[英]Why can't char** be the return type of the following function in C++?
I have the following function in C++ : 我在C ++中有以下功能:
char** f()
{
char (*v)[10] = new char[5][10];
return v;
}
Visual studio 2008 says the following: Visual Studio 2008表示如下:
error C2440: 'return' : cannot convert from 'char (*)[10]' to 'char **'
What exactly should the return type to be, in order for this function to work? 为了使这个功能起作用,返回类型究竟应该是什么?
char**
is not the same type as char (*)[10]
. char**
与char (*)[10]
。 Both of these are incompatible types and so char (*)[10]
cannot be implicitly converted to char**
. 这两者都是不兼容的类型,因此
char (*)[10]
不能隐式转换为char**
。 Hence the compilation error. 因此编译错误。
The return type of the function looks very ugly. 该函数的返回类型看起来非常难看。 You have to write it as:
你必须把它写成:
char (*f())[10]
{
char (*v)[10] = new char[5][10];
return v;
}
Or you can use typedef
as: 或者你可以使用
typedef
:
typedef char carr[10];
carr* f()
{
char (*v)[10] = new char[5][10];
return v;
}
Basically, char (*v)[10]
defines a pointer to a char
array of size 10. It's the same as the following: 基本上,
char (*v)[10]
定义了一个指向大小为10的char
数组的指针。它与以下内容相同:
typedef char carr[10]; //carr is a char array of size 10
carr *v; //v is a pointer to array of size 10
So your code becomes equivalent to this: 所以你的代码就等同于:
carr* f()
{
carr *v = new carr[5];
return v;
}
cdecl.org
helps here: cdecl.org
在这里帮助:
char v[10]
reads as declare v as array 10 of char
char v[10]
读作declare v as array 10 of char
char (*v)[10]
reads as declare v as pointer to array 10 of char
char (*v)[10]
读作declare v as pointer to array 10 of char
A pointer to pointers is not the same as a pointer to arrays. 指向指针的指针与指向数组的指针不同。
(In particular, notice how sizeof(*ptr1)
is sizeof(char)*6
, whereas sizeof(*ptr3)
is sizeof(char*)
— this has serious ramifications for pointer arithmetic.) (特别注意
sizeof(*ptr1)
是sizeof(char)*6
,而sizeof(*ptr3)
是sizeof(char*)
- 这对指针算法有严重的影响。)
new char[5][10]
gives you a char (*)[10]
(which has absolutely nothing to do with function pointers, by the way), because the pointers and chars are laid out in that fashion in memory (my second example). new char[5][10]
给你一个char (*)[10]
(顺便说一下,它与函数指针完全无关),因为指针和字符在内存中以这种方式排列(我的第二个)例)。
This is not the same as char**
(which represents a different layout), so a conversion between the two makes no sense; 这是不一样
char**
(代表一个不同的布局),因此两者之间的转换是没有意义的; hence, it is disallowed. 因此,它是不允许的。
So your function's return type must be char (*)[10]
: 所以你的函数的返回类型必须是
char (*)[10]
:
char (*f())[10] {
char (*v)[10] = new char[5][10];
return v;
}
Of course, this is really ugly, so you're far better off with a std::vector<std::string>
. 当然,这真的很难看,所以你用
std::vector<std::string>
会好得多。
This FAQ entry explains it best, under the title "Conversions". 此FAQ条目以“转换”标题解释最佳。
Because char**
and char (*)[10]
are two different types. 因为
char**
和char (*)[10]
是两种不同的类型。 char**
is a pointer to pointer(to char
), while char (*)[10]
is a pointer to an array(of size 10) of char
. char**
是指向指针(指向char
)的指针,而char (*)[10]
是指向char
的数组(大小为10)的指针。 Resultantly, the moving step of char**
is sizeof(void *)
bytes which is 4 bytes on win32 platform, and the moving step of char (*)[10]
is sizeof(char) * 10
bytes. 结果,
char**
的移动步骤是sizeof(void *)
字节,在win32平台上是4个字节,而char (*)[10]
的移动步骤是sizeof(char) * 10
个字节。
Example 例
char *c = NULL;
char **v = &c;
cout << typeid(v).name() << endl;
cout << (void*)v << endl;
v += 1;
cout << (void*)v << endl;
char d[10];
char (*u)[10] = &d;
cout << typeid(u).name() << endl;
cout << (void*)u << endl;
u += 1;
cout << (void*)u << endl;
Output 产量
char * *
0034FB1C
0034FB20
char (*)[10]
001AFC50
001AFC5A
To use char (*)[10]
as a function's return type(or as input/output parameter of the function), the easiest and most readable way is to use a typedef
: 要使用
char (*)[10]
作为函数的返回类型(或函数的输入/输出参数),最简单和最易读的方法是使用typedef
:
// read from inside-out: PTRTARR is a pointer, and, it points to an array, of chars.
typedef char (*PTRTARR)[10];
Note that it can easily be mixed up with typedef
of an array of pointers , if not careful: 请注意,如果不小心,它很容易与指针数组的
typedef
混淆:
// operator[] has higher precedence than operator*,
// PTRARR is an array of pointers to char.
typedef char *PTRARR[10];
Reference 参考
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