表达的大小& sizeof of type [英] sizeof of expression & sizeof of type
问题描述
为什么需要使用两种sizeof运算符：
* sizeof一元表达式，
* sizeof（类型名称）
？
他们的行为似乎没有什么不同（见下面的例子）。
------ C ++代码 - -----
#include< iostream>
使用命名空间std;
int main（）
{
int x = 100;
cout<< sizeof ++ x<< ENDL; //表达式sizeof
cout<< x<< endl;
x = 100;
cout<< endl;
cout<< sizeof（++ x）<< ENDL; //输入sizeof
cout<< x<< endl;
返回0;
}
---------------- ------
------运行------
4
100
4
100
------------- ----
Alex Vinokur
电子邮件：alex DOT vinokur AT gmail DOT com
http://mathforum.org/library/view/10978.html
http://sourceforge.net/users/alexvn
Why does one need to use two kinds of sizeof operator:
* sizeof unary-expression,
* sizeof (type-name)
?
Their behavior seem not to be different (see an example below).
------ C++ code ------
#include <iostream>
using namespace std;
int main()
{
int x = 100;
cout << sizeof ++x << endl; // expression sizeof
cout << x << endl;
x = 100;
cout << endl;
cout << sizeof (++x) << endl; // type sizeof
cout << x << endl;
return 0;
}
----------------------
------ Run ------
4
100
4
100
-----------------
Alex Vinokur
email: alex DOT vinokur AT gmail DOT com
http://mathforum.org/library/view/10978.html
http://sourceforge.net/users/alexvn
推荐答案
Alex Vinokur写道：
Alex Vinokur wrote:
为什么需要使用两种sizeof运算符：
* sizeof unary-expression，
* sizeof（类型名称）
？
因为你可能想在一个值上使用它来找出它的大小，或者你想要在一个类型上使用它来查找它的大小。
他们的行为似乎没有什么不同（见下面的例子）。
------ C ++代码------
＃包括< iostream>
使用命名空间std;
int main（）
{/> int x = 100;
cout< < sizeof ++ x<< ENDL; //表达sizeof
cout<< x<< endl;
x = 100;
cout<< endl;
cout<< sizeof（++ x）<< ENDL; //类型sizeof
（++ x）不是类型。 int将是一种类型。
cout<< x<< endl;
返回0;
}
----------------------
4
100
4
100
Why does one need to use two kinds of sizeof operator:
* sizeof unary-expression,
* sizeof (type-name)
?
Because you might want to use it on a value to find out its size, or you
want to use it on a type to find out its size.
Their behavior seem not to be different (see an example below).
------ C++ code ------
#include <iostream>
using namespace std;
int main()
{
int x = 100;
cout << sizeof ++x << endl; // expression sizeof
cout << x << endl;
x = 100;
cout << endl;
cout << sizeof (++x) << endl; // type sizeof
(++x) is not a type. int would be a type.
cout << x << endl;
return 0;
}
----------------------
------ Run ------
4
100
4
100
" Rolf Magnus" < RA ****** @ t-online.de>在消息新闻中写道：e6 ************* @ news.t-online.com ...
"Rolf Magnus" <ra******@t-online.de> wrote in message news:e6*************@news.t-online.com...
Alex Vinokur写道：
Alex Vinokur wrote:
[snip]
[snip]
int main（）
{
int x = 100;
cout<< sizeof ++ x<< ENDL; //表达sizeof
cout<< x<< endl;
x = 100;
cout<< endl;
cout<< sizeof（++ x）<< ENDL; // type sizeof
int main()
{
int x = 100;
cout << sizeof ++x << endl; // expression sizeof
cout << x << endl;
x = 100;
cout << endl;
cout << sizeof (++x) << endl; // type sizeof
（++ x）不是一种类型。 int将是一个类型。
(++x) is not a type. int would be a type.
[snip]
我认为，++ x在sizeof（++ x）中被解析为typename 。
-
Alex Vinokur
电子邮件：alex DOT vinokur AT gmail DOT com
http://mathforum.org/library/view/10978.html
[snip]
I think, ++x is parsed as typename in sizeof (++x).
--
Alex Vinokur
email: alex DOT vinokur AT gmail DOT com
http://mathforum.org/library/view/10978.html
http://sourceforge.net/users/alexvn
Alex Vinokur写道：
Alex Vinokur wrote:
" Rolf Magnus" < RA ****** @ t-online.de>在消息新闻中写道：e6 ************* @ news.t-online.com ...
"Rolf Magnus" <ra******@t-online.de> wrote in message news:e6*************@news.t-online.com...
Alex Vinokur写道：
Alex Vinokur wrote:
[snip]
[snip]
int main（）
{/> int x = 100;
cout << sizeof ++ x<< ENDL; //表达sizeof
cout<< x<< endl;
x = 100;
cout<< endl;
cout<< sizeof（++ x）<< ENDL; // type sizeof
int main()
{
int x = 100;
cout << sizeof ++x << endl; // expression sizeof
cout << x << endl;
x = 100;
cout << endl;
cout << sizeof (++x) << endl; // type sizeof
（++ x）不是一种类型。 int将是一个类型。
(++x) is not a type. int would be a type.
[snip]
我认为，++ x在sizeof（++ x）中被解析为typename。
[snip]
I think, ++x is parsed as typename in sizeof (++x).
不是它是一元表达式，因为一个主表达式也是一个表达式，而且（表达式）是一个主表达式并且很明显
++ x是一个表达式。
No it is a unary-expression because a primary-expression is also a
unary-expression and ( expression ) is a primary-expression and clearly
++x is an expression.
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