cpp 使用带有 size_t 的 abs() 函数答案

【问题标题】：cpp using abs() function with size_tcpp 使用带有 size_t 的 abs() 函数
【发布时间】：2018-10-09 01:04:10
【问题描述】：

我正在尝试检查两个字符是否是较大字符串中的邻居，但无法检查它们索引中差异的绝对值。以下是我的代码：

string state_string = "012345";
string goal_state_string = "125430";
for (int i = 0; i < state_string.length() - 1; i++)
{
    cout << abs(goal_state_string.find(state_string[i]) - goal_state_string.find(state_string[i + 1])) << endl;
}

当我运行时，我得到以下输出：

$ make test
g++ -std=c++11 test.cpp
test.cpp: In function ‘int main()’:
test.cpp:107:106: error: call of overloaded ‘abs(std::basic_string<char>::size_type)’ is ambiguous
         cout << abs(goal_state_string.find(state_string[i]) - goal_state_string.find(state_string[i + 1])) << endl;
                                                                                                          ^
In file included from /usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/cstdlib:75:0,
                 from /usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/ext/string_conversions.h:41,
                 from /usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/bits/basic_string.h:6349,
                 from /usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/string:52,
                 from /usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/bits/locale_classes.h:40,
                 from /usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/bits/ios_base.h:41,
                 from /usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/ios:42,
                 from /usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/ostream:38,
                 from /usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/iostream:39,
                 from test.cpp:1:
/usr/include/stdlib.h:70:5: note: candidate: int abs(int)
 int abs (int);
     ^~~
In file included from /usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/cstdlib:77:0,
                 from /usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/ext/string_conversions.h:41,
                 from /usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/bits/basic_string.h:6349,
                 from /usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/string:52,
                 from /usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/bits/locale_classes.h:40,
                 from /usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/bits/ios_base.h:41,
                 from /usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/ios:42,
                 from /usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/ostream:38,
                 from /usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/iostream:39,
                 from test.cpp:1:
/usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/bits/std_abs.h:56:3: note: candidate: long int std::abs(long int)
   abs(long __i) { return __builtin_labs(__i); }
   ^~~
/usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/bits/std_abs.h:61:3: note: candidate: long long int std::abs(long long int)
   abs(long long __x) { return __builtin_llabs (__x); }
   ^~~
/usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/bits/std_abs.h:70:3: note: candidate: constexpr double std::abs(double)
   abs(double __x)
   ^~~
/usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/bits/std_abs.h:74:3: note: candidate: constexpr float std::abs(float)
   abs(float __x)
   ^~~
/usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/bits/std_abs.h:78:3: note: candidate: constexpr long double std::abs(long double)
   abs(long double __x)
   ^~~
make: *** [Makefile:37: test] Error 1

hp user@DESKTOP-PGC8V9D /cygdrive/e/Google Drive/Phd_git/psvn/core/lessons/A-star
$ make test
g++ -std=c++11 test.cpp
test.cpp: In function ‘int main()’:
test.cpp:13:106: error: call of overloaded ‘abs(std::basic_string<char>::size_type)’ is ambiguous
         cout << abs(goal_state_string.find(state_string[i]) - goal_state_string.find(state_string[i + 1])) << endl;
                                                                                                          ^
In file included from /usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/cstdlib:75:0,
                 from /usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/ext/string_conversions.h:41,
                 from /usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/bits/basic_string.h:6349,
                 from /usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/string:52,
                 from /usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/bits/locale_classes.h:40,
                 from /usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/bits/ios_base.h:41,
                 from /usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/ios:42,
                 from /usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/ostream:38,
                 from /usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/iostream:39,
                 from test.cpp:1:
/usr/include/stdlib.h:70:5: note: candidate: int abs(int)
 int abs (int);
     ^~~
In file included from /usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/cstdlib:77:0,
                 from /usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/ext/string_conversions.h:41,
                 from /usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/bits/basic_string.h:6349,
                 from /usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/string:52,
                 from /usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/bits/locale_classes.h:40,
                 from /usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/bits/ios_base.h:41,
                 from /usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/ios:42,
                 from /usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/ostream:38,
                 from /usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/iostream:39,
                 from test.cpp:1:
/usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/bits/std_abs.h:56:3: note: candidate: long int std::abs(long int)
   abs(long __i) { return __builtin_labs(__i); }
   ^~~
/usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/bits/std_abs.h:61:3: note: candidate: long long int std::abs(long long int)
   abs(long long __x) { return __builtin_llabs (__x); }
   ^~~
/usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/bits/std_abs.h:70:3: note: candidate: constexpr double std::abs(double)
   abs(double __x)
   ^~~
/usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/bits/std_abs.h:74:3: note: candidate: constexpr float std::abs(float)
   abs(float __x)
   ^~~
/usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/bits/std_abs.h:78:3: note: candidate: constexpr long double std::abs(long double)
   abs(long double __x)
   ^~~
make: *** [Makefile:37: test] Error 1

我的问题如下：

不应发现 find 函数返回某种整数类型，即。一个无符号整数，如果是这样，为什么不呢。有什么意义拥有这种似乎对大多数人不起作用的替代类型它应该调用哪个函数？

使用此类型/避免此问题时的最佳做法是什么？我应该将其转换为 int 吗？

【问题讨论】：

正如您在编译器输出中看到的，abs 是为签名类型定义的。 std::string::size_type 未签名。要正确找到两个无符号值之间的距离，首先确定其中一个最大，然后从中减去另一个。
我之前已经发布了一个答案，但我现在看到你的方法的主要警告是，如果find 无法检索到搜索的字符串，它会返回std::string::npos，即-1，那对于无符号值意味着最大的可表示值。一旦你处理了这个问题，你就可以在这个 code review's post 中找到如何区分两个无符号整数
另见Is std::abs(0u) ill-formed?

标签： c++

【解决方案1】：

要了解`abs`是做什么的，我们必须了解`signed`和`unsigned`类型之间的区别。

考虑以下语句：

a = 10
b = -20
c = a + b

c 的结果是否取决于 a,b,c 是有符号还是无符号？

令人惊讶的是它没有。因为计算机使用二进制补码处理数字，所以如果两个数字相隔 2^N，则认为它们是同一个数字。

不仅add,subtract不关心符号，multiplication也不关心符号：

x*(2^N - y) = x*2^N - x*(-y) = x*(-y) mod 2^N

由于这种影响，有符号数和无符号数之间的限制被放宽了。

更多细节可以在X86架构的指令集中找到。实际上 X86 定义了一个统一加法，它同时处理有符号和无符号加法，有符号和无符号加法的唯一区别是载波位（也称为上溢/下溢位）。

有些运营商确实关心符号。喜欢

打印。人们肯定更喜欢 -1 而不是 4294967295，所以 printf 将有符号和无符号“%u”的符号分开
比较。 4294967295 > 0，但 -1
abs 函数
还有更多...

在使用关心符号的运算符时，您最好更加小心。这就是为什么abs 不允许带无符号参数的原因。

【讨论】：

现在我从来没有做过一个不需要的目标，但是 C++ 不需要对有符号整数进行补码。它只是该问题最常用的解决方案。
实际上将无符号值放入 abs 是没有意义的。因为它之后总是一样的，因为它是积极的。就像@user4581301 指出的溢出是未定义的行为，即使目标平台将其实现为二进制补码。编译器不必遵守，可以优化奇怪的东西。

要了解abs是做什么的，我们必须了解signed和unsigned类型之间的区别。

有些运营商确实关心符号。喜欢

要了解`abs`是做什么的，我们必须了解`signed`和`unsigned`类型之间的区别。