how can i avoid the compiler error: std::transform?

Question

Here is my C++ code (I'm using 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;
}

when I uncomment the line:

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

I got the error message:

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 ==========

but, the line of code:

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

can work. In fact, what I use is the same function: abs, I'm confused about the result. Besides, I want to know if it's possible to merge the following two lines of code into one(that is one std::transform call, with the same effect):

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

Anyone can help me about the two questions?

Answer 1

The problem is that std::abs has several overloads. That's why the compiler can't figure out which overload you are trying to use in the std::transform function call.

In order to fix it, you could do what @Xeo mentioned in his comment:

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

Or you could static cast to get the appropriate function address:

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

Regarding your last question, yes, lambdas work as a charm here:

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

Answer 2

abs is an overloaded function, not a single entity, so the call to std::transform cannot deduce the template argument because there is not a single function referred to by the name abs.

It is equivalent to this:

void foo(int) { }
void foo(const char*) { }

template<typename T>
int bar(T)
{ return 0; }

int i = bar(foo);  // which 'foo' ?

You can tell the compiler which abs you mean by explicitly converting it to the type of the function you mean:

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

Or by creating a variable referring to the function you want:

int (*p)(int) = abs;
std::transform(vec.begin(), vec.end(), vec.begin(), p);

To combine the two transformations into one you could use a lambda, as Xeo suggests, or use std::bind e.g.

using std::placeholders::_1;
int (*p)(int) = abs;
std::transform(vec.begin(), vec.end(), vec.begin(),
               std::bind(p, std::bind(std::minus<int>(), dpi, _1)));

Or for C++03 you could use boost::bind instead.