C++ * vs & 在函数声明中
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c++ * vs & in function declaration
提问by Paul Tarjan
Possible Duplicate:
Difference between pointer variable and reference variable in C++
可能的重复：
C++ 中指针变量和引用变量的区别
When should I declare my variables as pointers vs objects passed-by-reference? They compile to the same thing in assembly (at least run-time asymptotically) so when should I use which?
我什么时候应该将我的变量声明为指针和按引用传递的对象？它们在汇编中编译为相同的东西（至少在运行时渐近）所以我什么时候应该使用哪个？
void foo(obj* param)
void foo(obj& param)
采纳答案by Mykola Golubyev
My rule is simple: use * when you want to show that value is optional and thus can be 0.
我的规则很简单：当您想显示该值是可选的，因此可以为 0 时，请使用 *。
Excluding from the rule: all the _obj_s around are stored in containers and you don't want to make your code look ugly by using everywhere foo(*value);instead of foo(value);So then to show that value can't be 0 put assert(value);at the function begin.
从规则中排除：周围的所有 _obj_s 都存储在容器中，并且您不希望通过在任何地方使用foo(*value);而不是foo(value);So then 来使代码看起来丑陋，然后显示值不能为 0 放在assert(value);函数开始处。
回答by MahlerFive
I follow the Google style guide standardas it makes the most sense to me. It states:
我遵循Google 风格指南标准，因为它对我来说最有意义。它指出：
Within function parameter lists all references must be const:
void Foo(const string &in, string *out);In fact it is a very strong convention in Google code that input arguments are values or const references while output arguments are pointers. Input parameters may be const pointers, but we never allow non-const reference parameters.
One case when you might want an input parameter to be a const pointer is if you want to emphasize that the argument is not copied, so it must exist for the lifetime of the object; it is usually best to document this in comments as well. STL adapters such as bind2nd and mem_fun do not permit reference parameters, so you must declare functions with pointer parameters in these cases, too.
在函数参数列表中，所有引用都必须是 const：
void Foo(const string &in, string *out);事实上，在谷歌代码中输入参数是值或常量引用，而输出参数是指针，这是一个非常强的约定。输入参数可能是 const 指针，但我们绝不允许非常量引用参数。
您可能希望输入参数是 const 指针的一种情况是，如果您想强调该参数未被复制，因此它必须在对象的生命周期内存在；通常最好在评论中记录这一点。STL 适配器如 bind2nd 和 mem_fun 不允许引用参数，因此在这些情况下您也必须声明带有指针参数的函数。
回答by Greg Hewgill
One reason to use pointers is if it makes sense to pass a NULLvalue into the function. With a pointer, it's expected to be able to do this. With a reference, it is not expected to be able to do this.
使用指针的一个原因是NULL向函数传递一个值是否有意义。使用指针，预计能够做到这一点。有了参考，预计无法做到这一点。
(However, by doing tricky things it is still possible to pass a NULL into a reference parameter. You can expect that the called function may crash in this case.)
（但是，通过做一些棘手的事情，仍然可以将 NULL 传递给引用参数。在这种情况下，您可以预期被调用的函数可能会崩溃。）
Another convention is that if you pass a pointer into a function, the function may use the pointer to take ownership of the object (especially in a COM-like reference counted environment). If you pass a reference, then the called function can expect to use the object for the duration of the function call but not to keep a pointer to the object for use later.
另一个约定是，如果您将指针传递给函数，该函数可能会使用该指针来获取对象的所有权（尤其是在类似 COM 的引用计数环境中）。如果传递引用，则被调用的函数可以期望在函数调用期间使用该对象，但不会保留指向该对象的指针以供以后使用。
回答by quamrana
The other difference between pointers and references is that it is implied that you won't hold on to a reference, unless you pass one to a constructor. Passing pointers may mean that an object might hold onto it for a while, like a composite pattern object.
指针和引用之间的另一个区别是暗示您不会保留引用，除非您将引用传递给构造函数。传递指针可能意味着一个对象可能会保留一段时间，就像一个复合模式对象。
回答by elcuco
A good reason why I use pointers and not references in my C++ applications, is readability. When using a pointer you see what is really happening, and when using pointers the syntax tells you what's really happening.
我在 C++ 应用程序中使用指针而不是引用的一个很好的原因是可读性。使用指针时，您会看到实际发生的情况，而使用指针时，语法会告诉您实际发生的情况。
Same goes for "0" or "NULL". I prefer using "NULL", this way 3 months later when I see a code that looks like:
“0”或“NULL”也是如此。我更喜欢使用“NULL”，这样 3 个月后，当我看到如下代码时：
somevar_1 = 0;
somevar_2 = NULL;
I know that somevar_1is an int(or float) and somevar_2is some kind of pointer.
我知道这somevar_1是一个int（或float）并且somevar_2是某种指针。
