问题描述
我检查了 abs
和 fabs
在python
据我了解,关于速度和传递的类型有些区别,但是我的问题与VS上的本机c ++有关
关于VS
我在 Visual Studio 2013(v120)
上尝试了以下操作:
float f1 = abs(-9.2); // f = 9.2
float f2 = fabs(-9); //编译错误[*]
所以 fabs(-9)
会给我一个编译器错误,但是当我尝试执行以下操作时:
double i =- 9;
float f2 = fabs(i); //这可以正常工作
我从第一个代码中了解到,由于 fabs(-9)
需要加倍,并且编译器无法将-9转换为-9.0,但是在第二个代码中,编译器将转换 i =- 9
到 i = -9.0
,因此 fabs(i)
可以正常工作
还有更好的解释吗?
另一件事,为什么编译器不能接受 fabs(-9)
并将int值转换为像c#一样自动加倍?
[*]:
错误:一个以上的重载函数 fabs实例与参数列表匹配:
函数 fabs(double _X)
函数 fabs (float _X)
函数 fabs(long double _X)
参数类型为:(int)
在C ++中, std :: abs
对于有符号整数和浮点po均重载。 int类型。 std :: fabs
仅处理浮点类型(C ++ 11之前的版本)。请注意, std ::
很重要,出于遗留原因通常可用的C函数 :: abs
处理 int
!
<$ p $的问题p>
float f2 = fabs(-9);
不是说 int
( -9
的类型)到 double
的类型,但是编译器不知道选择哪个转换( int
-> float
, double
, long double
),因为这三个都各有一个 std :: fabs
。解决方法明确地告诉编译器使用 int
-> double
转换,这样就消除了歧义。
C ++ 11通过添加 double fabs(Integral arg);
来解决此问题,这将返回任何整数类型的abs
都转换为 double
。显然,这种过载在libstdc ++和libc ++的C ++ 98模式下也可用。
通常,只需使用 std :: abs
,它将做正确的事情。 (有趣的陷阱 / 1708801 / shafik-yaghmour> @ Shafik Yaghmour 。无符号整数类型在C ++中会很有趣。)
I checked the difference between abs
and fabs
on python here
As I understand there are some difference regarding the speed and the passed types, but my question related to native c++ on V.S.
Regarding the V.S.I tried the following on Visual Studio 2013 (v120)
:
float f1= abs(-9.2); // f = 9.2
float f2= fabs(-9); // Compile error [*]
So fabs(-9)
it will give me a compiler error, but when I tried to do the following:
double i = -9;
float f2= fabs(i); // This will work fine
What I understand from the first code that it will not compile because fabs(-9)
need a double, and the compiler could not convert -9 to -9.0, but in the second code the compiler will convert i=-9
to i=-9.0
at compile time so fabs(i)
will work fine.
Any better explanation?
Another thing, why the compiler can't accept fabs(-9)
and convert the int value to double automatically like what we have in c#?
[*]:
Error: more than one instance of overloaded function "fabs" matches the argument list:
function "fabs(double _X)"
function "fabs(float _X)"
function "fabs(long double _X)"
argument types are: (int)
In C++, std::abs
is overloaded for both signed integer and floating point types. std::fabs
only deals with floating point types (pre C++11). Note that the std::
is important, the C function ::abs
that is commonly available for legacy reasons will only handle int
!
The problem with
float f2= fabs(-9);
is not that there is no conversion from int
(the type of -9
) to double
, but that the compiler does not know which conversion to pick (int
-> float
, double
, long double
) since there is a std::fabs
for each of those three. Your workaround explicitly tells the compiler to use the int
-> double
conversion, so the ambiguity goes away.
C++11 solves this by adding double fabs( Integral arg );
which will return the abs
of any integer type converted to double
. Apparently, this overload is also available in C++98 mode with libstdc++ and libc++.
In general, just use std::abs
, it will do the right thing. (Interesting pitfall pointed out by @Shafik Yaghmour. Unsigned integer types do funny things in C++.)
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