是否保证嵌套std ::数据中的数据是连续的？ [英] Is the data in nested std::arrays guaranteed to be contiguous?

问题描述

数据是否在 std :: array< std :: array< T，N>，M> 例如：

  #include< array> 
 #include< cassert> 
 
 int main（）
 {
 enum {M = 4，N = 7}; 
 typedef std :: array< char，N>行; 
 typedef std :: array< Row，M>矩阵; 
 Matrix a; 
 a [1] [0] = 42; 
 const char * data = a [0] .data（）; 
 
 / * 1D数据数组的第8个元素应与第二行的
第1个元素相同。 * / 
 assert（data [7] == 42）; 
} 
  

断言是否成功？或者换句话说，我可以依靠 Row ？

结尾处没有填充> EDIT：为了清楚起见，在本例中，我希望整个矩阵的数据是连续的。

std :: array

code>保证是一个聚合，并且以这样一种方式指定，用于存储的底层数组必须是类型的第一个数据成员。

然而，不需要 sizeof（array ）== sizeof（T）* N ，也没有要求没有未命名的填充字节或者 std :: array 除了底层数组存储之外没有数据成员。 （但是，包含其他数据成员的实现最多只是不寻常的。）

Is the data in std::array<std::array<T,N>, M> guaranteed to be contiguous? For example:

#include <array>
#include <cassert>

int main()
{
    enum {M=4, N=7};
    typedef std::array<char,N> Row;
    typedef std::array<Row, M> Matrix;
    Matrix a;
    a[1][0] = 42;
    const char* data = a[0].data();

    /* 8th element of 1D data array should be the same as
       1st element of second row. */
    assert(data[7] == 42);
}

Is the assert guaranteed to succeed? Or, to put it another way, can I rely on there being no padding at the end of a Row?

EDIT: Just to be clear, for this example, I want the data of the entire matrix to be contiguous.

解决方案

No, contiguity is not guaranteed in this case.

std::array is guaranteed to be an aggregate, and is specified in such a way that the underlying array used for storage must be the first data member of the type.

However, there is no requirement that sizeof(array<T, N>) == sizeof(T) * N, nor is there any requirement that there are no unnamed padding bytes at the end of the object or that std::array has no data members other than the underlying array storage. (Though, an implementation that included additional data members would be, at best, unusual.)

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