为 new 运算符编写重载函数

发布于 2024-12-16 12:27:01 字数 300 浏览 1 评论 0原文

我编写了一个简单的函数，如下所示：

void *operator new(size_t size) throw(std::bad_alloc)
{
    void *p;
    p =  malloc(size);
    if(!p)
       throw bad_alloc();
    return p;
}

我还能做些什么来改进这个？ malloc 会比 new 更有效吗？如果我想写 new[] 我只需要更改函数签名吗？

原文

I have written a simple function as following:

void *operator new(size_t size) throw(std::bad_alloc)
{
    void *p;
    p =  malloc(size);
    if(!p)
       throw bad_alloc();
    return p;
}

What else can i do to improve this? Would malloc be more effective than new? If I want to write new[] do I only need to change just the function signature?

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绮筵 2024-12-23 12:27:01

这段代码将按原样工作，但是如果你这样做，你几乎需要编写一个匹配的::operator delete来使用它：

void operator delete(void *block) throw() {
    ::free(block);
}

就我个人而言，我可能会将您的代码修改为更像这样的内容：

void *operator new(size_t size) throw(std::bad_alloc)
{
    void *p =  malloc(size);
    if(!p)
       throw bad_alloc();
    return p;
}

我更喜欢初始化我能做的一切，而不是创建一个未初始化的变量，然后才为其赋值。在这种情况下，差异很小，但我认为这是一个值得培养的习惯。

至于比 operator new 的默认实现更有效，我想说的是，它不会更有效，而且很可能效率较低。您提供的基本上是在 C++ 诞生之前实现了多少个标准库，但从那时起，许多（大多数？）在 ::operator new 的实现上做了更多工作来定制它更接近于 C++ 中动态分配内存的使用方式（其中 malloc 主要面向 C 中的使用方式，通常至少有一点不同）。

就 new[] 而言，是的，这只是函数签名的更改。尽管用于单分配和多分配，但对operator new和operator new[]`的要求是相同的。

This code will work the way it is, but if you do this, you pretty much need to write a matching ::operator delete that will work with it:

void operator delete(void *block) throw() {
    ::free(block);
}

Personally, I'd probably modify your code to something more like:

void *operator new(size_t size) throw(std::bad_alloc)
{
    void *p =  malloc(size);
    if(!p)
       throw bad_alloc();
    return p;
}

I prefer to initialize everything I can, rather than create an uninitialized variable, and only later assign a value to it. In this case, the difference is pretty minor, but I consider it a habit worth cultivating.

As far as being more effective than the default implementation of operator new, I'd say chances are that no, it won't be more effective, and might well be less effective. What you've provided is basically how many standard libraries were implemented toward the dawn of C++, but since then, many (most?) have done more work on their implementation of ::operator new to tailor it more closely to how dynamically allocated memory tends to be used in C++ (where malloc is mostly oriented toward how it's used in C, which is typically at least a little different).

As far as new[] goes, yes, it's just a change of function signature. Despite being used for single vs. multiple allocations, the requirements on operator new and operator new[]` are identical.

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岁月打碎记忆 2024-12-23 12:27:01

如果您替换全局operator new，您还应该替换nothrow 变体。当然，还有全局运算符delete（普通和nothrow变体，因为如果对象的构造函数抛出异常，operator delete的nothrow变体code> 被调用为 nothrot operator new）。

我想在大多数实现中，内置的operator new 看起来或多或少与您的替代品完全相同。无论如何，一般来说，我不希望您能够击败 operator new 的内部实现。