的INT [n] [M],其中n和m是在运行时已知 [英] int[n][m], where n and m are known at runtime
问题描述
我经常需要创建宽度和高度的二维数组(让他们成为n和m),在编译时未知的,平时我写的:
矢量<&INT GT; ARR(N * M);
和我手动访问元素有:
改编[J * M + 1]
最近我告诉我可以代替写:
INT ARR [N] [M] // n和m仍然只在运行时称。
因此,这里有2个问题:
- 这是由行为的C ++标准? 允许
- 我应该如何传递这样一个数组功能? G ++报道,改编的类型为
INT(*)[N]
,但同样,n是动态的,在声明之外的功能尚不清楚(主
)。
则是问有关(其中尺寸仅取得在运行时知道)的特点是非标准扩展的C ++,但C的一个标准99(在C.11做成一个可选功能)。 (VLA)该功能被称为变长数组,链接是GCC的文档。
如果你正在使用gcc,那么你传递数组的长度作为参数传递给函数。
无效美孚(INT男,INT ARR [] [M]){
// ...
}
然而的,似乎有在任的编译器或文件中的错误,因为编译C code时,上面的函数原型语法只适用,不C ++(截至gcc版本4.8.2)。唯一的解决方法,我发现是使用无效*
参数,然后将其丢诠释函数体:
INT foo_workaround(INT男,无效* X)
{
INT(* ARR)[M] =的static_cast< INT(*)[M] GT;(x)的;
// ...
}
有其他的解决办法,如果你不希望依赖于编译器扩展。如果你不介意为每行一个单独的配置,你可以用向量的载体,例如:
的std ::矢量<的std ::矢量<&INT GT; > ARR(N,性病::矢量<&INT GT;(M));
不过,如果你想有一个单一的分配块就像你在你自己的例子证明,那么最好是创建一个围绕矢量
给你2-D的包装类喜欢的语法。
模板< typename的T>
一流的Vector2D { INT N_;
INT M_;
的std ::矢量< T> vec_; 模板< typename的I>
类vector2d_ref {
的typedef的std :: iterator_traits< I>性状;
的typedef typename的性状:: value_type的R_TYPE;
模板< typename的>友元类的Vector2D;
我P_;
vector2d_ref(I P):P_(p)的{}
上市:
R_TYPE&安培;运算符[](诠释J){返回*(P_ + J); }
}; 的typedef的std ::矢量< T> VEC;
typedef的vector2d_ref< typename的VEC:迭代> REF;
typedef的vector2d_ref< typename的VEC ::为const_iterator> CREF; 模板< typename的I>
vector2d_ref< I> REF(I P,int i)以{返回P +(我* M_); }上市: 的Vector2D(INT N,INT M):N_(N),M_(M),vec_(N * M){}
REF操作符[](int i)以{返回REF(vec_.begin(),I); }
CREF的operator [](int i)以常量{返回REF(vec_.begin(),I); }};
该包装的运算符[]
返回一个中间对象也重载运算符[]
允许2维数组使用时,包装语法。
的Vector2D< INT> V(N,M);
v [I] [J] = 7;
性病::法院LT&;< v [i] [j]的<<的'\\ n';
I often need to create a 2D array with width and height (let them be n and m) unknown at compile time, usually I write :
vector<int> arr(n * m);
And I access elements manually with :
arr[j * m + i]
I recently got told that I could instead write :
int arr[n][m] // n and m still only known at runtime.
So here are 2 questions :
- Is this behaviour allowed by the C++ Standard ?
- How should I pass such an array to a function ? g++ reports that arr has type
int (*)[n]
, but again, n is dynamic and not known outside the function where it is declared (main
).
The feature you are asking about (where the dimensions are only made known at runtime) is a non-standard extension of C++, but a standard one of C.99 (made into an optional feature in C.11). The feature is called variable-length array (VLA), and the link is the documentation for GCC.
If you are using GCC, then you are to pass the length of the array as a parameter to the function.
void foo (int m, int arr[][m]) {
//...
}
However, there seems to be a bug in either the compiler or the documentation, as the above function prototype syntax only works when compiling C code, not C++ (as of gcc version 4.8.2). The only work-around I found was to use a void *
parameter, and cast it int the function body:
int foo_workaround (int m, void *x)
{
int (*arr)[m] = static_cast<int (*)[m]>(x);
//...
}
There are other solutions if you do not want to rely on a compiler extension. If you don't mind a separate allocation for each row, you can use a vector of vectors, for example:
std::vector<std::vector<int> > arr(n, std::vector<int>(m));
However, if you want a single allocation block like you demonstrated in your own example, then it is better to create a wrapper class around vector
to give you 2-d like syntax.
template <typename T>
class vector2d {
int n_;
int m_;
std::vector<T> vec_;
template <typename I>
class vector2d_ref {
typedef std::iterator_traits<I> TRAITS;
typedef typename TRAITS::value_type R_TYPE;
template <typename> friend class vector2d;
I p_;
vector2d_ref (I p) : p_(p) {}
public:
R_TYPE & operator [] (int j) { return *(p_+j); }
};
typedef std::vector<T> VEC;
typedef vector2d_ref<typename VEC::iterator> REF;
typedef vector2d_ref<typename VEC::const_iterator> CREF;
template <typename I>
vector2d_ref<I> ref (I p, int i) { return p + (i * m_); }
public:
vector2d (int n, int m) : n_(n), m_(m), vec_(n*m) {}
REF operator [] (int i) { return ref(vec_.begin(), i); }
CREF operator [] (int i) const { return ref(vec_.begin(), i); }
};
The wrapper's operator[]
returns an intermediate object that also overloads operator[]
to allow 2-dimensional array syntax when using the wrapper.
vector2d<int> v(n, m);
v[i][j] = 7;
std::cout << v[i][j] << '\n';
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