c++ How to implement compose2

Question

#include <iostream>
#include <vector>
#include <cstdio>
#include <cstring>
#include <cassert>
#include <algorithm>
#include <ctime>
#include <iterator>
#include <string>
#include <numeric>

template <typename BinaryFunction, typename UnaryFunction1, typename UnaryFunction2>
struct compose2 {
    compose2(BinaryFunction binFunc, UnaryFunction1 unFunc1, UnaryFunction2 unFunc2)
        : m_binFunc(binFunc)
        , m_unFunc1(unFunc1)
        , m_unFunc2(unFunc2) 
    {}
    typedef typename BinaryFunction::return_type return_type;
    typedef typename UnaryFunction1::argument_type argument_type;
    return_type operator()(argument_type arg) {
        return m_binFunc(m_unFunc1(arg), m_unFunc2(arg));
    }

    BinaryFunction m_binFunc;
    UnaryFunction1 m_unFunc1;
    UnaryFunction2 m_unFunc2;
};

int main() {
    std::vector<int> v;
    v.push_back(1);
    v.push_back(75);
    v.push_back(10);
    v.push_back(65);
    v.push_back(15);
    v.push_back(78);
    v.push_back(14);
    v.push_back(19);
    int x = 10, y = 20;

    std::vector<int>::iterator it = std::find_if(v.begin(), v.end(), 
                    compose2(
                        std::logical_and<bool>(),
                        std::bind1st(std::less<int>(), x), 
                        std::bind1st(std::greater<int>(), y)
                    ));

    std::cout << (it - v.begin()) << std::endl;
}

I have tried to implement compose2 adaptor but this doesn't compile. I am getting main.cpp:43:29: error: missing template arguments before ‘(’ token and don’t know what template arguments I should pass. Why it doesn't detect the types.

I know that this is implemented in boost or other library or in new c++11 standard. But I only want to know why does my implementation fail. Thanks.

Answer 1

The compiler can only deduce template arguments for function templates, not class templates. That leaves you with a few choices: the most obvious (but often least convenient) is to specify the template parameters when you instantiate your compose2.

Marginally less obvious, but often more convenient is to create a function template that deduces the parameters, and creates a compose2 object using the deduced types:

template<class BinaryFunction, class UnaryFunction1, class UnaryFunction2>
compose2<BinaryFunction, UnaryFunction1, UnaryFunction2>
make_compose2(BinaryFunction binFunc, UnaryFunction1 unFunc1, UnaryFunction2 unFunc2) {
        return compose2_t<BinaryFunction, UnaryFunction2, UnaryFunction2>
            (binFunc, unFunc1, unFunc2);
}

Then the client code will use make_compose2 instead of compose2, and the template parameters can/will be deduced from the types of the passed parameters.