CODEWITHSUNDEEP
c++c++11templatesstlstd
mbed_dev
mbed_dev

Reputation: 1468

How to pass binary function to template function C++

I want to pass the standardized C++ binary functions to a template function, but somehow I didn't get it to work.

The following is my attempt to do it:

template<template <typename> typename Pred,typename T, typename Iterator>
void iota_stepa(Iterator begin, Iterator end, T startofSequence_, T threadStep)
{
    int currMaxThreads = startofSequence_;
    bool first = true;
    generate(begin, end,  Pred<T>(currMaxThreads, threadStep) );
}

and testing it with:

vector<int> tempVect_(10, 0);

iota_stepa<std::plus>(begin(tempVect_),end(tempVect_),1,thread::hardware_concurrency());

gives me unfortunately the errors:

Severity    Code    Description Project File    Line    Suppression State
Error   C2440   '<function-style-cast>': cannot convert from 'initializer list' to 'std::plus<int>'
Error   C2672   'generate': no matching overloaded function found   FractalCarpet   
Error   C2780   'void std::generate(_FwdIt,_FwdIt,_Fn0)': expects 3 arguments - 2 provided  FractalCarpet

The console output looks like the following:

1>  c:\users\mtunca\documents\esd\sps\fractalcarpet\main.cpp(55): note: see reference to function template instantiation 'void iota_stepa<std::plus,int,std::_Vector_iterator<std::_Vector_val<std::_Simple_types<float>>>>(Iterator,Iterator,T,T)' being compiled
1>          with
1>          [
1>              Iterator=std::_Vector_iterator<std::_Vector_val<std::_Simple_types<float>>>,
1>              T=int
1>          ]
1>c:\users\mtunca\documents\esd\sps\fractalcarpet\main.cpp(34): error C2672: 'generate': no matching overloaded function found
1>c:\users\mtunca\documents\esd\sps\fractalcarpet\main.cpp(34): error C2780: 'void std::generate(_FwdIt,_FwdIt,_Fn0)': expects 3 arguments - 2 provided
1>  c:\program files (x86)\microsoft visual studio 14.0\vc\include\algorithm(1532): note: see declaration of 'std::generate'

Could someone help me, how to solve this problem?

Upvotes: 2

Views: 898

Answers (2)

n. m. could be an AI
n. m. could be an AI

Reputation: 120051

Pred<T>(currMaxThreads, threadStep) );

Pred<T> is a type. You need to construct an actual callable object:

Pred<T>()(currMaxThreads, threadStep) );

This however cannot be the last argument to std::generate. The latter requires a callable object with no arguments, presumably holding a state (otherwise a call to std::fill woud suffice). It is unclear how an arbitrary binary function could be adapted to fill this role.

Upvotes: 0

sp2danny
sp2danny

Reputation: 7687

std::generate needs a generator, something that can be called like gen(). You could create one with a lambda, perhaps like this:

template<template <typename> class Pred, typename T, typename Iterator>
void iota_stepa(Iterator begin, Iterator end, T startofSequence_, T threadStep)
{
    bool first = true;
    T current;
    auto gen = [&]() -> T
    {
        if(first) {
            current = startofSequence_;
            first = false;
        } else {
            current = Pred<T>() ( current, threadStep );
        }
        return current;
    };
    generate(begin, end, gen );
}

Upvotes: 3

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