我怎样才能避免编译错误:std :: transform？

Question

这是我的C++代码(我使用的是Visual C++ 2010):

int absd(int t)
{
    return abs(t);
}
int main()
{
    try
    {
        int dpi = 137;
        int dpiCriterionAry[] = {100, 150, 200, 300, 400, 500, 600};
        std::vector<int> vec(dpiCriterionAry, dpiCriterionAry + _countof(dpiCriterionAry));
        std::transform(vec.begin(), vec.end(), vec.begin(), std::bind1st(std::minus<int>(), dpi));
        std::transform(vec.begin(), vec.end(), vec.begin(), absd);
        //std::transform(vec.begin(), vec.end(), vec.begin(), abs);
        copy(vec.begin(), vec.end(), ostream_iterator<int>(cout, "\t"));
        cout << endl;
    }
    catch(exception& e)
    {
        cerr << e.what() << endl;
    }
    return 0;
}

当我取消注释该行时:

//std::transform(vec.begin(), vec.end(), vec.begin(), abs);

我收到了错误消息:

1>------ Build started: Project: Console, Configuration: Release Win32 ------ 
1>Build started 2012/10/16 21:17:19. 1>InitializeBuildStatus: 1>  Creating "..\Intermediate\Console.unsuccessfulbuild" because "AlwaysCreate" was specified. 1>ClCompile: 1>  Console.cpp
1>Console.cpp(16): error C2780: '_OutIt std::transform(_InIt1,_InIt1,_InIt2,_OutIt,_Fn2)' : expects 5 arguments - 4 provided 
1>          D:\ProgramFiles\VS 2010\VC\include\algorithm(1155) : see declaration of 'std::transform'
1>Console.cpp(16): error C2914: 'std::transform' : cannot deduce template argument as function argument is ambiguous 
1>Console.cpp(16): error C2914: 'std::transform' : cannot deduce template argument as function argument is ambiguous 
1>Console.cpp(16): error C2784: '_OutIt std::transform(_InIt,_InIt,_OutIt,_Fn1)' : could not deduce template argument for '_OutIt' from 'std::_Vector_iterator<_Myvec>' 
1>          with 
1>          [ 
1>              _Myvec=std::_Vector_val<int,std::allocator<int>> 
1>          ] 
1>          D:\ProgramFiles\VS 2010\VC\include\algorithm(1051) : see declaration of 'std::transform' 
1> 
1>Build FAILED. 
1> 
1>Time Elapsed 00:00:02.48 ========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========

但是,代码行:

std::transform(vec.begin(), vec.end(), vec.begin(), absd);

能行得通.事实上,我使用的是相同的功能:abs我对结果感到困惑.此外,我想知道是否可以将以下两行代码合并为一行(即一次std::transform调用,效果相同):

std::transform(vec.begin(), vec.end(), vec.begin(), std::bind1st(std::minus<int>(), dpi));
std::transform(vec.begin(), vec.end(), vec.begin(), absd);

有人可以帮我解决这两个问题吗？

Answer 1

问题是std::abs有几个重载.这就是编译器无法确定您在std::transform函数调用中尝试使用哪个重载的原因.

为了解决它,你可以做@Xeo在他的评论中提到的:

std::transform(vec.begin(), vec.end(), vec.begin(), [&](int i){ return abs(i); });

或者你可以静态强制转换以获得适当的函数地址:

std::transform(vec.begin(), vec.end(), vec.begin(), static_cast<int(*)(int)>(abs));

关于你的最后一个问题,是的,lambdas在这里起了作用的作用:

std::transform(vec.begin(), vec.end(), vec.begin(), [&](int i) { return absd(dpi - i); } );