g++ compiler error: couldn't deduce template parameter ‘_Funct’

c++, foreach, templates

Solution

A names refers to an overload set. You'll need to specify which overload you want:

std::for_each(v.begin(), v.end(), (void (&)(S const&)) print_struct);

Another approach is to use a polymorphic callable function object as a helper:

struct PrintStruct
{
    template <typename T> void operator()(T const& v) const 
        { return print_struct(v); }
};

int main()
{
    PrintStruct helper;

    std::vector<S> sv;
    std::vector<int> iv;

    // helper works for both:
    std::for_each(sv.begin(), sv.end(), helper);
    std::for_each(iv.begin(), iv.end(), helper);

Problem

I'm trying to use an ANSI C++ for_each statement to iterate over and print the elements of a standard vector. It works if I have the for_each call a non-overloaded function, but yields a compiler error if I have it call an overloaded function. Here's a minimal test program to show where the compiler error occurs: ``` #include <algorithm> #include <iostream> #include <vector> struct S { char c; int i; }; std::vector<S> v; void print_struct(int idx); void print_struct(const struct S& s); // f: a non-overloaded version of the preceding function. void f(const struct S& s); int main() { v.push_back((struct S){'a', 1}); v.push_back((struct S){'b', 2}); v.push_back((struct S){'c', 3}); for (unsigned int i = 0; i < v.size(); ++i) print_struct(i); /* ERROR! */ std::for_each(v.begin(), v.end(), print_struct); /* WORKAROUND: */ std::for_each(v.begin(), v.end(), f); return 0; } // print_struct: Print a struct by its index in vector v. void print_struct(int idx) { std::cout << v[idx].c << ',' << v[idx].i << '\n'; } // print_struct: Print a struct by reference. void print_struct(const struct S& s) { std::cout << s.c << ',' << s.i << '\n'; } // f: a non-overloaded version of the preceding function. void f(const struct S& s) { std::cout << s.c << ',' << s.i << '\n'; } ``` I compiled this in openSUSE 12.2 using: ``` g++-4.7 -ansi -Wall for_each.cpp -o for_each ``` The full error message is: ``` for_each.cpp: In function ‘int main()’: for_each.cpp:31:48: error: no matching function for call to ‘for_each(std::vector<S>::iterator, std::vector<S>::iterator, <unresolved overloaded function type>)’ for_each.cpp:31:48: note: candidate is: In file included from /usr/include/c++/4.7/algorithm:63:0, from for_each.cpp:5: /usr/include/c++/4.7/bits/stl_algo.h:4436:5: note: template<class _IIter, class _Funct> _Funct std::for_each(_IIter, _IIter, _Funct) /usr/include/c++/4.7/bits/stl_algo.h:4436:5: note: template argument deduction/substitution failed: for_each.cpp:31:48: note: couldn't deduce template parameter ‘_Funct’ ``` I don't see any search results for this particular error on Stack Overflow, or on the web generally. Any help would be appreciated.

Original source