[英]Void pointers in C++
I have written this qsort: 我写了这个qsort:
void qsort(void *a[],int low,int high, int (*compare)(void*,void*));
When I call this on 当我打电话给你
char *strarr[5];
It says invalid conversion from char** to void**. 它表示从char **到void **的无效转换。 Why this is wrong? 为什么这是错的?
This is the code: 这是代码:
#include<cstdlib>
#include<cstdio>
#include<iostream>
using namespace std;
inline void strswap(void *a,void *b) {
char *t=*(char**)a;
*(char**)a=*(char**)b;
*(char**)b=t;
}
int strcompare(void *a, void *b) {
return strcmp(*(char**)a,*(char**)b);
}
void qsort1(void *a[],int low,int high, int (*compare)(void*,void*), void (*swap)(void*,void*)) {
if(low>=high)
return;
int q=low-1;
for(int i=low;i<=high-1;i++)
if((*compare)(&a[i],&a[high]) < 0)
swap(&a[i],&a[++q]);
swap(&a[high],&a[++q]);
qsort1(a,low,q-1,compare,swap);
qsort1(a,q+1,high,compare,swap);
}
int main() {
const int n=3;
//int a[n]={4,6,8,12,10,9,8,0,24,3};
char *strarr[5]={"abcd","zvb","cax"};
qsort1(strarr,0,n-1,strcompare,strswap);
for(int i=0;i<n;i++)
cout << strarr[i] << " ";
cout << endl;
return 0;
}
An implicit conversion from any pointer type to void *
is allowed, because void *
is a defined to be a pointer type that has a sufficient range that it can represent any value that any other pointer type can. 允许从任何指针类型到void *
的隐式转换,因为void *
被定义为具有足够范围的指针类型,它可以表示任何其他指针类型可以包含的任何值。 (Technically, only other object pointer types, which excludes pointers to functions). (从技术上讲,只有其他对象指针类型,它排除了指向函数的指针)。
This does not mean that void *
has the same size or representation as any other pointer type, though: Converting a pointer from another pointer type to a void *
does not necessarily leave the underlying representation unchanged . 这并不意味着void *
具有与任何其他指针类型相同的大小或表示,但是:将指针从另一个指针类型转换为void *
不一定会使底层表示保持不变 。 Converting from double *
to void *
is just like converting from double
to int
- it has to happen in full view of the compiler, you can't hide that conversion behind the compiler's back. 从double *
转换为void *
就像从double
转换为int
- 它必须在编译器的完整视图中发生,你无法隐藏编译器后面的转换。
So this implies that while void *
is a generic pointer, void **
is not a generic pointer-to-pointer. 所以这意味着虽然void *
是一个通用指针,但void **
不是一个通用的指向指针的指针。 It's a pointer to void *
- a void **
pointer should only ever point to real void *
objects (whereas void *
itself can point to anything). 它是一个指向void *
的指针 - 一个void **
指针应该只指向真正的void *
对象(而void *
本身可以指向任何东西)。
This is why there's no implicit conversions between type **
and void **
- it's for the same reason that there's no implicit conversions between double *
and int *
. 这就是为什么type **
和void **
之间没有隐式转换的原因 - 这是因为double *
和int *
之间没有隐式转换。
Now, there is one special case: for historical reasons, char *
is guaranteed to have the same size, representation and alignment requirements as void *
. 现在,有一个特例:由于历史原因, char *
保证具有与void *
相同的大小,表示和对齐要求。 This means that conversions between char **
(in particular) and void **
are actually OK, as an exception to the general rule. 这意味着char **
(特别是)和void **
之间的转换实际上是正常的,作为一般规则的例外。 So in your particular case, your code is correct if you add a cast to void **
when you pass strarr
to qsort1()
. 因此,在您的特定情况下,如果在将strarr
传递给qsort1()
时将strarr
添加到void **
,则代码是正确的。
However, your qsort1()
is only defined to correctly work on arrays of void *
or char *
(including unsigned char *
etc.). 但是,您的qsort1()
仅定义为正确处理void *
或char *
(包括unsigned char *
等)的数组。 You can't use it to sort an array of double *
pointers, for example (although it would actually work on most common environments today). 例如,您不能使用它来对double *
指针数组进行排序(尽管它实际上可以在当今大多数常见环境中使用)。
Any pointer can be implicitly converted to a void pointer. 任何指针都可以隐式转换为void指针。 But your first parameter isn't a void pointer - it's an array of void pointers, and there is no implicit conversion to that. 但是你的第一个参数不是一个void指针 - 它是一个void指针数组,并没有隐式转换。 You probably want to declare your function as: 您可能希望将您的函数声明为:
void qsort(void *,int low,int high, int (*compare)(void*,void*));
but it's difficult to say without seeing the code. 但是没有看到代码就很难说。
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