds and the Widget constructor throws an exception, the runtime system is responsible for undoing the allocation that operator new performed. The runtime system looks for a version of operator delete that takes the same number and types of extra arguments as operator new.

class Widget {
public:
    static void* operator new(std::size_t size, std::ostream& logStream) throw(std::bad_alloc);
    static void operator deleted(void* pMemory) throw();
    static void operator deleted(void* pMemory, std::ostream& logStream) throw();
};

Widget *pw =  new(std::cerr)Widget; // no leak 
// if no exception is thrown, the normal operator delete is invoked
delet pw; // invokes the normal operator delete

II.
Operator news in derived classes hide both global and inherited versions of operator new
C++ offers the following forms of operator new at global scope

void* operator new(std::size_t) throw(std::bad_alloc);
void* operator new(std::size_t, void*) throw();
void* operator new(std::size_t, const std::nothrow_t&) throw();

calss StandardNewDeletForms {
public:
    // normal new/delete
    static void* operator new(std::size_t size) throw(std::bad_alloc) { 
        return ::operator new(size); 
    }
    static void operator delete(void *pMemory) throw() {
        ::operator delete(pMemory);
    }

    // placement new/delete
    static void* operator new(std::size_t size, void* ptr) throw() {
        return ::opertator new(size, ptr); 
    }
    static void operator delete(void* pMemory, void* ptr) throw() {
        return ::operator delete(pMemory, ptr);
    }

    // nothrow new/delet
    static void* operator new(std::size_t size, const std::nothrow_t& nt) throw() {
        return ::operator new(size, nt);
    }
    static void operator delete(void* pMemory,const std::nothrow_t&) throw() {
        ::operator delete(pMemory);
    }
};

class Widget: public StandardNewDeleteForms {
public:
    using StandardNewDeleteForms::operator new;
    using StandardNewDeleteForms::operator delete;

    static void* operator new(std::size_t size, std::ostream& logStream) throw(std::bad_alloc);
    static void operator delete(void *pMemory, ostream& logStream) throw();
};