vector :: resize（）和内存分配 [英] vector::resize() and memory allocation
问题描述
---------
#include< vector>
using namespace std;
struct Foo
{
Foo（int）{}
};
int main（）
{
vector< Foo> v1;
v1.resize（1）;
v1 [0] = Foo（100）;
//检查点
返回0;
}
---------
多少Foo实例是在检查点分配的内存？
似乎是2个坚持？
如果是真的，那就意味着使用了vector :: resize（）是否需要分配额外的内存？
-
Alex Vinokur
http://mathforum.org/library/view/10978.html
http://sourceforge.net/users/alexvn
Alex Vinokur写道：---------
#include< vector>
使用命名空间std;
struct Foo
{
Foo（int）{}
};
int main（）
{
vector< Foo> v1;
v1.resize（1）;
v1 [0] = Foo（100）;
//检查点
返回0;
}
---------
对于在检查点分配的内存有多少个Foo实例？
似乎是2个坚持？
如果是是真的是否意味着使用vector :: resize（）涉及分配额外的内存？
该程序格式错误，因为''Foo''不合适要求
" Default-constructible"要求调整''调整大小''。它应该
不能编译。
现在，如果你错过了那个东西，实际上并不是要问
a技巧问题（据说''Foo''实际上有一个默认的c-tor），
然后未指定多少个对象''v1''实际上可以包含
请求调整大小后，因为它的初始容量为0
和''resize''_ will会导致重新分配以容纳您的对象。
分配的大小发生是未指定的（实现 -
定义，我相信）。
所以，在''Foo''给出一个默认的c-tor之后，回答到你的第一个
问题是未知或者实施定义，你的第二个问题的回答是我不明白，而你第三个问题的答案是b $ b ;是的，可能是。
Victor
" Victor Bazarov" <五******** @ comAcast.net>在消息新闻中写道：u7 *************** @ newsread1.dllstx09.us.to.ve rio.net ...Alex Vinokur写道：< blockquote class =post_quotes> ---------
#include< vector>
使用命名空间std;
struct Foo
{
Foo （int）{}
};
int main（）
{
vector< Foo> v1;
v1.resize（1）;
v1 [0] = Foo（100）;
//检查点
返回0;
}
---------
对于在检查点分配的内存有多少个Foo实例？
似乎是2个坚持？
如果是是真的是否意味着使用vector :: resize（）涉及分配额外的内存？
该程序格式不正确，因为''Foo''不符合要求
Default-constructible"要求调整''调整大小''。它应该不会编译。
[snip]
为了简化程序，我删除了Foo（）{} ;错误。
这是更新的程序。
---------
#include< vector>
使用命名空间std;
struct Foo
{
Foo（）{}
Foo（int）{}
};
int main（）
{
vector< ;富> v1;
v1.resize（1）;
v1 [0] = Foo（100）;
//检查点
返回0;
}
---------
-
Alex Vinokur
http://mathforum.org/library/查看/ 10978.html
http://sourceforge.net/users / alexvn
Victor Bazarov写道：
程序很糟糕因为''Foo''不符合要求而形成
Default-constructible要求调整''调整大小''。它应该不会编译。
编译器应该发出诊断信息。没有要求它不会编译代码。
-
Pete Becker
Dinkumware，Ltd。（ http://www.dinkumware.com ）
---------
#include <vector>
using namespace std;
struct Foo
{
Foo (int) {}
};
int main ()
{
vector<Foo> v1;
v1.resize(1);
v1[0] = Foo (100);
// Check point
return 0;
}
---------
For how many Foo instances is memory allocated at "Check point"?
It seems that for 2 insistences?
If it is truth does it mean that use of vector::resize() involves allocating extra memory?
--
Alex Vinokur
http://mathforum.org/library/view/10978.html
http://sourceforge.net/users/alexvn
Alex Vinokur wrote:---------
#include <vector>
using namespace std;
struct Foo
{
Foo (int) {}
};
int main ()
{
vector<Foo> v1;
v1.resize(1);
v1[0] = Foo (100);
// Check point
return 0;
}
---------
For how many Foo instances is memory allocated at "Check point"?
It seems that for 2 insistences?
If it is truth does it mean that use of vector::resize() involves allocating extra memory?
The program is ill-formed because ''Foo'' does not fit the requirement
"Default-constructible" needed for the call to ''resize''. It should
not compile.
Now, if you just missed that thing, and actually didn''t mean to ask
a trick question (and supposedly ''Foo'' actually has a default c-tor),
then it is unspecified how many objects ''v1'' can actually contain
after you asked to resize it because the initial capacity of it is 0
and ''resize'' _will_ cause reallocation to accommodate your object.
To what size the allocation happens is unspecified (implementation-
defined, I believe).
So, after ''Foo'' is given a default c-tor, the answer to your first
question is "unknown" or "implementation-defined", the answer to your
second question is "I don''t understand", and the answer to your third
question is "yes, possibly".
Victor
"Victor Bazarov" <v.********@comAcast.net> wrote in message news:u7***************@newsread1.dllstx09.us.to.ve rio.net...Alex Vinokur wrote:---------
#include <vector>
using namespace std;
struct Foo
{
Foo (int) {}
};
int main ()
{
vector<Foo> v1;
v1.resize(1);
v1[0] = Foo (100);
// Check point
return 0;
}
---------
For how many Foo instances is memory allocated at "Check point"?
It seems that for 2 insistences?
If it is truth does it mean that use of vector::resize() involves allocating extra memory?
The program is ill-formed because ''Foo'' does not fit the requirement
"Default-constructible" needed for the call to ''resize''. It should
not compile.
[snip]
To simplify the program I removed "Foo() {}" by mistake.
Here is the updated program.
---------
#include <vector>
using namespace std;
struct Foo
{
Foo () {}
Foo (int) {}
};
int main ()
{
vector<Foo> v1;
v1.resize(1);
v1[0] = Foo (100);
// Check point
return 0;
}
---------
--
Alex Vinokur
http://mathforum.org/library/view/10978.html
http://sourceforge.net/users/alexvn
Victor Bazarov wrote:
The program is ill-formed because ''Foo'' does not fit the requirement
"Default-constructible" needed for the call to ''resize''. It should
not compile.
The compiler should issue a diagnostic. There is no requirement that it
not compile the code.
--
Pete Becker
Dinkumware, Ltd. (http://www.dinkumware.com)
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