c++ - g++ 错误 : call of overloaded 'abs(unsigned int)' is ambiguous

Question

我正在尝试编译以下代码:

#include <stdlib.h>
static
unsigned iSqr( unsigned i )
{
    unsigned res1 = 2;
    unsigned res2 = i/res1;
    while( abs( res1 - res2 ) > 1 )
        {
        res1 = (res1 + res2)/2;
        res2 = i/res1;
        }
    return res1 < res2 ? res1 : res2;
}

使用 g++ test.cc -o test。

但是，g++ 编译器失败并出现以下错误:

test.cc: In function 'unsigned int iSqr(unsigned int)':                                         
test.cc:8:29: error: call of overloaded 'abs(unsigned int)' is ambiguous                        
     while( abs( res1 - res2 ) > 1 )            
                             ^                  
In file included from /usr/include/c++/6/cstdlib:75:0,                                          
                 from /usr/include/c++/6/stdlib.h:36,                                           
                 from test.cc:2:                
/usr/include/stdlib.h:735:12: note: candidate: int abs(int)                                     
 extern int abs (int __x) __THROW __attribute__ ((__const__)) __wur;                            
            ^~~         
In file included from /usr/include/c++/6/stdlib.h:36:0,                                         
                 from test.cc:2:                
/usr/include/c++/6/cstdlib:185:3: note: candidate: __int128 std::abs(__int128)                  
   abs(__GLIBCXX_TYPE_INT_N_0 __x) { return __x >= 0 ? __x : -__x; }                            
   ^~~                  
/usr/include/c++/6/cstdlib:180:3: note: candidate: long long int std::abs(long long int)        
   abs(long long __x) { return __builtin_llabs (__x); }                                         
   ^~~                  
/usr/include/c++/6/cstdlib:172:3: note: candidate: long int std::abs(long int)                  
   abs(long __i) { return __builtin_labs(__i); }                                                
   ^~~     

为什么会发生此错误以及如何解决？

g++版本为:gcc版本6.3.0

Answer 1

abs在 <stdlib.h> (自 c++11 起):http://www.cplusplus.com/reference/cstdlib/abs/

          int abs (          int n);
     long int abs (     long int n);
long long int abs (long long int n);

如果过载，调用将不明确分辨率 无法选择比此类不可微函数更好的调用匹配项。

您可以像这样显式地转换参数:

static_cast<int>(res1 - res2)