C ++类型转换带指针 [英] C++ type casting with pointers

问题描述

我来自C＃和Java的背景，我似乎不明白在C ++中用指针表示什么意思。

I come from a background of C# and Java and I can't seem to understand what casting with pointers means in C++.

例如：

int x = 1;
char c = *((char*)&x);

它做什么？它是什么有用的？

What does it do? What it is useful for?

推荐答案

在你的例子中，你犯错误使代码不编译。因此，我假设您尝试执行以下操作：

In both your examples you're making mistakes making the code not compile. So I'll assume you're trying to do the following:

int x = 1;
char c = *((char*)&x);

根据您的架构， c 具有 x 的最小或最高有效字节的值。在这个例子中，这将是0或1（这实际上可以用于检测字节排序）。

Depending on your architecture, c will now have either the value of the least or the most significant byte of x. In this example this would be either 0 or 1 (this can actually be used to detect the byte ordering).

你的第二个例子将不工作，因为你试图忽略 const 导致非法操作/错误转换（这也称为const正确性）。

Your second example won't work, cause you're trying to ignore the const resulting in an illegal operation/bad cast (this is also called "const correctness").

编辑：关于您对这是什么意思？的评论：

Regarding your comment about "what does it mean?":

在表达式中：
& somevariable 将返回 somevariable 的地址。
* somevariable 将假设 somevariable 的内容是实际值的地址， 。

In expressions: &somevariable will return the address of somevariable. *somevariable will assume the contents of somevariable are the address of the actual value, which is then returned.

在声明中：
datatype 是正常变量/对象。这是通过按值。
datatype& 是引用。这与Java / C＃中的正常变量完全相同，并通过引用传递。
datatype * 是一个指针。

In declarations: datatype is a normal variable/object. This is passed "by value". datatype& is a reference. This works exactly like normal variables in Java/C# and is passed by reference. datatype* is a pointer. This just contains the address where the actual value is located (see above) and is essentially passed by reference as well.

实际的转换工作与Java / C＃非常相似，但指针就是这样：它们指向实际值的位置。虽然这可能会让你困惑，C / C ++中的指针与Java / C＃中使用的标准变量/引用非常相似。

Actual casts work pretty much similar to Java/C#, but pointers are just that: They point to the location of the actual value. While this might confuse you, pointers in C/C++ work pretty much like the standard variables/references used in Java/C#.

看看：

MyClass x; // object of MyClass
MyClass *x; // pointer to an object of MyClass - the actual value is undefined and trying to access it will most likely result in an access violation (due to reading somewhere random).
MyClass *x = 0; // same as above, but now the default value is defined and you're able to detect whether it's been set (accessing it would essentially be a "null reference exception"; but it's actually a null pointer).
MyClass &x = MyClass(); // creating a new reference pointing to an existing object. This would be Java's "MyClass x = new MyClass();"

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