What should std::vector::data() return if the vector is empty?(如果向量为空, std::vector::data() 应该返回什么?)
问题描述
根据标准草案(23.3.6.4 向量数据)、data()指向底层数组且[data(), data() + size())
必须是有效范围:
According to the draft standard (23.3.6.4 vector data), data() points to the underlying array and [data(), data() + size())
must be a valid range:
T* data() noexcept;
const T* data() const noexcept;
1 Returns: A pointer such that [data(),data() + size()) is a valid range. For a non-empty vector,
data() == &front().
2 Complexity: Constant time
但是如果向量是空的呢?当我构造一个零大小的向量时:
But what if the vector is empty? When I construct a zero-size vector:
#include <vector>
#include <iostream>
int main() {
const int NUM = 0*10;
std::vector< double > v( NUM, 0.0 );
std::cerr << "V : "<< v.data() << std::endl;
}
MSVC 2010 返回 null,但在 Linux(使用 GCC 4.2.1 和 Intel 12.1)上我得到一个非 null 地址.
MSVC 2010 returns null, but on Linux (with GCC 4.2.1 and Intel 12.1) I get a non-null address.
是否允许 vector::data()
或者它应该返回 null?例如,实现是否可以进行默认大小的初始分配并返回指向它的(非空)指针?
Is vector::data()
allowed to or should it return null? Could an implementation, for example, do a default-size initial allocation and return a (non-null) pointer to it?
几个答案侧重于空范围的有效性.我完全同意.
Several answers focus on the validity of an empty-range. I fully agree there.
我真的很想看到一个很好的参考或解释:是否允许,必须返回空值或可能也返回非空?
I would really like to see a good reference or explanation for: Is it allowed to, must it return null or may it also return non-null?
推荐答案
范围的约定是[inclusive,exclusive)
,也就是说如果你遍历一个范围 [X,Y)
您将从概念上执行以下操作(伪代码):
The convention for ranges is [inclusive, exclusive)
, that is if you iterate over a range [X,Y)
you will conceptually do the following (pseudo-code):
for( iterator ii = X; ii != Y; ++ii) {
...
}
这允许将空范围表示为 [X,X)
.此外,这个空范围为每个地址定义得很好,无论它是有效还是无效.
This permits to express an empty range as [X,X)
. Also this empty range is perfectly well defined for each address, no matter if it is valid or invalid.
也就是说 data()
的要求是(强调我的):
That said the requirements for data()
are (emphasis mine):
23.3.6.4 [vector.data]
T* data() noexcept;
T* data() noexcept;
const T* data() const noexcept;
const T* data() const noexcept;
返回:一个指针,使得[data(),data() + size()) 是一个有效范围.为一个非空向量,data() == &front().
Returns: A pointer such that [data(),data() + size()) is a valid range. For a non-empty vector, data() == &front().
在我看来,唯一的无条件保证是 [data(),data() + size())
应该是一个有效范围.对于 size() == 0
成员函数 data()
可以返回任何值,范围将是一个有效的空范围.因此,我会说如果size()
为零,则允许实现返回非空指针.
It seems to me that the only unconditional guarantee is that [data(),data() + size())
should be a valid range. For size() == 0
the member function data()
may return any value and the range will be a valid empty range. Therefore I would say that an implementation is allowed to return a non-null pointer if size()
is zero.
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